CA1194578A

CA1194578A - Pressure monitoring and leak detection method and apparatus

Info

Publication number: CA1194578A
Application number: CA000422818A
Authority: CA
Inventors: Alan R. Kahn; Kenneth R. Clark; Dennis E. Bahr
Original assignee: CARDIOSEARCH Inc
Current assignee: CARDIOSEARCH Inc
Priority date: 1982-03-04
Filing date: 1983-03-03
Publication date: 1985-10-01
Also published as: EP0103624A1; WO1983003136A1; DE3377938D1; EP0103624B1; JPS59500430A; US4441357A; EP0103624A4; JPH0418614B2

Abstract

PRESSURE MONITORING AND LEAK DETECTION
METHOD AND APPARATUS
ABSTRACT
A closed pressure monitoring system is disclosed in which a pump delivers gas on a supply line through a first accumulator chamber to a load device and returns the gas on a return line through a second accumulator chamber to the pump. After pressures in the first and second chambers stabilize, the pressures are compared, and a leak warning signal is given if the compared pressures changed with respect to one another over time. The system is particularly adapted to use a pressure sensor as the load which is implanted within a human patient.
The pressure sensor receives air from a restrictor in the supply line and is operative to maintain the pressure in the supply line substantially equal to the pressure surrounding the sensor.

Description

PRESSURE MONITORING AN~ LEAK DETECTION
METHOD AND APPARATUS
TECHNICAL FIELD
This invention pertains generally to the field of pressure sensors and monitoring equipment and particularly to physiological sensing equipment wherein a sensor is implanted within the body of a patient to monitor internal pressures.
B~CKGROUND ART
AS an aid in the diagnosis and treatment of disease, it is often desirable to monitor the pressures at various positions within a patient's body or adjacent internal organs. A particu lar example is the measurement of the intracranial pressure with in a patient' 9 skull, since such measurements provide an indica-tion of abnormalities in perfusion pressure or fluid retention, and allow the effect of drugs on intracranial pressure to be accurately monitored for effective treatment.
Various types of sensors are now utilized to measure internal body pressures, including electrical pressure trans-ducers which transmit an electrical signal indicative of the pressure through transmission wires out of the patient to a recording device r and pressure sensing heads which expand or contact in response to the pressure within the patient and com-municate through a tube from the patient to a remote transducer which converts the pressure within the tube to a signal which can be displayed to the operator. The use of an electrical trans-ducer implanted within a patient carries the obvious risks of shocks and short circuits, as well as the noise and baseline drift problems associated with any electrical transducer of a size small enough to be implanted. Direct pressure transmitting syskems suffer from a lack of accuracy because of the distance that the pressure head must be kransmitted from khe pakient to the remote transducex. The connection of the implanted sensing head through a tube to an externa] transducer also presents the possibility of a rupture or leak which would release air into the patient and possibly provide a source of infectlon.
In another type of pressure sensing apparatus, air flow is directed to a pxessure sensor which includes a diaphragm covering a cavity within the sensor body. The diaphragm meters flow through an orifice by restrictinq or closing the orifice;
the flow of air through the sensor is thereby controlled to equalize the pressure on both sides of the diaphragm, allowing the pressure at the sensor to be accurately read by reading the pressure of the air flowing in the tube leading to the sensor.
In such a system~ as well as in those which use a pressure sensing head which transmits the pressure through a t~be to an external transducer, there is a small but definite risk that air (or other gas being used as the transmission medium) may leak from either the tubes or the sensor into enclosed places within the patient. Even leaks which occur outside the patient are detrimental to accurate measurement of the pressures within the patient.
Any closed, gas circulating system, in addition to those described above for physiological monitoring, may be subject to leakage which can affect system performance. A leak within such closed systems can result in abnormal pressures within the appa-ratus and erroneous data.
DISCLOSURE OF THE INVENTION
The apparatus of the present invention is particularly suited to monitoring pressures sensed by a pressure sensor which has been implanted within a human subject, allowing very accurate pressure readings to be obtained with a high ~ - 2 -degree of safety. The pressure sen.C;or utiLi%ed ls pref-erably oE ti)e type which has a cliauhragrrl coveriny a cavity within the sensor bocly and positioned to open and close an orifice within tt-)e cavity. ~ substantially constant air flow is pruvided to the CclVit~ with air being withdrawn through the orifice when it is uncovered b~ the diaphraym.
The metering of the orifice by the diaphragm maintains the pressure in the cavity substantially equal to the pressure on the outside of the diaphragm. The system of the invention which supplies air flow to the sensor and returns air from the sensor is completely closed and sealed. Air is circulated by a pump which directs it throuyh tubing to a first accumulator charnber and thence throuyh a restrictor to the sensor, and the air is returned from the sensor through tubing to a secùnd accurnulator chamber and thence back to the inlet of the purnp. All of these structures are sealed so that air cannot leak out of the system and potentially conta~ainated outsic3e air cannot be drawn in.
The pressure within the tubing between the restrictor and the sensor is measured by a load pressure transducer; this measured pressure is substantially equal to the pressure within the cavity in the sensor, and the pressure in the cavity is itself substantially equal to the pressure outside the sensor.
The two accumulator charnbers allow the apparatus to measure both positive and negative pressures arnbient to the pressure sensor~ Preferably, the second accumulator chamber is larger than the first, and the system will stabilize such that the gauge pressure within the second chamber will be a negative pressure. The magnitude of this negative pressure will be equal to the positive pressure ~ithin the first charnber times the ratio of the volume oE
the first chamber to the volurne of the second chamber.
Since a negative or "vacuum" pressure is maintained within the second chamber, the pressure sensor, when sealec3 off Erom the atmosphere, can detect and allow measurement of ne(Jative pressures up to the rnagnituclç of the negative pressure within the second accurnulator chamber.
Since the Elow rate through the first ancl second accumulator chambers must be equal if the systern remains sealed, a c3iEference in the flow rate throu~3h the two 5~
~, chambers inc3icates a leak into or Ollt of the system, and the difference in flow rate will show up as a change in the relative pressures in the two chambers. Under steady state conditions the ratio of pressures within the two chambers or the difference between the pressures in the two chambers must be a constant; therefore, a change in the ratio of pressures or the difference of the pressurés indicates a leak condition. The pressures within these chambers are measured and automatically compared by a controller which provides a signal iE the ratio or difference of pressures varies from a constant. A Waning may be given to the operator that a leak has occurred. In addition, a bypass valve is preferably connected around the pump and is responsive to the leak signal to open and shunt the output of the pUlllp back to its input. The pump may also be controlled to shut off in response to the leak signal. As the puJnp stops and the shunt valve opens, the pressures within the two accumulator chambers ~uickly equalize so that the flow of air to the sensor is cut off.
The utilization of the above described accurnulator chambers in gas supply and return lines can also be advan-tageous in other closed gas flow systelrls where it is desirable to be able to detect leaks~ Leaks can be de-tected in such systems by comparing the pressures in the two cha~nbers in the manner described above.
For maximum accuracy and response time, the pressure rneasured in the line between the restrictor and the pressure sensor may be utilized to control the input power to the pump so as to vary the displacement and flow rate of the pump in direct relation to the measured sensor pres-sure. By so controlling the power to the pump, the flow rate through the sensor may be maintained substantially constant by compensating for the decreased flow rate nor-mally occuring when the pressure at the sensor increases.
In adclition, if the pressure within the first charnber is kept constant by controlling the purnp, a system leak can be detecteci by monitoring only the pressure in the second charnber; this pressure will rernain constant unless a leak occurs.
~0 The mollitoring oE the pressure transducers, the com-parison of the accumulator chaJnber pressures, and the control of the pump and pump bypass valve is preferably done by a microcompu~er prograrnmed to perform the moni-toring and control tasks. The microcomputer controller is well adapted to display to an operator, in digital form or in a continuous read-out such as on a cathode ray tube or a strip chart recorder, the reading of the patient sensor pressure. The controller also preferably provides an audio warning siynal to the operator if the patient pressure sensor transducer shows a pressure reading above or below selected upper and lower pressure limits. The controller is also well adapted to initialize the system and obtain the zero readings for all of ~he transducers and an lnitial reading for the pressure sensor in ambient air; thus, the operator of the equipment requires little training and the equipment does not need significant operator attention during the time that it is in use.
Further objects, features and advantages will be apparent from the following detailed description taken in conjunction with the accompanying drawings showing a preferred embodilllent of apparatus for monitoring pressure and detecting leaks in accordance with the invention~

BRI~F DESCRIPTION OF THE DRAWINGS

In the drawings:
Fig. 1 is a schematic view of the apparatus of the invention.
Fic3. 2 is a schernatic view of one embodiment of a pump which can be utilized in the apparatus of the invention.
Fig. 3 is a block diagram showing the controller portion of the apparatus.
Fic3. ~ is a schematic circuit diagram of the proportional pump driver.
Figs. 5-19 are flow charts which show the operating steps in the programming oE the controller which carry out the monitoring and control tasks within the apparatus.
Fig. 20 is a bottom view of an embodiment of a pressure sensor adapted for implanting within a human patient, a portiorl thereof being broken away ~or illustration; and Fi~. 21 is a cross-sectional view of the pressure sensor of Fig. 20 taken along the lines 21-21 of Fig. 20.

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~6 ~
BE5T ~IODE ~O~ CARRYING OUT TIIE I NVENTION
With reference to the drawin~s, a schematic diagralrl of the air control components of the apparatus of the inven-tion is shown generally at 20 in Fig. 1, and a variable impedance load to be monitored i5 shown at 21. Forapplication in rnonit.oriny the pressures within a human patient, such as intracranial pressures, the load 21 would be a pressure sensor within the patient. A supply line 22 and a return line 23 extend from the sensor out of the patient's body to direct a stream of air, or other gas, to and from the sensor. The construction of a preferred pressure sensor adapted for use in the present apparatus is described in greater detail below. Generally the preferre~
pressure sensor 21 responds to changes in pressure within the patient by restricting or closing off the flow of gas from the line 22 when the patient pressure increases, and opening to reduce the impedence to the flow of gas in the line 22 when the pressure witt-lin the uatient decreases.
The flow rate within the line 22 is preferably maintained substantially constant, so that the increase or decrease in the impedellce to the flow by the sensor 21 is reflected in an increase or decrease in the pressure within the line 22, which is monitored by a load pressure transducer 25. When an equilibrium is reached , the pressure within the supply line 22, which is read by the load transducer 25, will be substantially equal to the pressure outside the sensor 21.
The source of the flow of gas (for e~ample, air) is a pump 27 whicll draws air in at an inlet from a return line 28 and forces it out at an outlet on a supply line 29. The air flow from the line 29 passes into a first accumulator chamber 30 and then out of the chamber on the supply line 22 through a bacteriological filter 31 and a flow re-strictor 32 and thence to the flow sensor 21. The flow of air is delivered back from the sensor through the return line 23 to a second accumulator charmber 34 which is connected to the return line 23 to returll ttle flow of air to the pump. The pressure within the first chamber 30 is rnonitored by a first pressure transducer 36 and the pressure witt~in the second chamber 34 is monitore~ by a ~econd transducer 37.

A normally closed solenoid operated bypass valve 40 is connected to the lines 23 and 29 to selectively stlllnt the flow out of the plllnp 27 back to its inlet, while a solenoid operated pressure release valve ~1 is connected to the line 28 to selectively vent the line 2S to arnbient air.
Prior to the start--up of operation, the valves 40 and 41 are opened to allow all of the comporlents within the system, such as the chambers 30 and 34 and the intercon-nected supply and return lines , etc., to reach ambient pressure. The valves 40 and 41 are then closed and the pump 27 is startedO
It ls clear frorn an examination ot Fig. 1 that the flo~
rate in the lines 23 and 29 must always be equal, and that the flow rate in the lines 22 and 23 must also always be equal. I-~owever, durin~ an initial period of time after the pump 27 is started, the flow rate in the lines 28 and 29 is not necessarily equal to the flow rate in the lines 22 and 23. This is true because air accumulates for a period of time within the charnber 30 to pressurize this chamber above arnbient, while air is withdrawn from the chamber 34 to reduce the pressure within this chamber below ambient.
Evelltually, the system reaches a steady state in which the pressures within the chambers 30 and 34 do not substan-tially change, an~ the flow rate within the lines 28 and 29 then equals the flow rate within the lines 22 and 23. If the air within the systern is assumed to act as an ideal gas, the relationship between the pressures in the two chambers can be simply expressed. Letting the steady state ~ressure within the chamber 30 be denoted as P while the volume ot the chamber is denoted as V , and le~ting the steady state pressure within the chamber 34 be denoted as P2 and the volume of the ctlamber be denoted as V2, the relationship between the pressure in chamber 30 and the pressure in chalnber 34 will be:

p = _p (vl/v2) or P2/P1 1 2 The volume of the chamber 34 is preferably made larger than that of the charnber 30 by some ir~teger ratio. For exaMple, if the volume of the chamber 34 is 10 times the volulne of the chanlber 30, then the following relationship 391~

.~ .
will exist between the yressures that are read by the transducers 36 and 37:

P2 = -Yl/10 or lOP ~ P = 0 s The foregoing equations will hold true if there are no leaks of air into or out of the system; the equations will not hold true if there are such leaks. For example, iE
air should escape from the line 22, the ~ressure in the chamber 30 will not change significantly, but the pressure in chamber 34 will drop because less air is flowing in through the line 23 than is being withdrawn through the line 28. If a leak develops in the line 23, and air is drawll into the line, the pressure within the chamber 34 will increase because more air is entering the chamber 34 from the line 23 than is being withdrawn from the line 28. Similar changes in the pressure within one or the other of the chalnbers 30 and 34 will occur if any of the lines are blocked.
By comparing the pressures in the chambers 30 and 34, it is possible to determine whether a leak has occurred For example, the quotient of the pressures should be equal to the inverse of the quotient of the respective chamber volumes, a constant. If the quotient of the pressures changes over time, a leak rnust exist. Alternatively, the comparison can be made by adding lO times the gauge pressure in the chamber 34 plus the gauge pressure within the chamber 30, and compariny the absolute value of the sum with soMe srnall constant; a leak or block is determined to have occurred iE the absolute value of the sum is yreater than the constant. For mAximum accuracy, the eEfective volumes of the supply and return lines must be included when determining Vl and V .
The vacuum that is maintained within the larger cham-ber 34 also is a particular advantage if the sensor 21 35 develops a leak since the chamber 34 will exert a vacuum clraw on the sensor 21 which will tend to initially draw in ~as or liquid from arourld the sensor.
The sensing of ~ressure within the chambers 30 and 34 a5~
, by the trans(~ucers 36 dll(i 3'7 carl be u3e~l by a controller respollsive to si~rlclls from the transducer~c3 to determine when a leak occurs; the controller may then translate this determinatioll into the action of turning off the pump 27 and opening up the solenoid valve ~0 as well as warnirlg an operator. The chamber 30, although under pressure, is isolateci froTn the sensor 21 hy the restrictor 32; and thus will exhaust its air through the valve 40 to the low pressure chamber 34 when tlle va:lve 40 is opened. The chamber 34, 'however, is directly connected to the sensor through a low impedence line and thus will tend to exert a vacuum draw on the sensor even as the valve 40 is opened, thereby tending to withdraw a:ir from the sensor rather than allowing the sensor to receive air under pressure which lS might otllerwise be injected to an area outside the sensor.
The negative pressure maintained within the chamber 34 also allows the sensor 21 to respond to pressures lower than arnbient air pressure. AS is apparent from Fig. 1, if the sensor 21 opens up when exposed to negative pressure so t`rlât it provides little or no impedance to the air flow, the pressure read by the transducer 25 will approâcll the pressure in the chamber 34, which is thus the lower limit of pressures that can be monitored by tile sensor 21.
The pump 27 preferably operates at an inlet to outlet pressure differential of about 10 to 14 psig while the pressure in the charnber 30 is maintained at about 10 psig.
If the volume of the cham~er 3~ is ten times that of the chamber 30, the pressure in the chamber 34 will theoret-ically be about 1 psig. However, because of the volume o~ the supply and return lines, the chamber 34, in actual practice, will be at a somewhat lower pressure. The pressure within the line 22r as read by the transducer 25, will generally be in the range oE -1 to ~3 psig (-50 to +150 mln llg.). Because the pressure within the chamber 30 is sul)stantially greater than the pressure within the line 22 as a result of the pressure drop across the restriction 32, which is preferably an ad~ustable orifice to allow the pressure drop to be varied, the flow through the line 22 will be subc3talltiaLly constant t~espite the variations in 4n resistarlce to flow caused ~y the sensor 21 as it responds to amL)ie~t pressure. :[t is noted, however, that the flow sensor 21 is initially calibrated by operating it in ambient air and defining the value read by the transducer 25 as zero pressure. For maximum accuracy, it is preferred that the flow rate which obtains during the initial cali-bration of the flow sensor be maintained during measurement of the changes in pressure when the flow sensor 21 is placed in a patient. Because the flow rate ghrough the sensor tends to decrease as the pressure within the line 22 increases, as read by the transducer 25, if the pump 17 has a controllable variable displacement it can be controlled by the signal from the transducer 25 to compensate by increasing its output pressure to maintain a substantially constant flow rate.
One type of pump which has been found particularly suited to this application is a WISA* bellows pump, a simplified schematic of which is shown in Fig. 2. As the bellows 46 within the pump body 45 moves in and out, air is drawn in through an inlet check valve 47 and is forced out through an outlet check valve 48. The bellows diaphragm 46 is moved outwardly by a solenoid 50 and springs back in-wardly to draw in air when the solenoid 50 is de-ener-gized. The solenoid 50 is typically drive by 60 Hz line voltage which is passed through a diode so that the solenoid is energized and de-energized 60 times a second to drive the bellows diaphragm 46 at a similar rate. The displacement of the pump can be simply adjusted by ad-justing the height of the voltage pulses provided to the solenoid. It is also apparent that other pumps which are responsive to a control signal to control the displacement or flow rate through the pump can also be utilized for this application.
The electronic control means components of the appa-ratus are shown in a schematic block diagram view in Fig.
3. The outputs of the patient transducer 25, the pressure transducer 36, and the vacuum transducer 37 are fed, 43-spectively, to amplifier units 52, 53, and 54, which provide impedence isolation, variable gain, and variable offsets. The outputs from the amplifiers 52, 53 and 54 are provided to an analog to digital converter 56 which directs an 8 bit output on a line 57 to a microprocessor 58. The microprocessor, or CPU, 58 controls the A to D converter *TRADEMARK

~94$~
-lL-56 through select lines 59 so as to receive data ~rorm a desired one o~ the trarlsducers 25, 36 OL 37. The analoy output of the patient transducer 25, a~ter passiny through the ampli~ier unit 52, is fed through a bu~fer amplifier ~0 and is available to an analog read out device such as a strip chart recorder or a CRT display.
Data is provided to the central processor 58 from a front panel display and keyboard unit 62 through a two-way serial converter 63. As explained further below, the data input from the keyboard rnay be commands to start up the system, to calibrate the various transducers, to set warning indicator lirnits to be used to warn of excessive over or under pressures read by the patient transducer, and for other purposes. The front panel display provides the user an indication of the pressure read by the patient transducer, preferably in base 10 digital readout, and also provides communication to the user during the various data input and status checking operations.
The system program is contained in a ROM unit 65 connected on a data bus to the CPU 58, and data storage is provided in a RAM unit 66 connected through a data bus to the CPU 5U. 'I'he CPU sends a siynal throu(31l an output port 68 to the bypass valve solenoid 40 and the exhaust valve solenoid 41 at the proper time to open these valves during ~5 initialization of the system, and will send a signal to open the valve 40 if a leak condition is noted as a result of monitoring the pressure transducer 36 and the vacuum transducer 37. As explained urther below, the CPU 58 also provides an output signal through a timer/counter 70 and thence to a audio speaker 71 to provide a warning to the operator whenever the pressure read by the patient/load pressure transducer 25, and interpreted by the CPU 58, exceeds a preprogramed over- or under-pressure limit.
As indicated above, it is desirable to maintain the 3s flow rate through the patient transducer at as constant a rate as possible, and this can be accomplished by varying the displacement of the air pump 27 in direct relationship to the pressure level read by the patient transducer. To accomplish this, the CPU 58 processes the patient trans-ducer signal and urovides an output to a digital to analogconvert~r 73 which provides an analog signal on a line 75 to a proportiorlal ~ump controller 7~. The pulnp controller whlch also receives an analog sigl-lal from the pressure transducer 36 on a :Line 76, modulates the magrlitude oE the AC signal provide(3 to tlle pUIilp 27.
To allow interconllectic)n to other data processin~
equiprnent, the CPU 58 also provic]es an o~tput sic~nal inclicating the level oE the patient transducer to a parallel to serial converter unit 77. ~rhis siyllal rnay then be interfaced with other data processing equipment for recording and subse~suent processin(J, or for real time processing .
A schematic diagram of the proportional pump controller 74 is shown i n Fig. 4. The siynal indicative of the reading of the pressure transducer 36 is passed on the line 76 through a buf Eer amplifier 80 and thence through a voltage divlder composed of resistors 81 and 82, with the resultiny voltage applied to the negative input of an operational amplifier 84. The analog output signal of the D to A converter 73 is applied from the line 75 through a potentiometer 85; the voltage taken oEf of the wiper of the potentiometer is transmitted on a line 86 to the positive input of the am?liXier 84. A feedback resistor 77 connects the output of the ampliEier 84 to its negative input. In this manner the signal from the pressure transducer 36, indicating the pressure within the chamber 23, is sub-tracted from th~ signal received from the digital to analog converter 73, a signal proport ional to the pressure read by the pat ient t ransducer 25. The amulif ier output is thus related to the àifference in pressure between the trans-ducers 36 and 25, which difference is proport ional to the flow rate through the restrictor 32. The output oE the amplifier ~34 passes through a series resistor 89 to a opto-isolater 90 which isolates the control portions of the circuitry Erom AC power. One terrninal of the opto-isolater 90 is connected by a conducting line 91 to the gate oE a field eEEect transistor 92 and the other terlninal of the opto-isolater 90 :i5 conrlected by a conducting line 93 to the source terlnillal of the FET 92. ~esistors 94 and 95 and capacitors 96 and 97 are connected across the lines 91 and ~10 93 to filter the output oE the opto-isolator. One of a pair of AC power lines 99 is connected to the source -l3-terminal of the FE~r, while the clrain terrninal of the E~ET
extends to the pulnp 21~ as does the othtr AC power line.
As noted above, ttle pump is preferdbly a solenoid driven diaphraglll-bellOWS pUlllp which receives half-wave AC power.
The output of the opto-isolater 90 moclulates the height of the pulses delivered through the F~r 92 to the pulnp so that the pulses vary in magnitude i.n proportior- to the output oE
the amplifier 84. The norrnal or steady state pulse magni-tude is selected to be sufficient to maintain a desired pressure level within the charnber 30 uncler initial or normal air flow conditions within the system. It is apparent frorn exarnination of the systern that an increase in the pressure sensed by the transducer 25 results in an increase in the magnituc~e of the pulses provided to the pump and therefore greater ciisplacement of the pump dia-phra~rn on every stroke, thereby tending to increase the pressure within the air chamber 30. As the pressure within the charnber 30 increases~ the mac;nit-lde of the pulses provided to the pump decreases toward the normal magnitucle, with proper adjustrllent of the system perameters, when the steady state is reached, the chamber 30 will be at a higher pr~ssure sufficierlt to provicle the desireci constant flow throuc3h the restrictor 32 despite the higher pressure observed at the pressure transciucer 25. Conversely, if the pressure transducer 25 senses a drop in pressure, the output of the amplifier 84 will decrease from the normal offset voltage level, thereby causing the FET 92 to decrease the magnitude of the pulses provided to the purnp 45 below the normal magnitude. The displacement of the diaphragln with each pulse will therefore be less than under normal conditions, allowing the air chamber 30 to bleed dowll in pressure through the restrictor 32 until it reaches a new lower level sufficient to maintain the desired flow rates throu(3h the restrictor.
It will be apparent to those skilled in the art that the control components shown in Fig. 3 are of standard design and the intercol1rlections therebetweel1 are readily apparen~. As an exanlple of commercial units satisfactory or im~le~TIlentillg the controller ~ the present invention, ~,~o the CPU 58 may comprise a Mosteh~M]~Y-CPU2, a Z80 based n~icroproce~C;or~ the I/O Serial input ancl output devices 63 ~ 7-~æ~D~

~4~

antl 77 may be Mostek MDX-SIO units, and the analo(J to digital converter 56 rnay be a Moste~ ~lD~-~IO, compatible with the CP~ unit. The RAM 66, compatible Wittl the afore-mentioned CPU, is pre~erably a ~K memory with an t3 bit word, while the ~OM 65 may contain the system prograrn within a 6K memory utilizing an ~ bit word. A display panel of the type manuEactured by ~urr-Browrl Research Corp.
under the name TM177 has been four-d suitable, and is utilized in the programming for the a~paratus described below. The pressure transducers 25, 36 and 37 may be proportional transducers such as those produced by Honey~
well Microswitch. Such transducers are available to read from, for example, -5 psig to ~5 psig, as appropriate for the load transducer 25, frorn 0 to -5 pSkJ as appropriate for the vacuum transducer 37, and from 0 to 15 psig, as appropriate for the pressure transducer 36. The afore-mentioned Microswitch transducers provide an output signalvarying from 1 to 6 volts about a null voltage midway in that range.
The above described control means of Fig. 3 is a preferred embodiment for carrying out the control functions of the system. It is quite apparent to those skilled in the art that there are other, equivalent embodiments for carryin~ out these control functions. For example, an analog signal circuit could easily be constructed to monitor the transducers 36 and 37 and provide a warniny if a leak is detected. A simple circuit for doing so could include a summillg amplifier which adds the output of the transducer 36 ant3 ten times the output of the transducer 37, with the summin9 amplifier output then bein~ compared in a comparator (or in two comparators, one for positive and one for neyative OlltpUtS) with a small offset voltaye, with a leak warning provided by the comparator if the offset is exceeded. The bypass valve solenoid may be operated by the output of such a comparator, and the pump 27 may be turned off by such a warnin-J signal. It is also quite ot)vious that the pump may be controlled completely with an(llo~ circuitry by simply connecting the line 75 to the output of the amplifier 52. trhe other control and warnin(J Eunct iOllS Inay also be ernl)odied in analog cir-cuitrr. '['he comparison of the pressures within the chambers 30 and 34 may even be per~ormed pneumatically, and a pneumatic pressure signal may be utilized to switch a bypass valve analogous to the valve 40.
Flowcharts illustrating the operations of the program which monitors the data from the various transducers and controls the pump and solenoid valves are shown in Figs. 5-19. The program is designed to switch between various major tasks at interrupt times, with a scheduler program distributing control to one of the major tasks at each interrupt. For example, in the program that follows, an interrupt clock frequency of 125 times a second has been found satisfactory to allow adequate processing of data by each of the major tasks. ~s shown in Fig. 51 the activation of the power on switch by the operator (block 100) causes the computer to initialize the R~ memory ~101), to initialize the counter/timer 70 (block 102), to initialize the serial input/output devices 63 and 77 (block 103), and to turn on the interrupts (block 104). The program, under the control of the scheduler, then distributes control at the sequential interrupt clock times to a command processor task, shown in Figs. 6 and 7, a leak task shown in Figs. 8A and BB, a data acquisition task shown in Fig. 9, and a solenoid control task shown in Fig. lO.
The connecting blocks shown in Fig. 5 at 6A, 8, 9 and 10 refer to the respective starting points on the flowchart shown in Figs. 6, 8, 9 and 10, respectively. Similar connection block numbering is used throughout the flowchart. In addition to the major tasks which are controlled by the interrupts, several subroutines are accessed by the tasks and are shown in separate figures.
Subroutines accessed by the command processor task are shown in Figs. 11 and 12, subroutines accessed by the leak task are shown in Figs. 13-18, and an analog to digital conversion subroutine which is accessed by the data acquisition task and communicates with the command processor task is shown in Fig. 19.
The command processor task shown in Fig. 6 begins with an initialization of the ~ront panel display 62 (block 106)~
Therea~ter, the program determines if there is input from the front panel (block 107). I~ not, the program recycles until input is received. If there is input from the front panel it is first determined if it is a system calibrate command (block 108); if so, a calibrate messa(Je is sent to the leak task (block 109) and the program proceeds to subroutine 14 (Fiy. 14) to await a reply from the leak task (block 110). If the systern calibrate cornmand is not present, it is deter~ninea whether the cornman(3 is a zero transducer command (block 111)S if not, the program recycles to check for input from the front panel (block 107); if so, a zero messaye is sent to the leak task (block 112) and the proyram goes to subroutine 14 to await a reply from thè leak task (block 110~. Subroutine 14, shown in Fiy. 14, is entered at block :L15 and requests a message ac3dressed to the task (block 116). If a rnessage is not available, the program cycles until a rnessaye does become available (block 117); if the messaye is available it is deterrnined whether the rnessage is from the right tas~
(block 118) -- in this case from the leak task -- and, if not, the rnessage is sent back to the OriyirlatinCJ task for retention and later processing (block 119). If the rnessaye is from the right task, return is rnade to the main proyram (block 120). The messaye from the leak task will either be that there is a leak (a leak messaye) or that the systern is up and operatiny satisfactorily (a system-up message). The cornmand processor procJram ShOWII ill Fiy. 6 first checks for the leak message (block 122) and if one is presentr the leak message is displayed on the front panel (block 123) and the prograrn returns to block 107 to check for further input frorn the frollt panel. If there is no leak message the program checks for a system up message (block 124); 'f none is available, the program returns to block 107 to check for input from the front panel. The lack of a system up rnessaye at this point would be in(3icative of the a failure of the system to properly calibrate during the performance of the leak task. If the system-up message is received, the prograln proceeds to the remainder of the command processor task, stlown in Fig. 7.
The proyram proceeds to set a one second timer which controls the updating of the display at the front panel ~0 (block 126). The progranl then proceeds through looys in two branctles depellding on whether it is tirne to update the 5~

display (block 12~). lf not, the intracranial pressure is requested from the data acquisition task (block 129) and the program then proceeds to subroutine 11 (Fig. 11) whictl processes any available front panel input data (block 130).
After entry into subroutine 11 (block 131), shown in Fig. 11, it is then deternlined whether there is input from the front panel (block 132); if not/ the system to the rnain command processor program (block 133). If there is input from the frollt panel it is then determined if there is a number which proceeds the command (block 134), indicatin~
that a high or low limit is beirl(3 set, and if such a number is receivedl it is stored (block 135) and a data available flag is set (block 136). After the data available flay has been set, or if no number preceeds the command, it is then determined if there is a hiyh a:Larm command (block 138) and, if so, the system proceeds to subroutine 12, shown in Fig. 12. If there is no hiyh alarrn command but there is a low alarm command (block 139), the system procéeds to subroutine 13 shown in Fiy. 13.
Subroutine 12, ~ig. 12, first determines if there is a new input value (block 140), and, if so, determines whether the value ls withirl bounds (block 141), and) if so, stores the new hic3h alarm set point (block 142) and then displays the stored high alarm set pvint (block 143). If there WdS
no new input value~ the proyram displays the previously stored hi~h alarm set point. If the value is not in bounds (block 141), an out-of-bounds message is displayed on the front panel (block 144). After the high alarm set point is displayed and stored, or if the out-of-bounds messa~e is displayed, the program proceeds to set a display hold fla~
(block 146), sets a display hold timer (block 147) -- which holds the pro(Jrarn for a few seconds to allow the operator to determine that the proper alarrn value has been set --, and then returns to ttle main commall(l processor progra (block 148).
Subroutine 13, showrl in Fi~J. 13, is accessed if a low alarm command is received, and first determines whether a new value input is received (block 150); iE so, it determines i~ the value is in bounds (block 151), and if so, stores the new low aLarm set point (block 152), an(i then displays the low alarm set point on the front panel (block 153). IE the value is not in bounds, an out-of-boun(ls messag~? is displclye(l (block 15~1). If no new valu(~
is received, the old value inE>ut is displayed (block 153).
After dis~?lay of the low alarm set point or displcly of the 5 out-of--bounds message, a display hold Elag is set (block 156), a dlsplay hold timer of several seconds is set (block 157), and the prograln returns (block 15B) to the main corn-mand processor program at block 130.
If neither a high alarm colnmand nor a low alarm cornmand 10 is reeeivèd at bloel;s 138 and 139, subroutine 11, Fig. 11, then determines whether an alarm silence eommand has been received (block 160); if so, the auc3ible alarrn is turned off (block 161), a display hold timer of two seconds is started (block 162), the alarm silenced flag is reset 15 (bloek 163), and the prograrn returns (bloek 133) to the main eommand processor proc;ra.n at block 130.
If no alarm silence command was receieved, it is then deterlnined whether a zero transducer eommand was received (bloek 165); if so, a zero message is sent to the leak task 20 (block 166) and the s~stem waits for the leak task reply (block 167), diverting into subroutine 14, Fig. 14, and thell returning after the leak task has replied. It is then determined if there is a leak message (bloek 16~), and, if so, the leak Inessage is displa~ed on t~e front panel (block 25 169) and the ~royram returns to the rnain command processor task at entry ~oint 6B and awaits further instruction frorn the front panel at block 107. If there is no leak message, it is then deterlnined if a system up message is available (block 170); if so, return (block 133) is made to the main 30 command processor program at block 130. If the system up message is not received, the prograrn diverts to subroutine 14 (block 167) to wait for a leak task reply having a valid message .
If a zero transducer comlnclnd was not received at block 35 165, a display hold flag is set (block 172), a display hold timer is set (block 173), and the flashing oE the display on the ~ront panel is turned off (block 174) before the uro(3ram returlls (bloek 133) to the main eornmarld processor prograrn at block 130. As explained below, the display of patiellt pressure is comrnanded to l~lash on and oEf if the intraeranial pressure whieh has been read is greater than the pro-~ra~ i(3h limit or L~ss thall the program low l:imit.
Input frOln tilt? frOllt parlt?l W}Jil'll iS lleLtht~r a hi(lll alarm command, a low alarlll commalld, an alarlll silel-lce commall(l, or a ~ero tralls(iucer colnlllarld, is reacJ as a comlnarld to turn oFf the ~lashing of the dispkly.
After cornpletion of subroutirle 11, at block 130, tlle command processor taslc, FiyO 7, sends a request for a message addressed to the task (block 180)~ and a check is made to see whether or not a message is availabe (block lBl); if not, the program returns to subroutine 11, block 130, to process front panel input data. If a message is available, it is checked to see if it is from the data acquisition task (block 182); if not, the message is sent back to the task from whicll it is received to be processed later (block 183) and the program returns to block 130l subroutine 11, to process any new front panel input data.
If a message is received from the data acquisition task it will be the patient pressure trallsducer data. Because the patient transducer will ~ick up transient readings of short duration, it is desirable to average out a few successive san~ples or sensor readings so as to provide a signal to the dis~lay at the time that the display is updated whicl) is not affected by the transients. Satisfactory results can be obtained if the latest pressure reading sample is averayed with the next three immediately received pressure readiny samples (block 185). The program then returns to block 128 and determines whether the timer at 126 has run and it is time to update the display. If not, the previ-ously described c~_le is repeated until it is time for display update, preferably at about one second intervals;
the display hold flag is then checked to see if it is set (block 1~6) and, if not, the proyram proceeds to determine determine whether the intracranial pressure reading that has been obtained is within the bounds that have been previously proyrallled (block 1~7)~ If the display hold flag has been set, it is Eirst deter]nirled whether the display hold timer is done (block 188), and, if not, proc3ram returns to block 126 and the timt?r for display update is reset. If the display hold has been released, or if the ~0 display hold timer had not been set, the program proceeds to reset the display hold Elac3 (block 189) and determines if the intracrclnial pressure value fourld is in t~ourlds (block 187). As in(licated above, these limits will t,~ set by the operator to provide an ~utomatic indication of dangerous pressure levels; typically, norlnal intracraniaL
pressure should be in the range of -25 mrn to -tl50 rnm of mercury, while a readirly outside this ran~e would be indicative of an a~normal or clanc3erous condition. If the pressure reading is not wittlirl these l)oull(3s, a check is made to see if the alarrn silenced flag has been set (block 190) ancl, if not, the averaged intracranial pressure is displayed on the panel (block 192), the display flash is turned on (block 193), and the proyrarn returns to block 126 to set the timer for new disp]ay update. If the alarm silenced flag has not been set, the audible alarm is turned on (block 194) by sending a siynal to the timer/counter 70 which drives the speaker 71 and the alarrn silenced fla~3 is reset (block 195) before the intracranial pressure reading is displayed (block 192) and the display flash is turned on (block 193).
~0 If the intracranial pressure was found to be within bounds at block 1~7, the audible alarm is turned off (block 197), if it had been on, the alarm silenced flag is set (block 198) the averaged intracranial pressure is displayed on the front panel (block 199~ and the flashinc3 of t-he display is turned off (block 200), if the display had been flashiny. The program then returns to set the timer for display update at block 126 and repeats the cycle.
It should be noted that the foregoing flow of the command processor task will be periodically interrupted and the other t-asks, described t)elow, will be performed in sequence. As noted from the description above, the command processor task, at various points irl the prograJn, serlds messages to the other tasks and asks for messages from the other tasks. In turn, the other tasks will run and be interruE)ted at some point in the prograrnming of the task, with each task beitl(3 restarted at the point in the task progralll at which the interrupt occurred.
The leak task is shown in Figs. ~A and B. UpOll the initial assignlllent to this task by the scheduler, the leak ~o task recluests a message whicll rnay be addresse(l to the task (block 2U2), and checks to see if a message is available -~21-(block 2~3); if no messa(Je is available, the program cycles ~ntil a messacJe does becorne avallable. When a rnessage is received, it is determined whether the rnessage is a cornrnand (block 204); if not, an error messacJe is sent to the task which originated the message (block 2~5) and the progral,l again requests a message addressed to the leak task (block 202). If a cornmand rnessa-3e is received, it is checked to see if it is a start UU colnmalld (block 206); if not, the prograrn returns to block 202 and recluests a message addressed to the leak task~ If the start up comlnand message is received, proyram proceeds to read the patient transducer zero (~lock 20~ in subroutine 17, Fig. 17.
ReEerring to Fiy. 17, after entry into subroutine 17 (block 210), a request is made for the patient transducer zero from the data acquisition task (block 211) and the program waits until the data acquisition task acknowledges the message (block 212); upon acknowledgment, return is ma~e (block 213) to the leak task subroutine at block 208.
After the patient transducer zero has been read, the patient sensor offset is then determined (block 215) in subroutine 18, Fig. 18.
Referrirlg to ~`ig. 18, after enter into the subroutine (bloclc 217), the sensor offset is requested from the data acquisition task (block 21~), and the prograr,l waits for the data acquisition task to acknowledge the message (block 219); upon acknowledgment of the message, return is made (block 22U) to the leak task program at block 215.
After the patient transducer zero and patient sensor offset readings have been obtained, a system up message is sent to the command processor (block 216) and the program then waits for five seconds before any further readings are taken (b:loclc 217)~ It is noted that the foregoing steps in the leak task are performed during the initial start up of the systern. In particular, at this time the pressure sensor 21 is being tested outside the body of a patient in amt)ient air to determine the patient sensor offset readings and ttle patient transducer zero readings under normal atlnosptleric conditions to establish a base line which can then be uxed to determine the actual pressure within a ~0 patiellt a~ter the sensor 21 has been inlplanted. The five second waiting period allows all of the pressures within ~9gl 5i~d~
-~2-th~ c~a~ t~r~ ~0 ~r~ c~ t~lro~l(J~ t ~ re~ ~f t~
system to stablize After the five second hold period has elapsed, the high pres~ure transducer is then read (block 220), Fig. ~B, in 5 subroutine 15, showrl in Fig. 15. After entry into this subroutine (block 221) a readiny of ti-le hicjh pressure transducer is re~uested frolll the data acquisition task (222) and the program waits until the high pressure reading is returned (block 223). This high pressure transducer readin(3 is then stored (block 224) and the program returns ~block 225) to the leak task at block 270.
The leak task then proceeds to read the low pressure transducer (block 230), by performing subroutine 16, shown in Fig. 16. After entry into the subroutine (block 231), the low transducer reading is requested from the data acquisition task (b]ock 232), and the prOgraJn waits until the low pressure transducer reading is returned (block 233). When this reading is received, it is stored (block 234), and the pro-Jram returns (block 235) to the leak task at block 230.
The initial high/low pressure transducer ratio is then calculated and stored (block 240) and a request is made for a Inessage addressed to the leak task (block 241). The leak task is now in its active monitoring made in which it will continue to morlitor the pressures within the high and low pressure chambers to determine if a leak occurs. This is done contirluously wllile the pressure sensor is beiny inserted in the patient and, of course, after insertion.
If no message is available to the leak task (block 242) the high pressure transducer is then read (block 243) in subroutine 15. After reading the high pressure transducer, the low ~ressure transducer is read (block 244) in sub-routine 16. The ratio of the high and low pressures is then calculated and compared to the initial start up high/low pressure transducer ratio (block 245). If no leaks in the system have occurrecl, the latest ratio should be idelltical or essentially iclentical to the initial higtl/low ratio. In the ~resent proyrarn it has been found convenient to express the high and low pressure transducer ~0 reaclin~Js as fixed point binary numbers, resultinc~ in a fixed i)oint (luotiellt Eor the higll/low ratio. The binar~

quotients so obtainecl froln tt~e initial hi~h/low ratio an(l the latest high/low ratio are cQmpared and the latest ratio is eonsidere(3 to be withirl boullds if it is the salne as the initial hi(Jh/low ratio or di-ffers Erom the initial high/low 5 ratio by no more than the least significant t;~:it. For the system pressures described above with a resulting flow throuyil the sensor of about 40 eubic ceiltirneters (cc) per minute, a one bit clifferenee in the ratios will correspond to a leaka(3e rate of less than 3 cc per rninute, so tnat 10 leaks of 3 ec per minute or rnore will be detected. Greater accuracy and lower leak rates rnay be detected silnply by utilizing floatinc; point division and ehoosing a bit differenee WhiCtl eorresponds to a desired maximurïl tolerable leakage flow rate.
If the high/low ratio is out of bounds, a leak messaqe is sent to the eornmand proeessor and the pump is turned off (bloek 246). The program returns to bloek 241 and requests a méssage a(idressed t~o the task.
If the high/low ratio is within bounds, the procJrarn 20 recyeles back to bloek 241 and requests a message addressed to the task and repeats the eycle If the message at block 242 is available, the message is eheeked to see if it is a stop eomnland (bloek 247), and if so, the prograrn returns to the beginning of the leak task (Fiy 8A) and waits for a 25 messacJe (bloeks 202 and 203). If no stop eor,lmand is reeeived, the leak task eheeks to see if a zero transdueer eommand ~las beell reeeived (block 248); and, if not, the program recyeles back to block 241 and requests a messac3e addressed to the task. If the zero transdueer eommand is 30 received, the patient transdueer zero is read (bloek 249) in subroutine 17, shown in Fi~. 17. It may be deslrable to periodieally read the patient transdueer zero level with the pressure sensor 11 ~laced within the patient, and the zero transdueer eornrnand frorn the keyboard eauses this to be 35 done. After the patient transdueer zero has been so read, the leak task returns to block 241 and continues to cycle arld will do so until a stop eomrnand is reeeived.
The data accluisition task, shown in Fiy. 9, begins initially by requesting a Inessaye addressed to the task 40 (block 250), cheekinc3 to see if a message is available (~loel< 251) al~cl, lf so, ehecking to see if it is a request --2~--for data (blocl; 252). Only a request Eor (iata is a proper message for the clata acqulsition taslc, so that if a message other thall a request Eor dclta is r~ceived, an error Inessaye is sent to the task ori~inatinc3 the message (block 253), 5 and retllrll is ma(Je to block 251) to acJain request a rnessaye.
If no message is available at block 251, the proqram switches the analog to dic~ital converter 56 to input the patient transducer (block 254), cornmands the start oE the analoc~ to diyital conversion (block 2S5), and waits untiL
10 the analoc3 to dic3ital conversiorl is done (block 256).
After the analoy to diyital conversion is completed, the patient transducer level obtained by the analog to digital eonverter is read in (block 257), a pulnp controller offset amount is added to it (block 258), and the sum is provided 15 to the pump controller through the digital to analoy converter 73 (block 259). An initial controller offset is required to provide a minimurn voltac~e level to the pump 17 through the yroportional pump controller 74 since the pump will not operate below a minimum voltage level. The con-20 troller ofset is adjusted to provide this minimum level.
After the output is provided to the purnp controller (block 259), the proc3ram recycles baek to bloek 250 to eheck for a request adc3ressed to the data acquisition task.
If a message is available at bloek 251 and if it is a 25 request for data as deterrninec3 at block 252, the program saves the name of the transducer which is to be read (block 260) and cllecks to see if it is a zero patient transciucer request (bloek 261). If not, it must be a request to read the patient transclucer, anc3 thereEore the patient trans-30 ducer is read and the data is sent to the task which requestec3 it (block 262) by perforl,lincj subroutine 19.
After subroutine 19 is cornpleted, the proyrarn recycles hack to block 250 to check Eor requests ad~3ressed to the data aequisition task.
Sul)routine 19, showrl in l~i~. 19, after entry at block 270, deterlnil-les whether the request is a request to read the low pressure chamber t ransclucer 27 (block 271), and, if so, it switches the input of the analoy to digital cor~-verter 5~ to the low pressure cllalnl)er i-ransducer (block ~o 272) alld starts the analo~ to digital conversion (block ~73). IE at block 271, a request to read the low pressure ....

-~s~-t~ ducer is not r~ceive~J, it i-; t1~e11 (ieter1nir1ed if a hic3h pressure cha111ber trar1s(lucer 26 readin(J request is received (block 274), and, if so, the ar1alo(~ to digital converter 56 is switchec3 to thtl hic~h uressure cha111ber transducer (block 5 275), and begins the analo-; to diyital co1lversio11 (block 273). If a high transducer request has not been received, it must be a request to read the patient transducer 25 and the converter 56 is switched to the patient transc3ucer (block 276), and the analoc3 to di~3ital conversion is started. T~e program checks t:o determine when the analoc~
to digital conversion is complete (block 277), and then reads and stores the data frorn the converter (block 278).
After the data has bee1l read and stored at block 278, a check is made to see if the request was a read patient transclucer request (block 280); if so, the transducer zero offset is subtracted from the patient transducer reading (block 281) and the sensor offset is substracted from the difference ~reviously calculated (block 282) and the data is sent back to the requestiny task (block 283). If a patient transducer request was not received, it is next determined if a zero transducer request was received (block 28~). If so, the new patient transducer æero reading is stored (block 285) and the data on the new transducer zero reading is sent baek to the requesting task at bloek 283.
If a zero transducer request had not been made, a check is made to determine if a sensor ofset request was reeeived (block 286), and, if so, the transducer zero reading is subtracted from the sensor offset reading (block 287) and the difference is stored as the new sensor offset (block 288) and the sensor data offset is sent back to the requesting task. If a sensor offset request had not been received as determined at block 286, then the request must have been for readinc3 the low or hiyh transducer, and this data is sent directly back to the requesting task at 283.
After tl1e ddta i~as been sent to the reqllestin~ task, return is made (block 289) back to the data acquisition task (Fic~.
9) at block 26_.
If a deter1ninatlon is made at block 26l that a æero patient transducer request has been received, the pumE) is turned o~f (block 2')0) by turnin(3 oEf the offset volta~Je ~rovided throu~3h the D/A converter 63, an(~ a zero messac3e is serlt to the solenoid valve control task (block 291j.
The pro-Jraln therl wail s for a r~ply frc)lll the solenoi(l control task (block 292) by perforlrling subroutine 1~.
After re~ly is receive~l frorn the solenoid tasls, the zero 5 patient transducer is read (block 293) by performing subroutine 19 and the data is sent back to the task which has requested the patient transducer zero. The patient transc3ucer comman(3 is then sent to the solenoic3 control task (block 294) and a wait is mac3e for the solenoic3 task 10 reply (block 295) by performitl(J subroutine 14. After tne reply from the solenoid task, the program recycles back to block 250 to cl)eck for a messaye addressed to the data acquisition task.
rllhe solenoid control task is shown in Fiy. 10. Initial 15 entry into this task begins with a request tor a message addressed to the task (block 300), and the proyrarTl waits until the message is available (block 301). When the message becomes available, it is checked to see if it is a command (block 302), and if not, an error messaye is sent 20 to the task originating the messaye (block 304) and the proyra~n returns to block 300 to request a messaye addressed to tlle task.
If a co~ land is founc3 at block 302, it is checkec3 to see if it is a ~atient sensor offset command (t~lock 303), 25 and, if so, comlllan(ls are sent through the output port 68 to turn off the solenoids 40 and 41 to seal the chambers, and a command is sent throuc3h the digital to analog converter 63 to the pUMp controller to turn on the pump (block 305).
The program then waits 12 seconds to allow the ~urnp to 30 begin operating and pressurize the chambers (block 306), waits another 5 seconds (block 307) to allow the system to completely stabliæe, and then sends an acknowledyment of th~ commdncl to the task of origin (block 308) indicating that the system is up and running and is ready to be tested 35 for patiellt s~nsor oftset. The prc)gram then waits at block 300 and 301 for another message acldressed to the task.
If no patient sensor offset command was received at block 30~, a check is then made to see if a æero transducer COllllllalld wa9 receive(l (block 310); and, if not, return is 40 made throuqh block 30~, selldillg an error rnessage to the task th<lt ha(l oriyinated the comlnanc3, and the proyralTl -27~-proceeds back to block 300 to wait for another messacJe ad(iresse(3 to the task. If the ~ero trcln~ cer colnrllall(i was reeeived, the~ proyralll then provi~ies an output siynal throuyh the output port 68 to the so:Lelloi~3s 4~ an(i 41 to open up the valves controlle~ by the solerlc)icls an(l also provides an outpul tllrough the converter 73 to the pump eontroller 74 to turn off the punlp (block 31:L). The prograrn thell waits 5 seeonds at block 307, and thereaftt~r sends an aclcnowledyment of the receipt of the eomrnand to the task of oriyirl an(i proeeecls back to bloek 300 to wait for another messacJe adclressed to the solenoid eontrol task.
After the ehambers are opened up and the pUlilp iS tUrlled off, all of the chalnbers and the entire system should settle dowll to ambiel-lt prexsure so that any of the pressur transducers can be read to determine their level under ambient air eonditions.
A particular elnbodilnent oE a pressure sensor whietl can aet as the loacJ in the systern of ~`iy. 1 is shown yenerally at 321 in Fiy. 20. Ttle pressure sensor 32] ineludes a eup-shaped housin(J 322 formed as a eireular dise of plastie having a eircular open mouth 323 forlned in one surface thereof surroundecl by a peripheral annular faee 32~. A
flexible diaphragm 326 is seeured by glue or other adhesive or by sonic welding, as desired, to the peripheral faee 324 of the housiny to define a plenurn in the mouth 323 under the diaphracJT,l. A portion of the diaphragrn 326 is shown bro~en away in Fig. 20 to illustrate the strueture of the pressure sensor under the diaphraym. This structure inclu~es a eireular exhaust tube 327 eentrally mounted in the mout~ 323 arld terminating -- at a position adjaeent the inner side of the diaphrayln 326 -- in an annular face which surrounds the open bore 328 of the exhaust tube. As shown in F`i(3. 21, the annular face oE the exhaust tube is forme~
substantially eoplanar with the peripheral faee 324 of the housill(J. A thin w.llled r,letal outlet pi~e 330 has its inner bore in c:ommunieation Wittl the bore 328 of the exhaust tube and exterlds outwardly throucJh the outer wall of the housiny 322 an~ is eonneeted to plastic tubiny 331 eorrespondiny to the return line 23 shown in Fi(J. 1. An inlet pipe 333, also Eorn~ed of a thin walled metal pipe, extencls throuyh the outer walL Oe the housing 322 sueh that its bore is in 5~
_~fl_ communication wlth the plenuln 323, with the outer erlc3 of the inlet pipe 333 being connected to plastic tubing 334 correspon(iing to the supply line 22 of Fic~. 1. The outlet pipe 330 and the inlet pipe 333 are preferably mounted closely adjacent to one another at approximately the same elevation in the outer wall of the housing 322 to minimize the space taken up by the pipes and by the tubing connected to them.
As best shown in Fig. 21, the outlet pipe 330 passes through the plenum 323 and the wall of the exhaust tube 327 to the bore 328 of the exhaust tube. As is illustrated in Fig. 21, the to~ wall 336 of the sensor 321 need only be thick enough to provide structural strength and integrity.
For example, the height of the pressure sensor 321, i.e., the distance from the outer surface of the diaphragm 326 to the surface o the top wall 336 of the housiny, may be in the range of 1.5 mm., to thereby minimize the space within the patient that is occupied by the sensor. The structure of the sensor 321 also has the advantage of allowin~ the plastic tubes 331 and 334 to extend from the sensor in closely spaced, parallel relation. The inlet and outlet pipes 333 anci 330 are preferably located at a position in the wall of the sensor housirly above the bottorn of the mouth of the housing such that the top wall of the sensor housing can be made as thin as desired.
The housing of the pressure sensor 321 is preferably for~ned of biocompatible plastic material such as silicone or polyurethane, and similar materials are used for the diaphragm 326 and tubincJ 331 and 334. A stiffer diaphragm (less elastic) may be provided, if desired, ~y molding a nylon mesh withirl the material of the diaphragrn.
In operation, the diaphragm 326 closes off the bore 328 of the exhaust tube until the pressure inside the plenurn, and thus in the line 334, is slightly greater than the pressure outside the sensor; at this pressure, the diaphragm is moved out to uncover the bore 328 and allow air to escape through the exhaust tube 330 and return line 331. Release of the air pressure withill the plenum allows the diaphra~m to move bac~ and close the bore until sufficient pressure builds up to move the diapl-lragm away agaill. T?tle pressure ~ithitl the line 334 (corresponding to

-2)-the l:ine 12 of E`ig. 1) thus t.LIlctu~ltes sLi(Jhtly but rapidly about the value of the pres~sure outside the sensor.
It is ullderstood that the invelltion is not confined to the particular embodiments here.in illustrated and de-scribed, but embraces SUCil modified forms thereof as comewithin the scope of the followirl~ claims.

Claims

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. In a closed circulating gas system having a pump which draws gas at an inlet from a return line and delivers gas under pressure at an outlet to a supply line, and also having a load device which receives gas from the supply line and delivers it to the return line, apparatus for detecting leaks in the system comprising:
(a) a first accumulator chamber connected in the supply line;
(b) a second accumulator chamber connected in the return line;
(c) first pressure transducer means for measuring the pressure in the first accumulator chamber and providing an output signal indicative thereof;
(d) second pressure transducer means for measuring the pressure in the second accumulator chamber and providing an output signal indicative thereof;
(e) control means responsive to the output signals of the first and second pressure transducer means for comparing the pressure measured in the first accumulator chamber with the pressure measured in the second accumulator chamber and providing a leak warning output signal if the compared pressures change with respect to one another in excess of a selected range, thereby indicating that the flow through the two accumulator chambers is not equal and a leak in the system has occurred;
(f) a load device comprising a pressure sensor connected between the supply and return lines which includes:
(1) a cup-shaped housing having an open mouth and having an inlet and an outlet formed therein;
(2) a flexible diaphragm sealed over the open mouth of the housing to define a plenum between the walls of the mouth in the housing and the diaphragm, and wherein the inlet formed in the housing is in communication with the plenum;
(3) an exhaust tube centrally mounted in the housing within the plenum with an end thereof terminating adjacent the inner side of the diaphragm and connected to be in communication with the outlet from the housing, the supply line being connected to the inlet to the housing and the return line being connected to the outlet from the housing;
(g) a flow restrictor connected in the supply line between the first accumulator chamber and the pressure sensor;
(h) load pressure transducer means for measuring the pressure in the supply line between the restrictor and the pressure sensor and providing an output signal indicative thereof which is supplied to the control means; and (i) the control means also providing a display to an operator of the pressure measured by the load pressure transducer means.

2. The system of claim 1 including a bypass valve connected around the pump and responsive to the leak warning signal from the control means, the bypass valve being operative to open when the leak warning signal is received to shunt the outlet of the pump back to its inlet and to relieve pressure within the system including equalizing the pressures within the first and second accumulator chambers.

3. The system of claim 2 including a pressure release valve connected to the return line, the pressure release valve being responsive to a signal from the control means to open at the same time that the bypass valve opens and to place the return line in communication with the ambient atmosphere.

4. The system of claim 1 wherein the volume of the second chamber is substantially greater than the volume of the first chamber.

5. The system of claim 1 wherein the control means also provides an audio signal when the pressure measured by the load pressure transducer means exceeds a selected high pressure or is below a selected low pressure.

6. The system of claim 1 wherein the pump is responsive to an input signal to vary the displacement of the pump in direct relation to the input signal, and wherein the control means varies the input signal to the pump in proportion to the output pressure measured by the sensor pressure transducer means so as to control the displacement of the pump in direct relation to the pressure at the pressure sensor.

7. The system of claim 1 including means for converting the output signals of the first and second pressure transducer means to digital data signals, and wherein the control means compares the digital pressure signals from the first and second transducer means by periodically dividing one of the digitized output signals into the other to provide an initial digital ratio which is stored and a latest ratio, the control means comparing the latest ratio with the initial ratio and providing a leak warning output signal if the latest ratio deviates from the initial ratio in excess of a selected tolerance amount.

8. The system of claim 7 wherein the ratios of the transducer means output signals are provided in fixed point binary digital form and the leak warning output signal is provided if the latest ratio differs from the initial ratio by more than the least significant bit.

9. The system of claim 1 wherein the pump is responsive to an input signal to vary the displacement of the pump in direct relation to the input signal, and wherein the control means varies the input signal to the pump in proportion to the difference between the load pressure transducer means signal and the first pressure transducer means signal to control the pump so that the flow rate through the restrictor is maintained substantially constant.

10. Pressure monitoring apparatus comprising:
(a) a pump which draws gas in at an inlet and delivers gas under pressure at an outlet;
(b) a pressure sensor including:
(1) a cup shaped housing having an open mouth and having an inlet and outlet formed therein;
(2) a flexible diaphragm sealed over the open mouth of the housing to define a plenum between the walls of the mouth in the housing and the diaphragm, and wherein the inlet formed within the housing is in communication with the plenum;
(3) an exhaust tube centrally mounted in the housing within the plenum with an end thereof terminating adjacent the inner side of the diaphragm and connected to be in communication with the outlet from the housing;
(c) a supply line connected from the outlet of the pump to the inlet in the sensor housing to supply gas under pressure to the sensor and a return line connected from the outlet in the sensor housing to the inlet of the pump;
(d) a first accumulator chamber connected in the supply line;
(e) A second accumulator chamber connected in the return line;
(f) a flow restrictor connected in the supply line between the first accumulator chamber and the inlet to the pressure sensor housing;
(g) load pressure transducer means for measuring the pressure in the supply line between the restrictor and the pressure sensor and providing an output signal indicative thereof, whereby changes in ambient pressure at the pressure sensor will result in movement of the diaphragm to alternately open and close the exhaust tube to automatically maintain the pressure within the plenum approximately equal to the ambient pressure, and whereby the load pressure transducer means will measure a pressure which is approximately equal to the pressure surrounding the sensor including pressures lower than the ambient atmospheric pressure which are no lower than the pressure within the second accumulator chamber.

11. The apparatus of claim 10 wherein the pump is responsive to an input signal to vary the displacement of the pump in direct relation to the input signal, and including control means responsive to the signal from the load pressure transducer means so as to control the displacement of the pump in direct relation to the pressure at the pressure sensor.

12. The apparatus of claim 10 including:
(1) first pressure transducer means for measuring the pressure in the first accumulator chamber and providing an output signal indicative thereof;
(2) second pressure transducer means for measuring the pressure in the second accumulator chamber and providing an output signal indicative thereof;
and (3) control means responsive to the output signals of the first and second pressure transducer means for comparing the pressure measured in the first accumulator chamber with the pressure measured in the second accumulator chamber and providing a leak warning output signal if the compared pressures change with respect to one another in excess of a selected range, thereby indicating that the flow through the two accumulator chambers is not equal and a leak has occurred.

13. The apparatus of claim 12 wherein the pump is responsive to an input signal to vary the displacement of the pump in direct relation to the input signal, and including control means responsive to the signals from the load pressure transducer means and the first pressure transducer means for varying the input signal to the pump in proportion to the difference between the load pressure transducer means signal and the first transducer means signal to maintain the flow rate through the restrictor substantially constant.

14. The apparatus of claim 12 including a bypass valve connected around the pump and responsive to the leak warning signal from the control means, the bypass valve being operative to open when the leak warning signal is received to shunt the outlet of the pump back to its inlet and to relieve pressure within the system including equalizing the pressures within the first and second accumulator chambers.

15. The apparatus of claim 14 including a pressure release valve connected to the return line the pressure release valve being responsive to the signal from the control means to open at the same time that the bypass valve opens and to place the return line in communication with the ambient atmosphere.

16. The apparatus of claim 12 wherein the volume of the second chamber is substantially greater than the volume of the first chamber.

17. The apparatus of claim 12 wherein the control means is responsive to the output signal from the load pressure transducer means and provides a display to an operator of the pressure measured by the load pressure transducer means and wherein the control means also provides an audio output signal when the pressure measured by the sensor pressure transducer means exceeds a selected high pressure or is below a selected low pressure.

18. The apparatus of claim 12 including means for converting the output signals of the first and second pressure transducer means to digital data signals and wherein the control means compares the digital pressure signals from the first and second transducer means by periodically dividing one of the digitized output signals into the other to provide an initial digital ratio which is stored and a latest ratio, the control means comparing the latest ratio with the initial ratio and providing a leak warning output signal if the latest ratio deviates from the initial ratio in excess of a selected tolerance amount.

19. The apparatus of claim 18 wherein the ratios of the transducer means output signals are provided in fixed point binary digital form and the leak warning output signal is provided if the latest ratio differs from the initial ratio by more than the least significant bit.

20. The apparatus of claim 18 wherein the control means includes a microprocessor which periodically calculates the ratios of the output signals of the first and second pressure transducer means and provides the leak warning signal if the deviation from the initial ratio is in excess of the selected tolerance amount.

21. In a closed circulating gas system having a pump which draws gas at an inlet from a return line and delivers gas under pressure at an outlet to a supply line, and also having a load device which receives gas from the supply line and delivers it to the return line, apparatus for detecting leaks in the system comprising:
(a) a first accumulator chamber connected in the supply line;
(b) a second accumulator chamber connected in the return line;
(c) first pressure transducer means for measuring the pressure in the first accumulator chamber and providing an output signal indicative thereof;
(d) second pressure transducer means for measuring the pressure in the second accumulator chamber and providing an output signal indicative thereof;
(e) control means responsive to the output signals of the first and second pressure transducer means for comparing the pressure measured in the first accumulator chamber with the pressure measured in the second accumulator chamber and providing a leak warning output signal if the compared pressure change with respect to one another in excess of a selected range, thereby indicating that the flow through the two accumulator chambers is not equal and a leak in the system has occurred;
(f) a bypass valve connected around the pump and responsive to the leak warning signal from the control means, the bypass valve being operative to open when the leak warning signal is received to shunt the outlet of the pump back to its inlet and to relieve pressure within the system including equalizing the pressures within the first and second accumulator chambers; and (g) a pressure release valve connected to the return line, the pressure release valve being responsive to a signal from the control means to open at the same time that the bypass valve opens and to place the return line in communication with the ambient atmosphere.

22. In a closed circulating gas system having a pump which draws gas at an inlet from a return line and delivers gas under pressure at an outlet to a supply line, and also having a load device which receives gas from the supply line and delivers it to the return line, apparatus for detecting leaks in the system comprising:
(a) a first accumulator chamber connected in the supply line;
(b) a second accumulator chamber connected in the return line;
(c) first pressure transducer means for measuring the pressure in the first accumulator chamber and providing an output signal indicative thereof;
(d) second pressure transducer means for measuring the pressure in the second accumulator chamber and providing an output signal indicative thereof;
(e) control means responsive to the output signal of the first and second pressure transducer means for comparing the pressure measured in the first accumulator chamber with the pressure measured in the second accumulator chamber and providing a leak warning output signal if the compared pressures change with respect to one another in excess of a selected range, thereby indicating that the flow through the two accumulator chambers is not equal and a leak in the system has occurred; and (f) means for converting the output signals of the first and second pressure transducer means to digital data signals, and wherein the control means compares the digital pressure signals from the first and second transducer means by periodically dividing one of the digitized output signals into the other to provide an initial digital ratio which is stored and a latest ratio, the control means comparing the latest ratio with the initial ratio and providing a leak warning output signal if the latest ratio deviates from the initial ratio in excess of a selected tolerance amount.

23. The system of claim 22 wherein the control means includes a microprocessor which periodically calculates the ratios of the output signals of the first and second pressure transducer means and provides the leak warning signal if the deviation from the initial ratio is in excess of the selected tolerance amount.