WO1994007127A1

WO1994007127A1 - Apparatus and method for measuring chemical concentrations

Info

Publication number: WO1994007127A1
Application number: PCT/US1993/008651
Authority: WO
Inventors: Michael M. Gifford; Bruce T. Dalton
Original assignee: Sippican, Inc.
Priority date: 1992-09-14
Filing date: 1993-09-14
Publication date: 1994-03-31

Abstract

A chemical concentration measurement device comprises a probe assembly adapted for real-time, in-situ measurement of the concentration of a predetermined chemical or class of chemical compounds in a medium, an electronic processor module for receiving a first signal related to the concentration of the predetermined chemical or class of chemical compounds in the medium from the probe assembly and for generating a second signal indicative of the concentration of the predetermined chemical or class of chemical compounds in the medium, and means for providing the first signal to the electronic processor module. The probe assembly comprises a sensor, generates the first signal, and is interchangeable with other such probe assemblies for measurement of the concentration of other predetermined chemicals or classes of chemical compounds. The medium may include liquid, vapor, or other media.

Description

APPARATUS AND METHOD FOR MEASURING CHEMICAL CONCENTRATIONS

Field of the Invention

This invention relates to a device and method for measuring the concentration of a chemical or class of chemical compounds in a body of liquid, vapor, or other media, and more particularly, to a probe assembly adapted to be placed in-situ in a body of liquid, vapor, or other media and to transmit a signal related to a detected concentration to a signal processing unit.

Background of the Invention

It is known to use a fiber optic cable as a chemical sensor to detect the presence of a chemical in, e.g., a water sample. Many other types of chemical sensors are also known and used by those of ordinary skill in the art.

In the prior known systems, a sample typically is removed from the body of water of interest and transported in some type of container to a device which tests the sample and determines the concentration of a particular chemical in the sample. Prior devices typically include a chemical sensor which, regardless of its type, is brought into contact with the sample either by placing the sensor in the container holding the sample or by transferring the sample from the container into a receptacle located in the device. Summary of The Invention In general, in one aspect, the invention features a chemical concentration measurement device comprising a probe assembly adapted for real-time, in-situ measurement of the concentration of a predetermined chemical or class of chemical compounds in a liquid, vapor, or other media, the probe assembly comprising a sensor and generating a first signal related to the concentration of the predetermined chemical or class of chemical compounds in the medium; an electronic processor module for receiving the first signal and for generating a second signal indicative of the concentration of the predetermined chemical or class of chemical compounds in the liquid, vapor, or other media; and means for providing the first signal to the electronic processor module, the probe assembly being interchangeable with other such probe assemblies for measurement of the concentration of other predetermined chemicals or classes of chemical compounds. Some embodiments of this aspect of the invention include the following features. The first signal may comprise an optical signal which may be converted to a current or voltage signal and then to, for example, an intermediate frequency (IF) signal, an infrared (IR) signal, a radio frequency (RF) signal, or a digital signal. The second signal may comprise a digital signal. The providing means may comprise an electrical cable connected to the probe assembly at one end and to the electronic processor module at the other end, or alternatively, the providing means may comprise a transmitter. The probe assembly further may comprise a sealed probe housing in which the sensor is disposed such that at least a portion of the sensor is exposed to the medium. The probe assembly may comprise an optics assembly and a circuit board which are both disposed within the sealed probe housing, the optics assembly being electrically connected to the circuit board and adapted to receive the sensor. The probe assembly also may comprise a controllable current source for controlling an output intensity level of a light source to maintain the output at a predetermined intensity level. Also, the probe assembly may cooperate with the electronic processor module to automatically calibrate the probe assembly. The probe assembly may further generate a signal identifying, to the electronic processor module, the predetermined chemical which the probe assembly is adapted to measure. In general, in another aspect, the invention features a method for measuring chemical concentration comprising the steps of (i) providing a probe assembly for placement in a medium, the probe assembly adapted for real-time, in-situ detection of the concentration of a predetermined chemical or class of chemical compounds in the medium, the probe assembly comprising a sensor and being interchangeable with other such probe assemblies for measurement of the concentration of other predetermined chemicals or classes of chemical compounds; (ii) generating, by the probe assembly, a first signal related to the concentration of the predetermined chemical or class of chemical compounds in the medium; (iii) providing the first signal to an electronic processor module; and (iv) generating, by the electronic processor module, a second signal indicative of the concentration of the predetermined chemical or class of chemical compounds in the medium. In general, in another aspect, an optics assembly is disclosed for use within a probe assembly to facilitate real-time, in-situ detection of the concentration of a predetermined chemical or class of chemical compounds in a medium, the optics assembly comprising a light source, a measurement detector for detecting light generated by the light source, a reference detector forming a feedback path to maintain the intensity of light generated by the light source, a fiber optic element having a first end for transmitting light to the measurement detector and a second end for receiving light from the light source, a first support member for receiving the first end of the fiber optic element and for receiving and holding the measurement detector adjacent to the first end of the fiber optic element, and a second support member for receiving the second end of the fiber optic element and for receiving and holding the light source adjacent to the second end of the fiber optic element and for receiving and holding the reference detector at an angle to the length of the fiber optic element.

In general, in another aspect, the invention features a circuit for use in a probe assembly adapted for real-time, in-situ measurement of the concentration of at least one predetermined chemical in a medium, the probe assembly being a component of a chemical concentration measurement device, the chemical concentration measurement device further comprising an electronic processor module for receiving at least one signal from the probe assembly. The circuit comprises a controllable current source for controlling an output intensity level of a light source within the probe assembly to maintain the output at a predetermined intensity level, and the circuit is adapted to cooperate with the electronic processor module to automatically calibrate the probe assembly. The interchangeable probe assemblies provide flexibility and cost-effectiveness. Once a user identifies the specific chemicals and/or class of chemical compounds which are to be detected, the user only needs to use the probe assemblies designed to detect those specific chemicals and/or classes of chemical compounds.

The chemical concentration measurement device is compact and easy to use. After charging, it can be used as a portable tester.

The in-situ operation allows concentration readings to be obtained directly on location at the medium (e.g., liquid, vapor, or other media) of interest. Also, the readings are obtained in real time. Thus, the chemical concentration measurement device is adapted to provide rapid, low cost analysis of a variety of chemicals and/or classes of chemical compounds which may be contained in, e.g., a liquid, vapor or other media. Other aspects, features, and advantages of the invention will become apparent from the following description and from the claims.

Brief Description of the Drawings FIG. 1 is a block diagram of a measurement device according to the invention.

FIG. 1A is a block diagram of an alternate embodiment of the measurement device according to the invention.

FIG.2 is a schematic diagram of an embodiment of a measurement device according to the invention.

FIG. 3 is a perspective view of the embodiment of FIG. 2, including a set of interchangeable probe assemblies.

FIG. 4 is a block diagram of an embodiment of electronics contained within a meter housing. FIG. 5 is a view of a probe assembly. FIG. 6 is a side diagramatic view of some of the components of the probe assembly of FIG. 5.

FIG. 7 is a view of the probe assembly taken along line 7-7 of FIG. 6.

FIG. 8 is a side view of a circuit board assembly adapted to be housed within the probe assembly of FIG. 6.

FIG. 9 is a top view of the circuit board assembly, taken along line 9-9 of FIG. 8.

FIG. 10 is a bottom view of the circuit board assembly, taken along line 10-10 of FIG. 8.

FIG. 11 is a block diagram of an embodiment of a circuit contained within the probe assembly of FIG. 5. FIG. 12 is a top view of an optics assembly, with portions broken away. FIG. 13 is a diagramatic view of some components of the optics assembly of FIG. 12. Detailed Description Referring to FIG. 1, the invention relates to a probe assembly 11 having a chemical sensor 13 for measuring the concentration of a chemical or class of chemical compounds in a medium 15 and for generating a signal related to the concentration of the chemical or class of chemical compounds. The medium 15 may include liquid (e.g., water), vapor, or other media. The chemical sensor 13 may be within the probe assembly 11 or connected to the probe assembly 11. The invention further includes a meter 17 for receiving the signal from the probe assembly 11 and for generating another signal indicative of the concentration of the chemical or class of chemical compounds. An electronic processor module 19 within or connected to the meter 17 receives the signal from the probe assembly 11 and generates the signal indicative of the concentration of the chemical or class of chemical compounds in the medium 15. The signal generated by the probe 11 is transmitted to the electronic processor module 19 via a transmission channel 21. In addition to the probe assembly 11, a plurality of similar probe assemblies 23 are provided. Each of the plurality of probe assemblies is interchangeable with the probe assembly 11. The probe assembly 11 and each of the plurality of probe assemblies 23 are preferably adapted to measure the concentration of a different chemical or class of chemical compounds in the medium 15.

Referring to FIGS. 2 and 3, an embodiment of the invention includes a chemical concentration measurement device 10 comprising a probe assembly 12 connected to a meter housing 16 via an electrical cable 14. Instead of the cable 14, the transmission channel may be, as shown in FIG. 1A, free space 201, in which case no physical cable is needed. In the embodiment of FIG. 1A, a transmitter 200 modulates and sends a signal generated by the probe assembly 12 to a receiver 202 which receives and demodulates the signal. The signal generated by the probe assembly 12 may be, e.g., a digital signal, an intermediate frequency (IF) signal, a radio frequency (RF) signal, an infrared (IR) signal, or an optical signal. The transmitter 200 may be housed within the probe assembly 12, or the transmitter 200 may be external to the probe assembly 12. In either embodiment, the receiver 202 can be located either within the meter housing 16 or separate from the meter housing 16.

As shown in FIGS. 2 and 3, the probe assembly 12 may be submerged in a body of liquid 18. Note that while references are made hereinafter only to the liquid 18 or the body of liquid 18, all such references should be interpreted to include aqueous liquid (such as an ocean, a harbor, or a lake), non-aqueous liquid, vapor, or other media. With the probe assembly 12 submerged in the body of liquid 18, the concentration (i.e., amount per unit volume) of a chemical or class of chemical compounds (typically a pollutant or pollutants) in the liquid 18 can be determined and displayed to a user of the measurement device 10, preferably in parts per million (ppm) or parts per billion (ppb). Typically, the probe assembly 12 is designed to only detect the concentration of one particular chemical or class of chemical compounds. Accordingly, in this embodiment, the probe assembly 12 is detachable from the cable 14 and a set of interchangeable probe assemblies 24 are provided, where each of the plurality of probe assemblies is preferably designed to detect the concentration of a different chemical or class of chemical compounds. While FIG. 3 shows six probe assemblies in the set 24, the set could include any number of probe assemblies.

An end connector 20 at the distal end of the cable 14 and a mating connector 22 on each of the plurality of probe assemblies 24 facilitate attachment and detachment of the various probe assemblies.

The measurement device 10 is adapted to determine the concentration of a particular chemical or class of chemical compounds in the body of liquid 18 with the probe assembly 12 located in-situ. That is, the measurement device 10 provides readings of chemical concentration with the probe assembly 12 located in the body of liquid 18, and without the need to first remove a sample from the body of liquid 18. This in-situ operation eliminates complicated sample preservation and handling procedures which inherently involve the risk of sample contamination and generally unnecessarily complicate the testing procedure. Applications for which the disclosed measurement device 10 is specifically suited include industrial and municipal wastewater monitoring, groundwater monitoring, and surface water monitoring. In industrial and municipal wastewater monitoring applications, the measurement device 10 can, for example, trace chemical discharges to their source, and monitor process and treatment operations. In groundwater monitoring applications, the measurement device 10 can, for example, measure chemical concentrations during site assessment, test the efficacy and progress of remediation, and monitor the effluent of pump-and-treat operations. In surface water monitoring applications, the measurement device 10 can, for example, measure chemical concentrations in rivers, lakes, estuaries, or wastewater lagoons. Other applications may include the testing of soil and enclosed volumes such as storage tanks, the latter application being an example of testing liquid and/or vapors.

In the disclosed embodiment, the probe, assembly 12 is placed in the body of liquid 18 by selectively drawing in and paying out the cable 14 from the meter housing 16 through a cable opening 25. The cable 14 is drawn in and paid out by turning a knob 26 on a rotatable section 28 of the meter housing 16 with one hand while firmly grasping a handle 30 formed integrally with a non-rotatable section 32 of the meter housing 16 with the other hand. The cable 14 is preferably about twenty-five feet in length but may be shorter or longer depending upon the particular application.

In the disclosed embodiment, when the probe assembly 12 enters the body of liquid 18, the liquid 18 (typically water) completes an electrical circuit in probe assembly 12 and an audible "beep" is produced at the meter housing 16 to indicate to a user that the probe assembly 12 is in place. This feature is useful in situations where the user loses sight of the probe assembly 12 as it is being paid out from the meter housing 16.

When inserted in the liquid 18, the probe assembly 12 continuously or repeatedly generates signals related to the concentration of the particular chemical or class of chemical compounds it is designed to detect. Preferably, the probe assembly 12 also continuously or repeatedly generates signals related to, for example, (i) the type of chemical or class of chemical compounds that the probe assembly 12 is designed to detect (i.e., a "chemical identifier" signal which is preferably stored in the probe assembly), (ii) ambient light rejection, (iii) the sensitivity of the probe assembly 12, and (iv) the temperature of the liquid 18. In the disclosed embodiment, these signals are analog signals. The signals may be intermediate frequency (IF) signals, radio frequency (RF) signals, infrared (IR) signals, or optical signals, to name a few. The signals also may be digital signals. Regardless of the type/format of the signals, the probe assembly 12 preferably generates the signals related to the concentration of the chemical or class of chemical compounds of interest by detecting optical signals. Generally, all of the signals generated by the probe assembly 12 are sent to the meter housing 16 via the cable 14 where they are processed by an electronic processor module 40 within the meter housing 16. A control panel 34 located in the rotatable section 28 of the meter housing 16 allows a user to control the electronics 40 (and, in general, the measurement device 10) via a tactile keypad 36 and to view the concentration of the chemical or class of chemical compounds being detected on a display 38 in, e.g., parts per million (ppm) or parts per billion (ppb), and typically temperature in Celsius degrees. The display 38 is updated in real time to provide the user with the most accurate information about the chemical concentration in the liquid 18 at any given instant. The only delay of any significance is the time necessary for the probe assembly 12 to generate the signals related to the concentration and for the electronics 40 to process the signals received from the probe assembly 12. The electronic processor module 40 may be provided on a circuit card assembly adapted for insertion into a personal computer. In this embodiment, the meter housing would be eliminated and the personal computer would utilize the card to receive data from the probe assembly and process the data. A monitor connected to the personal computer may be used to display information such as the concentration and temperature values. Also, in this embodiment, the data from the probe assembly may be transmitted and received in a variety of formats (e.g., digital signal, intermediate frequency (IF) signal, radio frequency (RF) signal, infrared (IR) signal, or optical signal) and via a variety of transmission channels (e.g., free space, electrical cable, optical cable).

Thus, the measurement device 10 is adapted for real time, in-situ measurement of the concentration of a variety of chemicals and/or classes of chemical compounds in a medium (e.g., liquid, vapor, or other media) by utilizing a plurality of interchangeable probe assemblies that are each designed to detect the concentration of one particular chemical or class of chemical compounds, typically a chemical pollutant or pollutants. The measurement device 10 may include a rechargeable battery pack (not shown) disposed in the meter housing 16. The battery pack may include, for example, six AA NICAD rechargeable batteries which would allow more than forty hours of continuous operation on a single battery charge. A 9 volt DC battery charger (not shown) may also be provided with the measurement device 10.

The meter housing 16 is designed to be portable and weather resistant. In one embodiment, it measures approximately 8 inches by 10 inches by 7 inches, weighs less than about five pounds (including twenty-five feet of cable 14), and is made of a rugged, chemical resistant high density polyethylene (HDPE). In one embodiment, the probe assembly 12 is about 16 cm in length. The measurement device 10 preferably has an operating temperature range of 0 to 50 Celsius degrees, and a storage temperature range of -20 to 60 Celsius degrees.

As shown in the embodiment of FIGS. 2 and 3, the control panel 34 includes the keypad 36 and the display 38, and is preferably sealed to be weather resistant. In the disclosed embodiment, the display 38 is a generally rectangular, sixteen-character alphanumeric liquid crystal display (LCD). While the probe assembly 12 is submerged in the body of liquid 18, the display 38 continuously displays the temperature (in Celsius degrees) of the body of liquid 18 as well as the concentration in, preferably, parts per million (ppm) or parts per billion (ppb). The keypad 36 preferably includes tactile (i.e., touch-sensitive) keys having alphanumeric markings printed on their face. The various keys allow a user to perform various functions. In one embodiment, a user may store and recall data points, set an audible alarm to "ring" when the concentration exceeds a specified limit, invoke an autoscaling feature to automatically scale the units of the concentration reading on the display 38 between, for example, parts per million (ppm) and parts per billion (ppb), "freeze" the readout, clear the readout, and verify an entry, all via the keys on the keypad 36.

Having described the basic structure of and functions which can be performed by the measurement device 10, a more detailed description of the various aspects of the measurement device 10 follows. Referring to FIG. 4, in the disclosed embodiment, the electronics 40 in the meter housing 16 include a microprocessor 166 (preferably a model 87C51 available from Intel). In the circuitry currently used, the microprocessor 166 is connected to an input frequency conditioner circuit 168, a tri-state line driver circuit 170, a non-volatile memory 174, a water detection oscillator circuit 172, a low battery detector circuit 180, a random access memory (RAM) 182, a keypad interface circuit 184, a test port 186, an LCD contrast adjustment circuit 188, and an RS 232 I/O circuit 190. The low battery detector 180 is connected to the voltage regulator circuit 178 which is connected to the on/off toggle circuit 176. The lines 160, 162, and 164 extend to a circuit described below with reference to FIG. 11. The signals on the various lines connected to the microprocessor 166 are preferably digital signals.

It should be readily apparent to one of ordinary skill in the art that other electronic topographies will also work and that the specific details of each of the electronic blocks are well-known.

In the disclosed embodiment, the input frequency conditioner 168 accepts a signal 118 (described below) generated by the probe assembly 12 and provides an associated digital output signal to the microprocessor 166. The microprocessor 166 contains software that implements a different algorithm for each particular chemical or class of chemical compounds being tested. Each algorithm utilizes the various signals received from the probe assembly 12 (e.g., as described previously, signals related to the type of chemical or class of chemical compounds the probe assembly is designed to detect, ambient light rejection. sensitivity, and temperature, as well as the signal 118) to determine a concentration level. Parameters used in the various algorithms are retained in an on¬ board non-volatile memory 158 located within the probe assembly 12 and are retrieved whenever a probe assembly 12 is connected to the meter housing 16 and the electronic processor module 40. (The memory 158 is described more fully below with reference to FIG. 11.) In general, by execution of the various algorithms contained within the microprocessor 166, the electronic processor module 40 generates a digital signal indicative of the concentration of the particular chemical or class of chemical compounds being tested. This digital signal ultimately is displayed as a numerical value in typically either parts per million (ppm) or parts per billion (ppb) on the display 38 (FIGS. 2 and 3).

The tri-state line driver 170 conditions digital signals from the microprocessor 166 that are bound for the probe assembly 12. The non-volatile memory 174 operates in conjunction with the tri-state line driver 170 and the microprocessor 166 to retain various system parameters and/or coefficients the microprocessor 166 uses in processing data received from the probe assembly 12 and maintaining the display contrast. The memory 174 also is used to store integration periods and as a scratchpad storage area for the user. The memory 174 may comprise a serial read only memory (ROM) chip. The water detection oscillator circuit 172, which may include a threshold adjustment circuit for accommodating various "qualities" of water (e.g., various levels of conductivity), may provide water detection capabilities. Two water-detection electrodes 94 and 95 may be used in conjunction with the circuit 172 to indicate when the probe assembly 12 is submerged in the liquid 18.

The low battery detector 180 is adapted to detect when a battery pack (not shown), if included, is low, and report the condition to the microprocessor 166. The voltage regulator 178 maintains its output voltage essentially constant despite variations in its input voltage. The on/off toggle circuit 176 may comprise an on/off switch and a charging current limiter circuit. The on/off toggle circuit 176 is designed to interrupt system power and provide a single-key on/off feature.

The RAM 182 may comprise a 4096x8 bit memory chip. Some digital data output lines of the RAM 182 connect to the keypad interface 184, which interfaces with the keypad 36 on the control panel 34 of the meter housing 16. Some digital data output lines of the RAM 182 are connected to the test port 186 which allows diagnostics to be performed on the electronics 40 during assembly. All digital data output lines of the RAM 182 are connected to the microprocessor 166.

The LCD contrast adjustment 188 interfaces with the display 38 on the control panel 34 and controls the contrast of the display via a digital signal from the microprocessor.

The RS 232 I/O 190 operates in accordance with the Institute of Electrical and Electronics Engineers, Inc. (IEEE) RS 232 protocol standard. Accordingly, the RS 232 I/O 190 accepts inputs that follow the standard and provides associated digital signals to the microprocessor 166. Also, the RS 232 I/O 190 receives digital signals from the microprocessor 166 and provides outputs in accordance with the IEEE standard. Referring to FIGS. 5, 6, and 7, in the disclosed embodiment, the probe assembly 12 includes a sealed probe housing 42 having an aperture 80 formed generally through its center. Preferably, the aperture 80 is oblong in shape. The aperture 80 is designed to expose a chemical sensor disposed within the probe housing 42 to the liquid 18 in which the probe assembly 12 is submerged.

In one embodiment, the chemical sensor comprises a fiber optic element 82. The fiber optic element 82 detects chemical concentrations in liquid by methods well-documented and well-known to those of ordinary skill in the art to which the invention pertains. Briefly, the fiber optic element 82 typically includes a chemical coating or outer layer chosen to react with or have an intermolecular affinity for a particular chemical or class of chemical compounds in the medium and thereby change the total internal reflection characteristics of the fiber optic element 82. The change in the transmission of light in the fiber optic element 82 relates to the concentration of the particular chemical or class of chemical compounds in the medium. This technology commonly is referred to as fiber optic chemical sensor technology. Using fiber optic chemical sensor technology, each probe assembly 12 provided in the set 24 is capable of detecting a very small amount of a particular chemical or class of chemical compounds in the liquid 18. Two examples of the types of chemicals and/or classes or chemical compounds that can be detected with the probe assemblies include aromatic hydrocarbons and halogenated hydrocarbons. Preferably, the concentration levels of the detected chemicals or classes of chemical compounds are displayed to a user on the display 38 (FIGS. 2 and 3) in either parts per million (ppm) or parts per billion (ppb).

Also, when fiber optic chemical sensor technology is employed, because each probe assembly in the set of interchangeable probe assemblies 24 (FIG. 3) is designed to detect a different chemical or class of chemical compound, each probe assembly typically includes a fiber optic element having a different chemical coating or outer layer.

Other chemical sensors may also be used in the probe assembly 12. Essentially any sensor or array of sensors having an output which varies with the concentration of a particular chemical or class of chemical compounds in the medium can be used as the chemical sensor in the probe assembly 12. Examples of possible sensors include (i) a chemically-sensitive field effect transistor (chemFET) such as ISFET and ENFET, (ii) a surface acoustic wave (SAW) device, (iii) acoustic wave devices such as a Lamb Wave device, an acoustic plate mode device, and a microbalance, (iv) a Raman or surface enhanced Raman spectrometry (SERS) device, (v) an optical reflectance device such as a polymer swell micromirror, (vi) an optical planar waveguide used for spectrometry, refractometry, attenuated total reflectance (ATR) or fluorescence, (vii) a miniature fluorescence liquid reservoir cell, (viii) a miniature optical absorbance liquid reservoir cell, (ix) a solid optical structure for optical absorbance or fluorescence measurements such as a thin film, waveguide or porous fiber made of polymer or glass, (x) an electrochemiluminescence device, (xi) an electrical capacitance device utilizing polymer swell, immunochemical and enzyme surface bonding schemes. (xii) a planar or fiber optic waveguide grating coupler device, (xiii) an electrochemical sensor such as an ion sensitive electrode (ISE) or a miniaturized solid state electrochemical sensor, and (xv) an electrical resistance device such as a thin or thick film resistor. Alternative chemical sensor technologies and many variations of the above-listed sensors exist. A person of ordinary skill in the art will know of the various alternatives and variations which may be applicable to the present invention.

Still referring to FIGS. 5, 6, and 7, in one preferred embodiment, the probe housing 42 is made of nickel-plated brass for environmental resistance, is generally cylindrical in shape, and has a bore (indicated by dotted lines in FIG. 6) extending from a top end to a bottom end. The top end of the probe housing 42 is designed to accept a top cap 86, and the bottom end of the probe housing 42 is designed to accept a bottom cap 84. The top cap 84 is designed to be combined with a nut 88, a washer 90, and the male connector 22 before being inserted into the top end of the probe housing 42. The end view of FIG. 7 shows twelve electrical receptacles in the male connector 22, numbered one through twelve. The probe housing 42 is preferably about 16 cm in length with an outside and inside diameters of about 2.5 cm and 2.4 cm, respectively. The aperture 80 preferably measures approximately 5.08 cm by 1.02 cm. To assemble the probe assembly 12, the bottom cap 84 is pressed into the bottom end of the probe housing 42 and a circuit board assembly 44 (FIGS. 8, 9, and 10) is placed in the probe housing 42 through the opening in its top end. A two-part epoxy (not shown) is used to encapsulate both ends of the circuit board assembly 44 within the probe housing 42. The epoxy does not encapsulate the chemical sensor (e.g., the fiber optic element 82). That is, the aperture 80 is generally free of any epoxy. Two dotted lines 76, 78 indicate the areas inside the probe housing 42 which contain epoxy and those which do not contain epoxy. When the probe assembly 12 is inserted in a medium, the medium directly contacts the chemical sensor via the aperture 80 but the medium does not enter the probe housing 42 or contact the encapsulated portions of the circuit board assembly 44.

Prior to the encapsulation process, the male connector 22 is placed in the top cap 86, the washer 90 is placed over the portion of the male connector 22 extending through the top cap 86, and the nut 88 is screwed onto the male connector 22 over the washer 90. Some wires 96 extending from the circuit board assembly 44 (described below) are then inserted into the numbered electrical receptacles (FIG. 7) in the male connector 22, and the combination of the nut 88, the washer 90, the top cap 86, and the male connector 22 is secured in the probe housing 42 by pressing the top cap 86 into the top end of the probe housing 42.

A probe identification label 77 is typically affixed to the outside of the probe housing 42 to identify the type of chemical or class of chemical compounds this particular probe assembly is designed to detect. Typically, as described previously, each probe assembly in the set of interchangeable probe assemblies 24 (FIG. 3) is designed to detect a different chemical or class of chemical compounds, and therefore such an identifying label is generally helpful.

Referring to FIGS. 8, 9, and 10, in the disclosed embodiment, the circuit board assembly 44 of the probe assembly 12 includes an optics assembly 45 and a printed circuit board 46. As previously described, the circuit board assembly 44 is adapted to be disposed within the sealed probe housing 42.

In the disclosed embodiment, the printed circuit board 46 of the circuit board assembly 45 is made using known printed circuit board (PCB) technology. The optics assembly 45 is soldered to the printed circuit board 46 via the plated holes E10 through E15 (FIG. 10) in the printed circuit board 46. Two mounting screws 92 and 93 further secure the optics assembly 45 to the printed circuit board 46 when screwed through mounting holes MH1 and MH2 (FIG. 11) into the optics assembly 45. The wires 96 extend from a connector 98 and have plug-in pins 100 on their ends for connection to the receptacles of the male connector 22. The connector 98 is secured to plated holes El, E2, E3, E4, E7, E8, and E9 of the printed circuit board 46.

In the disclosed embodiment, two water-detection electrodes 94 and 95 extend from plated holes E5 and E6 on the bottom of the printed circuit board 46. In addition to exposing the chemical sensor in the probe assembly 12 to the liquid 18 in which the probe assembly 12 is submerged, the aperture 80 exposes the electrodes 94 and 95 to the liquid. (Note that because the epoxy from the encapsulation process does not enter into the area of the aperture 80, the electrodes 94 and 95, like the chemical sensor, are not encapsulated by the epoxy.) The liquid provides an electrical path between the electrodes 94 and 95. Accordingly, a closed electrical circuit is created in the probe assembly 12 when the aperture 80 of the probe assembly 12 is submerged in the liquid. Thus, in this embodiment, the electrodes 94 and 95 provide an on/off feature controlled by the presence or absence of liquid. The audible "beep" described previously is triggered when the electrodes 94 and 95 are electrically connected by submersion of the aperture 80 in liquid. In the disclosed embodiment, the optics assembly 45 includes (i) the fiber optic element 82, (ii) an optics support member 102 having an aperture 104 (preferably oblong in shape) formed therethrough, (iii) a source light-emitting diode (LED) 110, (iv) a detector photodiode 112, and (v) a reference photodiode 114. The aperture 104 is designed to allow the fiber optic element 82 to contact any medium in which the probe assembly 12 is inserted.

Typically, each probe assembly in the set includes a source LED which emits light of a particular wavelength. Generally, in the disclosed embodiment, the source LED 110 emits light which is transmitted through the fiber optic element 82 and detected by the detector photodiode 112. The reference photodiode 114 detects light emitted by the source LED 110 and provides a feedback path which maintains the intensity of the light emitted by the source LED 110 at a desired level. The changes in the transmission of light (i.e., changes in an optical signal) which occur in the fiber optic element 82 when the chemical or class of chemical compounds of interest is present in the medium (as briefly described previously) are detected by the arrangement of the source LED 110, the detector photodiode 112, and the reference photodiode 114. In the disclosed embodiment, a circuit 47 on the printed circuit board 46 translates these detected changes into the signal 118 which is related to concentration of the chemical or class of chemical compounds of interest in the medium. Referring to FIG. 11, the circuit 47 transforms a signal 116 (current or voltage) related to the concentration of a chemical or class of chemical compounds into the signal 118 which is also related to the concentration. The signal 116 typically is an analog output from the detector photodiode 112 and is related to changes in the transmission of light (i.e., changes in an optical signal) in the fiber optic element 82 which occur when the medium in which the probe assembly 12 is inserted contains the chemical or class of chemical compounds the probe assembly 12 is designed to detect. The signal 118 typically is an analog signal and may be an intermediate frequency (IF) signal, a radio frequency (RF) signal, an infrared (IR) signal, or an optical signal. The signal 118 may be a digital signal. In general, the signal 118 may be any type of signal known to those of ordinary skill in the art and may be transmitted to the electronics 40 (FIG. 4) in any of a variety of ways/formats, as described previously.

In the arrangement currently used, the circuit 47 includes a detector amplifier circuit 150, a temperature switch and thermistor circuit 152, a voltage-to-frequency converter 154, an LED servo driver circuit 156, and the on-board non-volatile memory 158. It should be readily apparent to one skilled in the art that other circuit configurations will also work. The detector amplifier 150 is adapted to receive and amplify the signal 116. The detector amplifier 150 may include an offset adjustment circuit to allow the level of amplification to be adjusted.

The temperature switch and thermistor circuit 152 receives the amplified signal from the detector amplifier 150 and is adapted to detect and provide temperature information to the electronic processor module 40. The voltage-to-frequency converter 154 transforms a signal at its input to signal 118 and provides the signal 118 as an output on the line 160. The signal 118 on the line 160 is the input for the input frequency conditioner 168 (FIG. 4). As described previously, the signal 118 may be any of a variety of signal types/formats known to those of ordinary skill in the art.

The LED servo driver 156 controls the source LED 110 and receives the output of the reference photodiode 114. The reference photodiode 114 aids in stabilizing the output of the source LED 110 and maintaining the output at a constant intensity level by dynamically compensating for variations in the output due to, e.g., aging, temperature, and voltage fluctuations. By maintaining the output of the source LED 110 at a constant intensity level, any variations of optical energy propagating through the fiber optic element 82 and detected at the detector photodiode 112 are assumed to be caused by the presence of the particular chemical or class of chemical compounds of interest.

The measurement system (i.e., the circuit 47, the source LED 110, the detector photodiode 112, and the reference photodiode 114) typically requires periodic adjustment of the output intensity level of the source LED 110 to maintain the output of the detector amplifier 150 in an acceptable "window" during operation. In the disclosed embodiment, the output level of the source LED 110 is adjusted by a current controller circuit 157 which is a controlled current source whose through current may be set. The current controller 157 may include a 12-bit, serial digital-to- analog (D/A) converter whose output current may be set by providing a digital word to an input port of the D/A. The digital word may be provided by the electronic processor module 40, specifically by the microprocessor 166.

In the disclosed embodiment, the LED servo driver 156 includes an amplifier. The output of the reference photodiode 114 is a current that is added to the inverting input of the amplifier of the LED servo driver 156. Output current from the current controller 157 flows from the inverting input of the amplifier of the LED servo driver 156 to ground. Consequently, the output of the current controller 157 offsets the output of the reference photodiode 114. The amplifier of the LED servo driver 156 has a very high closed-loop gain (e.g., greater than about twenty million). Any current difference at the input of the amplifier will be multiplied by the value of the closed-loop gain. Thus, the output intensity of the source LED 110 will result in an output from the reference photodiode 114 equal to the value of the output current from the current controller 157. The current controller 157 thus has direct control over the output intensity of the source LED 110. That is, the output intensity level of the source LED 110 is controlled by the output of the current controller 157 and is unaffected by, e.g., temperature, aging characteristics of the source LED, and battery pack voltage.

In a typical use, a user connects a probe assembly 12 to the meter housing 16 and turns on the measurement device 10 via the keypad 36. The user then presses the "zero" key to indicate that the electronic processor module 40 should interpret the received signal as a "blank" or baseline value from which all subsequent measurements are to be referenced. If the electronics 40 determine the received signal is outside of an acceptable range, the electronics 40 will send a control signal to the current controller 157 (e.g., a digital word sent to the input of the D/A) to change the output of the current controller 157 and achieve an acceptable output intensity level of the source LED 110. An acceptable intensity is reached when the output of the detector amplifier 150 is centered within a predetermined operating range. Whether the output is centered typically is determined by the microprocessor 166 in the electronic processor module 40. (This center point generally is referred to as the "set point.") Note that minor deviations from the center of the operating range are corrected mathematically by the microprocessor 166. The device 10 thus provides an automatic "zeroing" feature which automatically sets the output intensity of the source LED 110 to a desired level. With this feature, while the measurement system (i.e., the circuit 47, the source LED 110, the detector photodiode 112, and the reference photodiode 114) may have a very high level of gain, problems such as component drift, manufacturing tolerances, and the accumulation of debris on the probe assembly are minimized or eliminated.

In general, the automatic zeroing feature may be performed anytime but typically is performed prior to each use of the device 10 or each time a different probe assembly 12 is connected to the meter housing 16. As described previously, the automatic zeroing feature may be activated by turning on the device 10 and pressing the "zero" key on the keypad 36. The automatic zeroing feature adjusts the set point to, e.g., compensate for the accumulation of dirt and other debris on the probe assembly 12, and generally automatically calibrates the probe assembly 12. The set point value typically is determined by the microprocessor 166 in the meter housing 16 and, in general, the microprocessor 166 utilizes the set point value and other signals received from the probe assembly 12 to determine the concentration level of the particular chemical or class of chemical compounds which the probe assembly 12 is designed to detect.

In addition to being used in the automatic zeroing feature, the current controller 157 is used to extract out the effect of interfering ambient light on the optical components (i.e., the fiber optic element 82, the source LED 110, the detector photodiode 112, and the reference photodiode 114). During a measurement, the output intensity of the source LED 110 is modulated by a precise amount that depends on the particular fiber optic element employed. Due to the very high gain of the measurement system, it is not possible to turn off completely the source LED 110. Completely shutting down the source LED 110 would drive the detector amplifier 150 and the amplifier in the LED servo driver 156 out of their operating ranges. The source LED 110 is modulated by a small amount roughly equivalent to the total change in light attenuation that the particular fiber optic element would experience when exposed to a chemical or class of chemical compounds it is designed to detect. This small amount of modulation allows the output intensity of the source LED 110 to remain "on scale" and still provide two discrete frequencies at different light levels to minimize the effects of interfering ambient light. Control of the current controller 157 allows precise modulation of the source LED 110 to be achieved accurately and allows a high degree of ambient light rejection. The frequencies output by the source LED 110 before and after modulation are supplied to an algorithm implemented by the microprocessor 166 which determines what portion of the received signal is due to chemical influences and what part is due to ambient light fluctuations.

When the probe assembly 12 is not being utilized for measurements (an idle state) , the current controller 157 is caused to produce an output current which results in a minimum output intensity from the source LED 110. This feature tends to extend the useful life of the battery pack.

Still referring to FIG. 11, the LED servo driver 156 is connected to the on-board non-volatile memory 158 and to the line 162. The line 162 connects to the tri-state line drivers 170 (FIG. 4). The on-board non¬ volatile memory 158 is also connected to the line 164 which connects to the tri-state line drivers 170 (FIG. 4). The memory 158 is referred to as "on-board" because it is located on the printed circuit board 46. The memory 158 may retain parameters specific to each of the various probe assemblies 12 such as parameters computed during a probe calibration procedure and/or the automatic zeroing procedure described previously. The memory 158 may comprise a serial read only memory (ROM) chip.

Referring to FIGS. 12 and 13, in the disclosed embodiment, the optics support member 102 of the optics assembly 45 has several passages formed in it. One bore, for accepting the fiber optic element 82 (shown by dotted lines), extends through the support member 102. This bore is preferably about 1 mm in diameter. Another bore is formed in the support member 102 to accept and hold the source LED 110. This second bore is preferably about 5 mm in diameter. Another bore extends at an oblique from the previous bore, and is designed to accept and hold the reference photodiode 114. This bore preferably has a diameter of about 5 mm. Another bore is formed in the support member 102 to accept and hold the detector photodiode 112. This bore preferably has a diameter of about 5 mm. Two other bores extend perpendicularly through the support member 102. These two bores are each preferably about 1.8 mm in diameter, and are designed to accept the mounting screws 92 and 93 (FIG. 8). The aperture 104 (FIG. 8) which extends through the support member 102 is indicated by two dotted lines 106 and 108 in FIGS. 12 and 13. The optics support member 102 preferably is made of aluminum. The optics support member 102 is designed to hold all optical components (i.e., the fiber optic element 82, the source LED 110, the detector photodiode 112, and the reference photodiode 114) securely in place and in proper alignment with each other, and to expose the fiber optic element 82 via the aperture 104 (indicated by the dotted lines 106 and 108) to any medium in which the probe assembly 12 is inserted. The optics support member 102 is easy and inexpensive to manufacture. It is a single unit that facilitates fast and accurate assembly of the optics assembly 45 without the need for a highly-skilled technician.

Referring to FIG. 13, in the disclosed embodiment, the optics assembly 45 is assembled as follows. One end 138 of the fiber optic element 82 and the face 139 of the source LED 110 are sand-etched to diffuse the light that emits from the source LED 110 and enters the fiber optic element 82. The source LED 110 is inserted into the bore in the support member 102 and is held in place by a press fit. The fiber optic element 82 is inserted into the bore extending through the length of the support member 102. A cap 141 is placed over the detector photodiode 112 and the combination is inserted into the bore in the support member 102 and held in place by glue. The cap 141 is used to compensate for variations in the length of the fiber optic element 82. The fiber optic element 82 is preferably approximately 5.08 cm in length and 1 mm in diameter. The portion of the fiber optic element 82 which is exposed by the aperture 104 (and, ultimately, the aperture 80 in the sealed probe housing 42 when the probe assembly 12 is completely assembled) is preferably about 3 cm in length. The reference photodiode 114 may be inserted into its respective bore at any point in the assembly process, and is held in place by a press fit.

As shown in the embodiment of FIG. 12, when the optics assembly 45 is assembled, one end of the fiber optic element 82 contacts the face 139 of the source LED 110 and the other end of the fiber optic element 82 contacts the face 140 of the cap 141. This arrangement provides the necessary stabilization for the fiber optic element 82 for proper operation of the probe assembly 12. With the source LED 110 press fit into place and the detector photodiode 112 glued into place, the fiber optic element 82 is held securely and accurately in the desired operational position relative to the source LED 110, the detector photodiode 112, and the reference photodiode 114.

Other modifications and implementations will occur to those skilled in the art without departing from the spirit and the scope of the invention as claimed. Accordingly, the invention is to be defined not by the preceding illustrative description, but by the following claims. What is claimed is:

Claims

1. A chemical concentration measurement device comprising: a probe assembly adapted for real-time, in-situ measurement of the concentration of at least one predetermined chemical in a medium, said probe assembly comprising a sensor and generating a first signal related to the concentration of said at least one predetermined chemical in the medium, an electronic processor module for receiving said first signal and for generating a second signal indicative of the concentration of said at least one predetermined chemical in the medium, and means for providing said first signal to said electronic processor module, wherein said probe assembly is interchangeable with other such probe assemblies for measurement of the concentration of other predetermined chemicals.

2. The device of claim 1 wherein said first signal comprises an optical signal.

3. The device of claim 2 wherein said optical signal is converted by said probe assembly to an intermediate frequency signal, a radio frequency signal, or an infrared signal.

4. The device of claim 2 wherein said optical signal is converted by said probe assembly to a digital signal.

5. The device of claim 1 wherein said providing means comprises an electrical cable connected to said probe assembly at one end and to said electronic processor module at the other end.

6. The device of claim 1 wherein said providing means comprises a transmitter.

7. The device of claim 1 wherein said probe assembly further comprises a sealed probe housing, said sensor disposed within said sealed probed housing, at least a portion of said sensor being exposed to the medium.

8. The device of claim 7 wherein said probe assembly further comprises an optics assembly and a circuit board which are both disposed within said sealed probe housing, said optics assembly being electrically connected to said circuit board and adapted to receive said sensor.

9. The device of claim 1 wherein said probe assembly further generates a signal identifying said at least one predetermined chemical for which it is adapted to measure, said identifying signal being delivered to said electronic processor module via said providing means.

10. The device of claim 1 wherein said probe assembly further comprises a controllable current source for controlling an output intensity level of a light source to maintain the output at a predetermined intensity level.

11. The device of claim 1 wherein said probe assembly cooperates with said electronic processor module via said providing means to automatically calibrate said probe, assembly. 12. A method for measuring chemical concentration, comprising: providing a probe assembly for placement in a medium, said probe assembly adapted for real-time, in- situ detection of the concentration of at least one predetermined chemical in the medium, said probe assembly comprising a sensor, said probe assembly being interchangeable with other such probe assemblies for measurement of the concentration of other predetermined chemicals, generating, by said probe assembly, a first signal related to the concentration of said at least one predetermined chemical in the medium, providing said first signal to an electronic processor module, and generating, by said electronic processor module, a second signal indicative of the concentration of said at least one predetermined chemical in the medium.

13. A probe assembly adapted for real-time, in- situ detection of the concentration of at least one predetermined chemical in a medium, comprising: a sealed probe housing, a sensor mounted within said housing, at least a portion of said sensor being exposed to the medium, and a circuit disposed within said sealed probe housing for converting an optical signal related to the concentration of said at least one predetermined chemical in the medium to a second signal related to the concentration of said at least one predetermined chemical in the medium. 14. The probe assembly of claim 13 further including means for providing said second signal to an electronic processor module.

15. An optics assembly for use within a probe assembly to facilitate real-time, in-situ detection of the concentration of at least one predetermined chemical in a medium, comprising: a light source for generating light at substantially one predetermined frequency; a measurement detector for detecting light generated by said light source; a reference detector for forming a feedback path for light generated by said light source to maintain the intensity of light generated by said light source at a substantially constant, predetermined value; a fiber op ic element having a first end for transmitting light to said measurement detector and a second end for receiving light from said light source; a first support member including a first recess for receiving said first end of said fiber optic element, and a second recess for receiving said measurement detector and holding said measurement detector adjacent to said first end of said fiber optic element; and a second support member including a third recess for receiving said second end of said fiber optic element, a fourth recess for receiving said light source and holding said light source adjacent to said second end of said fiber optic element, and a fifth recess for receiving said reference detector and holding said reference detector at an angle to the length of said fiber optic element. 16. A circuit for use in a probe assembly adapted for real-time, in-situ measurement of the concentration of at least one predetermined chemical in a medium, the probe assembly being a component of a chemical concentration measurement device, the chemical concentration measurement device further comprising an electronic processor module for receiving at least one signal from the probe assembly, said circuit comprising: a controllable current source for controlling an output intensity level of a light source within the probe assembly to maintain the output at a predetermined intensity level, said circuit adapted to cooperate with the electronic processor module to automatically calibrate the probe assembly.

AMENDED CLAIMS

[received by the International Bureau on 24 February 1994 (24.02.94); original claim 9 cancelled; original claims 1, 3-8, 10,11,13 and 14 amended; new claims 17-20 added; other claims unchanged (6 pages)]

Claims

1 1. A chemical concentration measurement device,

2 comprising:

3 a probe for real-time, in-situ measurement of the

4 concentration of one or more predetermined chemicals in

5 a medium, said probe comprising

6 a probe housing having an opening therein,

7 a sensor for generating a first signal which

8 varies with the concentration of the one or more

9 predetermined chemicals, said sensor disposed within

10 said probe housing such that the opening exposes at

11 least a portion of said sensor to the medium, and

12 a circuit disposed within said probe housing

13 and coupled to said sensor for converting said first

14 signal to a digital signal and for generating a probe

15 identification signal indicative of the one or more

16 predetermined chemicals which the probe is adapted to

17 measure;

18 microprocessor-based electronics for receiving said

19 probe identification signal and said digital signal and

20 for using said digital signal to determine the

21 concentration of the one or more predetermined

22 chemicals; and

23 means for providing said digital signal and said

24 probe identification signal to said microprocessor-

25 based electronics.

1 2. The device of claim 1 wherein said first signal

2 comprises an optical signal.

1 3. The device of claim 1 wherein said circuit

2 includes a voltage-to-frequency converter which converts said first signal to said digital signal, and wherein said digital signal comprises a set of pulses having a frequency proportional to said first signal.

4. The device of claim 1 wherein said probe further comprises a transmitter for modulating and transmitting said digital signal, and wherein said microprocessor-based electronics comprises a receiver for receiving and demodulating said modulated digital signal.

5. The device of claim 1 wherein said providing means comprises an electrical cable which includes a connector at one end for removably coupling said probe to said microprocessor-based electronics.

6. The device of claim 1 further comprising a plurality of said probes, each said probe for real- time, in-situ measurement of the concentration of different predetermined chemicals in the medium.

7. The device of claim 6 wherein said providing means comprises means for removably coupling any one of said plurality of probes to said microprocessor-based electronics, and wherein said microprocessor-based electronics uses said probe identification signal to determine which of said plurality of said probes is coupled thereto.

8. The device of claim 1 wherein said probe further comprises a mounting assembly disposed within said probe housing for mounting said sensor and for electrically coupling said circuit to said sensor. 10. The device of claim 1 wherein said circuit comprises a light-emitting device and a controllable current source for controlling an output intensity level of the light-emitting device to maintain the output of said light-emitting device at a predetermined intensity level.

11. The device of claim 1 wherein said microprocessor-based electronics determines the concentration of the one or more predetermined chemicals relative to a baseline value which is adjustable.

12. A method for measuring chemical concentration, comprising: providing a probe assembly for placement in a medium, said probe assembly adapted for real-time, in- situ detection of the concentration of at least one predetermined chemical in the medium, said probe assembly comprising a sensor, said probe assembly being interchangeable with other such probe assemblies for measurement of the concentration of other predetermined chemicals, generating, by said probe assembly, a first signal related to the concentration of said at least one predetermined chemical in the medium, providing said first signal to an electronic processor module, and generating, by said electronic processor module, a second signal indicative of the concentration of said at least one predetermined chemical in the medium.

13. A probe for real-time, in-situ detection of the concentration of one or more predetermined chemicals in a medium, comprising: a probe housing having an opening therein; a sensor for generating a first signal which varies with the concentration of said one or more predetermined chemicals, said sensor disposed within said housing such that the opening exposes at least a portion of said sensor to the medium; and a circuit disposed within said probe housing and coupled to said sensor for converting said first signal to a digital signal and for generating a probe identification signal indicative of the one or more predetermined chemicals which the probe is adapted to measure.

14. The probe of claim 13 further comprising a transmitter for modulating and transmitting said digital signal and said probe identification signal to microprocessor-based electronics.

15. An optics assembly for use within a probe assembly to facilitate real-time, in-situ detection of the concentration of at least one predetermined chemical in a medium, comprising: a light source for generating light at substantially one predetermined frequency; a measurement detector for detecting light generated by said light source; a reference detector for forming a feedback path for light generated by said light source to maintain the intensity of light generated by said light source at a substantially constant, predetermined value; a fiber optic element having a first end for transmitting light to said measurement detector and a second end for receiving light from said light source; a first support member including a first recess for receiving said first end of said fiber optic element, and a second recess for receiving said measurement detector and holding said measurement detector adjacent to said first end of said fiber optic element; and a second support member including a third recess for receiving said second end of said fiber optic element, a fourth recess for receiving said light source and holding said light source adjacent to said second end of said fiber optic element, and a fifth recess for receiving said reference detector and holding said reference detector at an angle to the length of said fiber optic element.

16. A circuit for use in a probe assembly adapted for real-time, in-situ measurement of the concentration of at least one predetermined chemical in a medium, the probe assembly being a component of a chemical concentration measurement device, the chemical concentration measurement device further comprising an electronic processor module for receiving at least one signal from the probe assembly, said circuit comprising: a controllable current source for controlling an output intensity level of a light source within the probe assembly to maintain the output at a predetermined intensity level, said circuit adapted to cooperate with the electronic processor module to automatically calibrate the probe assembly.

17. The probe of claim 13 further comprising a temperature sensor disposed within said housing.

18. The probe of claim 13 wherein said circuit includes a voltage-to-frequency converter which converts said first signal to said digital signal, and wherein said digital signal comprises a set of pulses having a frequency proportional to said first signal.

19. The probe of claim 13 wherein said probe further comprises a mounting assembly disposed within said probe housing for mounting said sensor and for electrically coupling said circuit to said sensor.

20. The probe of claim 13 wherein said circuit comprises a light-emitting device and a controllable current source for controlling an output intensity level of the light-emitting device to maintain the output of said light-emitting device at a predetermined intensity level.