WO2001092867A1

WO2001092867A1 - Gas bubble detector

Info

Publication number: WO2001092867A1
Application number: PCT/GB2001/002124
Authority: WO
Inventors: Paul Robertson; David Peall
Original assignee: Ab Technology Cambridge Ltd
Priority date: 2000-05-30
Filing date: 2001-05-14
Publication date: 2001-12-06
Also published as: GB0013003D0; AU2001279936A1

Abstract

An apparatus and method for detecting the presence of gas bubbles within a liquid involving transmitting an ultrasonic signal through a liquid filled conduit and detecting fluctuations in the amplitude of the received signal or a deviation in the phase shift of the received signal relative to the transmitted signal.

Description

GAS BUBBLE DETECTOR

This invention relates to apparatus for detecting gas bubbles in a liquid, for example for detecting the presence of gas bubbles within the liquid refrigerant of a refrigeration system or air conditioner.

A vapour compression refrigeration system comprises four main parts: two heat exchangers, a compressor and an expansion valve. The two heat exchangers are normally referred to as the evaporator and the condenser. The system is charged with a refrigerant which circulates around the system transferring heat from the cold evaporator to the warm condenser.

Figure 1 shows the evaporator 1, condenser 3, compressor 2 and expansion valve 4 of a compression refrigeration system. These are also shown schematically in Figure 2 for clarity.

The cycle begins with the evaporation of liquid refrigerant into a low pressure vapour in evaporator 1. This low pressure vapour is drawn away and pressurised by compressor 2. The resulting high pressure vapour then passes into condenser 3. Since the temperature of the vapour is higher than ambient, heat is transferred via the condenser 3 to ambient and the refrigerant condenses into a high pressure liquid. This liquid is allowed to expand through expansion valve 4 to become a low pressure liquid with a corresponding drop in temperature. This low pressure liquid undergoes further adiabatic expansion in evaporator 1 and, due to the Joule-Thomson effect, as the liquid evaporates the temperature of evaporator 1 is reduced. As such, the temperature of the refrigeration system compartment in which evaporator 1 is contained is reduced by transferring heat from the refrigeration system compartment to ambient via evaporator 1 and condenser 3. A recognised problem of compression refrigeration systems is that refrigerant may be lost by leakage. Aside from possible environmental damage, this has the effect of reducing the efficiency of the refrigeration system and hence increasing its running costs. Depletion of refrigerant allows partial expansion of the liquid refrigerant leading to the presence of gas bubbles, known as "flash gas", in conduit 5. Other fault conditions may also lead to the presence of gas bubbles.

The determination of the correct refrigerant charge level in a compression refrigeration system is conventionally performed using a sight glass. This is fitted in the conduit carrying the high pressure liquid refrigerant from the condenser to the expansion valve. A sight glass typically comprises a metal casing with a clear glass aperture allowing a user to see the liquid refrigerant as it flows through the conduit. A deficiency in the refrigerant charge causes bubbles to appear in the sight glass. A sight glass 60 is shown in Figure 1.

This technique for determining a deficiency of the refrigerant charge carries with it several disadvantages. Amongst these are that constant human assessment is necessary and this is both costly and unreliable. The unreliability stems from the fact that • monitoring of refrigerant charge levels in this fashion is prone to human error. For example, it is possible to mistake a condition in which the system is fully charged with one in which the refrigerant is so depleted that there is no liquid visible in the sight glass. Another disadvantage of this type of monitoring system is that it is necessarily invasive, requiring the conduit to be cut for insertion of the sight glass . Due to the economic and environmental consequences of leakage of refrigerant, there exists a need to be able to detect automatically the loss of a small amount of refrigerant from a refrigeration system so that remedial action can be taken expediently. Non-invasive detectors for loss of refrigerant have been disclosed in US-A-4, 138, 879 and US-A-4 , 235, 095. These describe a pair of ultrasonic transducers fixed to the circumference of a liquid filled conduit and a driver amplifier for driving one transducer in response to the electrical output of the other. An automatic gain control (AGC) circuit adjusts the gain of the driving amplifier and hence, maintains the system on the limits of oscillation. An indicating circuit is provided for detecting modulation of the driving signal. Bubbles passing through the conduit force the AGC circuit to increase the gain in order to maintain the system on the limit of oscillation and hence, these bubbles are detected as modulations of the driving signal .

A disadvantage with this technique is that it requires maintaining the circuit in a marginal condition of oscillation. This condition is easily disturbed by electrical noise or other transient phenomena and may lead to false readings.

In accordance with one aspect of the present invention there is provided apparatus for detecting gas bubbles within a liquid contained in a conduit, the apparatus comprising an acoustic transmitter, an acoustic receiver, a signal generator, an amplitude monitor and a phase comparator, the signal generator producing a first electrical signal for driving the acoustic transmitter such that it transmits an acoustic signal through the liquid to the acoustic receiver, the acoustic receiver producing a second electrical signal corresponding to the received acoustic signal, the amplitude monitor being configured to monitor fluctuations in the amplitude of the second electrical signal and the phase comparator being configured to monitor the phase shift between the first and second electrical signals, wherein the presence of a gas bubble is determined by at least one of : a. a fluctuation in amplitude of the second electrical signal ; or b. a fluctuation in phase shift between the first and second electrical signals. In accordance with a second aspect of the present invention, there is provided a method for detecting gas bubbles within a liquid contained in a conduit, the method comprising generating a first electrical signal, converting the electrical signal into a corresponding acoustic signal directed through the liquid, converting the acoustic signal into a corresponding second electrical signal after it has passed through the liquid, monitoring the amplitude of the second electrical signal, monitoring the phase shift between the first and second electrical signals and determining the presence of a gas bubble by at least one of: a. detecting a fluctuation in the amplitude of the second electrical signal; or b. detecting a fluctuation in the phase shift between the first and second electric signal . The frequency of operation of the acoustic transmitter and receiver depends, in part, on the smallest size of bubble which is required to be detected. In order to achieve a measurable phase shift from a small acoustic impedance change (i.e. a small bubble) a short wavelength is required. Further, small transmitters have high frequency resonance modes so, for efficient acoustic signal generation, a compact transmitter must operate at an ultrasonic frequency. However, at too high a frequency, transmitter and receiver efficiency decreases and the electronics become more complex and costly.

Hence, it is preferable for the detection of smaller bubbles that the acoustic transmitter and receiver operate at ultrasonic frequencies.

Clearly, any electroacoustic transducer may be used as the acoustic transmitter and receiver but it is envisaged that these will normally comprise piezoelectric elements. The amplitude monitor may be any device which can respond to a change in amplitude such as an RMS voltmeter but typically it will comprise a peak detector to improve the response to rapidly varying signal changes. A preferred embodiment incorporates the phase comparator and signal generator in a phase-locked loop so that the phase-locked loop can maintain the phase shift between the first and second electrical signals at a nominal predefined value by adjusting the output from the signal generator. The presence of a gas bubble will cause a transient deviation of the phase shift between the first and second electrical signals from the nominal predefined value before the phase-locked loop can restore the phase shift to the nominal predefined value. This transient deviation in phase shift can be detected and used to indicate the presence of a gas bubble.

It is desirable for the apparatus to comprise a control system, such as a microprocessor, to process the outputs of the amplitude monitor and phase comparator in accordance with a predetermined algorithm in order to determine the presence of a gas bubble .

The apparatus may comprise a selection device for placing the control system in different modes, at least some of which are self-learning modes where the algorithm measures the outputs of the amplitude monitor and the phase comparator with varying quantities of gas bubbles present in the liquid and sets thresholds against which to compare these outputs when in an operational mode. Typically, the control system will process digitised representations of the outputs of the amplitude monitor and phase comparator and in one embodiment, these digitised representations are obtained by pulse width modulation of the outputs of the amplitude monitor and phase comparator. It is advantageous for the apparatus to be able to communicate with a remote computer and for this reason, the apparatus may comprise a communication system to enable communication with a remote computer using a predefined communication protocol. The communication system may incorporate a network adapter connected to a local area network via which the apparatus may communicate with the remote computer. Alternatively, the apparatus may communication with the remote computer via the Internet and in this instance, the communication system may connect to the Internet via any of the usual means including a modem, a leased line or a direct connection. The mechanism for connecting to the Internet, such as a modem, may be incorporated into the communication system.

Typically, the control system will be configured to issue an alarm upon detection of a gas bubble and this alarm may be either audible or visible or both. Alternatively, or additionally, the alarm may be an alarm message sent to a remote computer via the communication system. In this case, the remote computer may record information relating to the alarms issued. For example, the time and/or date that an alarm was issued may be recorded.

In accordance with a third aspect of the present invention, there is provided an apparatus for detecting gas bubbles within a liquid contained in a conduit, the apparatus comprising a sensor for sensing gas bubbles within the liquid and a processor for processing the output from the sensor and issuing an alarm upon detection of a gas bubble, wherein the processor is configured to inhibit the issuing of an alarm upon detection of gas bubbles produced during certain predetermined transient conditions.

Typically, the issuing of an alarm will be inhibited for a predefined period after detection of gas bubbles produced during certain predetermined transient conditions.

In accordance with a fourth aspect of the present invention, there is provided a method for detecting gas bubbles within liquid contained in a conduit, the method comprising sensing gas bubbles within the liquid and issuing an alarm upon detection of a gas bubble, wherein the issuing of the alarm is inhibited upon detection of gas bubbles produced during certain predetermined transient conditions . 2124

The invention is useful in a variety of applications, particularly for the detection of gas bubbles within the liquid refrigerant of refrigeration systems and air conditioners . An example of a gas bubble detector according to the present invention will now be described, with reference to the accompanying drawings, in which:

Figure 1 shows a vapour compression refrigeration system; Figure 2 is a schematic representation of a vapour compression refrigeration system;

Figure 3 shows the general arrangement of a gas bubble detector in use;

Figure 4 is a block diagram of a gas bubble detector's signal processor;

Figure 5 is a block diagram of a signal processing algorithm;

Figure 6 shows a clamp for removably mounting the transducers to the conduit; and, Figure 7 is a block diagram of another embodiment of a gas bubble detector's signal processor.

In order to determine the presence of bubbles in conduit 5, acoustic energy may be transmitted from a transmitter arranged on the circumference of conduit 5 to a corresponding receiver arranged diametrically opposite.

Since the speed of sound in the material used to form conduit 5 is likely to be much higher than that of the refrigerant fluid contained therein, the portion of the transmitted energy transferred from the transmitter to the receiver via conduit 5 itself will arrive at the receiver well in advance of the energy transmitted through the refrigerant liquid. It is estimated that around 70% of the energy is transmitted via the walls of conduit 5 and the remaining 30% is transmitted via the refrigerant fluid. However, the energy transmitted via conduit 5 does not need to be filtered since the component transmitted through the fluid is both detectable and varies with the presence of gas bubbles. When both components, which will be the result of multi-path reflection, combine at the receiver, a fluctuation in phase shift and/or amplitude of the ultrasound can be detected. If a bubble passes between the transmitter and receiver, then the amplitude of the received signal and its phase relative to the transmitted signal are modified by the presence of this bubble.

An example of such a detector is shown in Figure 3 where an ultrasound transmitter 7 and an ultrasound receiver 8 are fixed on the circumference of conduit 5 such that they are diametrically opposite. Typically, the ultrasound transmitter 7 and ultrasound receiver 8 comprise piezoelectric elements . The ultrasound transmitter 7 and ultrasound receiver 8 may be permanently attached to the conduit 5 using a suitable adhesive. However, in a preferred embodiment they are removably mounted as shown in Figure 6. Ultrasound transmitter 7 and ultrasound receiver 8 are shown held against conduit 5 by a clamp.

The clamp comprises a U-shaped metal bar 50 with threaded ends 52a and 52b, two nuts 51a and 51b with corresponding threads and two blocks 53 and 54. Block 53 is shaped to fit into the curved portion of U-shaped metal bar 50 and to provide a bearing surface for forcing ultrasound transmitter 7 against conduit 5. Block 54 provides a similar bearing surface for forcing ultrasound receiver 8 against conduit 5 in a position diametrically opposite to ultrasound transmitter 7. Two holes are provided in block 54 so that it can slide over the threaded ends 52a and 52b of U-shaped metal bar 50. Nuts 51a and 51b can then be tightened to provide the clamping force required to hold the transducers in contact with conduit 5. This clamping arrangement is particularly advantageous as it allows the ultrasound transmitter 7 and ultrasound receiver 8 to be appropriately fixed to a range of different diameters of conduit . Ultrasound transmitter 7 is excited by an electrical signal generated by a signal processor 6. This electrical signal is converted to an acoustic signal by ultrasound transmitter 7 and the acoustic signal is propagated through the liquid in a direction orthogonal to the longitudinal axis of conduit 5. A corresponding signal is received by ultrasound receiver 8 which is monitored by signal processor 6. On detection of a gas bubble within conduit 5, signal processor 6 will issue an alarm. In a simple case, this may be an audible or visible alarm such as, for example, activation of a buzzer or a LED or both.

However, as an alternative or in addition, the alarm may be issued as an alarm message sent to a remote computer 9 via a communication channel 10. Communication channel 10 may utilise any of a wide variety of technologies including: a serial communications link such as RS232 or RS485, a local area network such as Ethernet, a high speed serial communications link such as USB or Firewire (IEEE- 1394) , the Internet or alternatively, it may be a wireless connectivity technology such as Bluetooth.

Several such detectors may be connected to the same remote computer 9. Figure 3 shows further signal processors 11 and 13 connected to remote computer 9 via communication channels 12 and 14 respectively. This allows for the installation of the detectors to be extremely versatile. For example, a large supermarket may have several refrigeration units, each with its own gas bubble detector. It may be that the supermarket requires to use only one computer to monitor alarms issued by all the relevant signal processors and this could be achieved by connecting all signal processors to the remote computer using a local area network.

A large chain of supermarkets could expand this further by connecting each local area network of signal processors via a router to a wide area network or the

Internet and hence to a central remote computer for monitoring alarms from a nationwide group of stores. P T/GB01/02124

10

However, a small store may only have one or two refrigeration units and these can be connected to an on- site remote computer using an appropriate technology such as Bluetooth, RS232 or USB. Alternatively, a third party organisation may provide the detectors and arrange for the monitoring of alarms to be performed by an off-site remote computer using any or a combination of the above technologies.

The transmission of an alarm message to a remote computer also enables the signal processor 6 to call for a service technician to be sent to examine the refrigeration unit if appropriate.

A further method of deployment of the detectors would be for ultrasound transducers to be installed at a variety of locations and the detection of gas bubbles to be performed periodically by a mobile technician with just one signal processor.

A block diagram of one embodiment of signal processor 6 is shown in Figure 4. Signals received by ultrasound receiver 8 are amplified by an input amplifier 20 before being passed to one input of a phase comparator 21a and to the input of a peak detector 23. Phase comparator 21a is incorporated in a phase-locked loop 21 along with a low- pass filter 21b and a voltage controlled oscillator 21c. Phase comparator 21a adjusts the output of voltage controlled oscillator 21c via low-pass filter 21b such that its two inputs maintain a constant average phase shift. The other input (not shown) to phase comparator 21a is directly connected to the output of voltage controlled oscillator 21c and hence, the phase shift of the input to phase comparator 21a is maintained at a constant value with respect to the output of voltage controlled oscillator 21c which is used to drive ultrasound transmitter 7 via an output driver 22. The output impedance of output driver 22 is partially matched to the impedance of ultrasound transducer 7 to improve the transfer of power to the ultrasound transducer 7. P T/GB01/02124

11

If a gas bubble disturbs the transmission of ultrasound energy from ultrasound transmitter 7 to ultrasound receiver 8 there will be a corresponding fluctuation in amplitude and relative phase of the signal presented to phase comparator 21a and peak detector 23.

Phase-locked loop 21 will adjust the output of voltage controlled oscillator 21c so that the relative phase of the signal input to phase comparator 21a is restored to its nominal value. However, in the meantime there will be a fluctuation in the output signal produced by phase comparator 21a. The output signal is presented as an input to a pulse width modulator 24b and the output of peak detector 23 is presented to the input of a pulse width modulator 24a. Pulse width modulators 24a and 24b compare their inputs against a continuous triangle wave generated by a triangle wave generator 25 and as such produce digital pulse width modulated representations of the phase shift between the input signal to phase comparator 21a and the output of voltage controlled oscillator 21c and the amplitude of the received signal. These pulse width modulated representations are subsequently passed to a control system 26.

Control system 26 performs further processing on the signals in accordance with a predefined algorithm such that the detection of gas bubbles can be achieved reliably.

A block diagram depicting a circuit for performing a suitable algorithm is shown in Figure 5. This algorithm may be performed using either a hardware or, more conveniently, a software implementation. As such, Figure 5 also represents a flow diagram for a software implementation of the algorithm.

The pulse width modulated representations of the phase shift between the input signal to phase comparator 21a and the output of voltage controlled oscillator 21c and the amplitude of the received signal are input to comparators 40 and 41 respectively. These compare the signals against 12 two independent thresholds which may take any value from 0 to 127. The outputs of these comparators are combined in a logical OR operation 46 such that in the event of either threshold being exceeded, the quantity held by an event counter 42 is incremented by one.

Event counter 42 is held in a reset state on initialisation by a start timer 44 for a predetermined period between 0 and 59 seconds so that no events are counted when power is first applied to the signal processor. The output of a gating timer 45 is logically ORed 47 with the output of start timer 44. Gating Timer 45 periodically resets event counter 42 with a predetermined period of 1 to 59 seconds. When an event is counted by event counter 42 this is signified by activation of an event monitor 29, which may be, for example, an LED or a buzzer.

Finally, the value held by event counter 42 is compared with a predetermined value of between 0 and 65,535 by an event threshold comparator 43. If the predetermined value is exceeded before event counter 42 is reset by gating timer 45 then an alarm is issued. An alarm is indicated by activation of an alarm monitor 30, which may be, for example, an LED or a buzzer.

An alarm message may be sent to a remote computer via communication system 28 which may incorporate any of the technologies previously mentioned such as RS232, USB, the Internet, a local area network or Bluetooth.

A real time clock 27 is provided so that the exact times, dates and quantities of alarms may be recorded by control system 26 or by the remote computer. In this way, the remote computer can gather data on the quantities, times and dates of alarms issued by all signal processors connected to it and subsequent analysis may be performed on this data if desired. The real time clock can also be used to adjust the thresholds of the above mentioned algorithm in accordance with changes in environmental or usage conditions as may occur, for example, between night and day, between different seasons or between weekdays and weekends.

Further, a remote computer may use communication system 28 to adjust the values held by comparators 40 and 41, start timer 44, gating timer 45 and event threshold comparator 43.

In another embodiment, the algorithm features a self- learning mode. Figure 7 shows a block diagram similar to that shown in Figure 4 with the addition of a selection device 60 which can be operated by a user. Parts corresponding to those shown in Figure 4 have the same reference numerals.

The selection device 60 is used to place the control system 26 in difference modes. For example, if the selection device 60 is a switch having three positions, the first may be used to place the control system 26 in a first learning mode where the algorithm can measure the pulse width modulated representations of the phase shift between the input signal to phase comparator 21a and the output of voltage controlled oscillator 21c and the amplitude of the received signal when the refrigerant is depleted (by- removing a suitable quantity of refrigerant) . The second position may be used to place the control system 26 in a second learning mode where the algorithm can measure the pulse width modulated representations of the phase shift between the input signal to phase comparator 21a and the output of the voltage controlled oscillator 21c and the amplitude of the received signal when the system is fully charged with refrigerant. In the third position, selection device 60 places control system 26 in its normal operational mode and in this mode the algorithm can compare the pulse width modulated representations of the phase shift between the input signal to phase comparator 21a and the output of the voltage controlled oscillator 21c and the amplitude of the received signal with suitable thresholds determined when control system 26 was in the first and second learning modes . Clearly, the switch may have more positions to place control system 26 in different learning modes. For example, an extra position of selection device 60 may be used to teach the algorithm when 5% of the refrigerant is depleted and another position when 10% is depleted.

Selection device 60 may be a potentiometer. This can be used in place of a multi-position switch where different positions of the switch are represented by different degrees of rotation of the potentiometer. Alternatively, this may be used to set a simple analogue threshold. For example, a proportion of the refrigerant may be removed from the refrigeration system and the thresholds against which the pulse width modulated representations of the phase shift between the input signal to phase comparator 21a and the output of voltage controlled oscillator 21c and the amplitude of the received signal are compared may be adjusted by the user using the potentiometer such that an alarm is issued. The system can then be recharged with refrigerant and if the refrigerant falls below the preset level an alarm will be issued.

The remote computer may also reprogram control system 26 so that improvements to the signal processing algorithm can be remotely downloaded after installation of the detector. In addition, a power supply 31 for the signal processor is shown and this provides for supply of power either from the mains or from a battery. Two outputs are provided to control system 26. The first is a pulse width modulated representation of the battery voltage so that the state of charge of the battery can be monitored. The other output provides an indication of whether the power supply 31 is receiving power from the mains. As such, the control system 26 can enter a low power mode of operation when mains power is not being received in order to conserve the battery's charge.

It is known that production of gas bubbles during starting of the compressor of a compression refrigeration system is normal and hence, a further embodiment of this invention will feature a control system 26 adapted such that bubbles produced during this and other transient events are ignored and issuing of alarms is inhibited upon detection of such bubbles.

The control system 26 can monitor when the compressor is running using the output from power supply 31 which indicates whether the power supply 31 is receiving power from the mains. If the compressor and power supply 31 are both provided with the same mains supply, then this output will also indicate whether the compressor is running.

In this way, the control system 26 can determine when the compressor has started and inhibit the issuing of alarms for a predetermined period of time. This predetermined period of time may be adjusted by a remote computer via communication system 28.

However, if the compressor is powered from a three phase supply then the above technique cannot be used. In this instance, power supply 31 will be provided with an independent mains supply and a signal will be provided to the control system 26 to indicate that the compressor is running. This signal may be produced by a pair of relay contacts which close or open when the compressor is running. Alternatively, a transformer could be used where one winding is connected to the three phase power supply of the compressor and another winding provides a signal to the control system 26.

Claims

1. Apparatus for detecting gas bubbles within a liquid contained in a conduit, the apparatus comprising an acoustic transmitter, an acoustic receiver, a signal generator, an amplitude monitor and a phase comparator, the signal generator producing a first electrical signal for driving the acoustic transmitter such that it transmits an acoustic signal through the liquid to the acoustic receiver, the acoustic receiver producing a second electrical signal corresponding to the received acoustic signal, the amplitude monitor being configured to monitor fluctuations in the amplitude of the second electrical signal and the phase comparator being configured to monitor the phase shift between the first and second electrical signals, wherein the presence of a gas bubble is determined by at least one of: a. a fluctuation in amplitude of the second electrical signal; or b. a fluctuation in phase shift between the first and second electrical signals.

2. Apparatus according to claim 1, wherein the acoustic transmitter and receiver operate at ultrasonic frequencies.

3. Apparatus according to either of the preceding claims, wherein the acoustic transmitter and acoustic receiver comprise piezoelectric elements.

4. Apparatus according to any of the preceding claims, wherein the amplitude monitor comprises a peak detector.

5. Apparatus according to any of the preceding claims, wherein the apparatus further comprises a phase-locked loop incorporating the phase comparator and signal generator, the phase-locked loop maintaining the phase shift between the first and second electrical signals at a nominal predefined value, the presence of a gas bubble causing a transient deviation of the phase shift between the first and second electrical signals from the nominal predefined value before the phase-locked loop can restore the phase shift to the nominal predefined value.

6. Apparatus according to any of the preceding claims, wherein the apparatus further comprises a control system for processing the outputs of the amplitude monitor and the phase comparator according to a predetermined algorithm in order to determine the presence of a gas bubble.

7. Apparatus according to claim 6, wherein the apparatus further comprises a selection device for placing the control system in different modes, at least some of which are self-learning modes where the algorithm measures the outputs of the amplitude monitor and the phase comparator with varying quantities of gas bubbles present in the liquid and sets thresholds against which to compare these outputs when in an operational mode.

8. Apparatus according to claim 6 or claim 7, wherein the apparatus further comprises two pulse width modulators for generating pulse width modulated representations of the outputs of the amplitude monitor and phase comparator, the control system processing the pulse width modulated representations of the outputs of the amplitude monitor and phase comparator according to the predetermined algorithm.

9. Apparatus according to any of the preceding claims, wherein the apparatus further comprises a communication system to enable communication with a remote computer using a predefined communication protocol .

10. Apparatus according to claim 9, wherein the communication system incorporates a network adapter and the apparatus communicates with the remote computer using a local area network to which the network adapter is connected.

11. Apparatus according to claim 9, wherein the apparatus communicates with the remote computer via the Internet.

12. Apparatus according to claim 11, wherein the communication system incorporates a modem.

13. Apparatus according to any of the preceding claims, wherein the control system is configured to issue an alarm upon detection of a gas bubble.

14. Apparatus according to claim 13 , wherein the alarm is an audible alarm.

15. Apparatus according to claim 13 or claim 14, wherein the alarm is a visible alarm.

16. Apparatus according to any of claims 13 to 15 when dependent on any of claims 9 to 12, wherein the alarm is an alarm message sent to the remote computer by the communication system.

17. A refrigeration system having a cooling system, the cooling system having a conduit to which apparatus according to any of claims 1 to 16 is fitted.

18. Apparatus for detecting gas bubbles within a liquid contained in a conduit, the apparatus comprising a sensor for sensing gas bubbles within the liquid and a processor for processing the output from the sensor and issuing an alarm upon detection of a gas bubble, wherein the processor is configured to inhibit the issuing of an alarm upon detection of gas bubbles produced during certain predetermined transient conditions.

19. A refrigeration system having a cooling system, the cooling system having a compressor and a conduit to which apparatus according to claim 18 is fitted, wherein the processor inhibits issuing of an alarm upon detection of gas bubbles produced by the starting of the compressor.

20. A refrigeration system according to claim 19, wherein the processor inhibits issuing of an alarm for a predefined period after detection of gas bubbles produced by the starting of the compressor.

21. A refrigeration system according to claim 17 or either of claims 19 or 20, wherein the cooling system contains a liquid refrigerant.

22. A method for detecting gas bubbles within a liquid contained in a conduit, the method comprising generating a first electrical signal, converting the electrical signal into a corresponding acoustic signal directed through the liquid, converting the acoustic signal into a corresponding second electrical signal after it has passed through the liquid, monitoring the amplitude of the second electrical signal, monitoring the phase shift between the first and second electrical signals and determining the presence of a gas bubble by at least one of : a. detecting a fluctuation in the amplitude of the second electrical signal; or b. detecting a fluctuation in the phase shift between the first and second electrical signals.

23. A method according to claim 22, wherein the acoustic signal has a frequency in the ultrasonic spectrum.

24. A method according to claims 22 or claim 23, wherein representations of the fluctuation in the amplitude of the second electrical signal and the phase shift between the first and second electrical signals are processed according to a predetermined algorithm in order to determine the presence of a gas bubble.

25. A method according to claim 24, wherein the representations of the fluctuation in the amplitude of the second electrical signal and the phase shift between the first and second electrical signals are pulse width modulated representations.

26. A method according to any of claims 22 to 25, wherein an alarm is issued upon detection of a gas bubble.

27. A method according to claim 26, wherein the alarm is an audible alarm.

28. A method according to claim 26 or claim 27, wherein the alarm is a visible alarm.

29. A method according to any of claims 26 to 28, wherein the alarm is an alarm message sent to a remote computer.

30. A method according to claim 29, wherein the remote computer records information relating to the alarms issued.

31. A method according to claim 30, wherein the information recorded is the time and/or date that an alarm was issued.

32. A method according to any of claims 22 to 31, wherein the acoustic signal propagates through the liquid in a direction orthogonal to the longitudinal axis of the conduit .

33. A method for detecting gas bubbles within a liquid contained in a conduit, the method comprising sensing gas bubbles within the liquid and issuing an alarm upon detection of a gas bubble, wherein the issuing of the alarm is inhibited upon detection of gas bubbles produced during certain predetermined transient conditions.

34. A method according to any of claims 22 to 33, wherein the , conduit forms part of the cooling system of a refrigeration system.

35. A method according to claim 34, wherein the cooling system contains a liquid refrigerant.