DETERMINING THE PARTIAL PRESSURE OF A GAS, CALIBRATING A PRESSURE SENSOR

Abstract

There is disclosed a method and system for determining the partial pressure of at least one gas in a mixture of gasses contained in a pressure vessel, the mixture being pressurised to a level which is above local atmospheric pressure. The method comprises the steps of positioning a gas analysis sensor (14) within a pressure vessel (10); exposing the sensor to the mixture of gasses at the pressure level found in the pressure vessel; operating the sensor to measure the actual partial pressure of the at least one gas in the mixture contained in the vessel; and periodically calibrating the sensor by directing a calibrating gas mixture (20, 22) to the sensor in the chamber, the calibrating gas mixture being breathable by a human being.

Claims

1. A method of determining the partial pressure of at least one gas in a mixture of gasses contained in a pressure vessel, the mixture being pressurised to a level which is above local atmospheric pressure, in which the method comprises the steps of: positioning a gas analysis sensor within the pressure vessel; exposing the sensor to the mixture of gasses at the pressure level found in the pressure vessel; operating the sensor to measure the actual partial pressure of the at least one gas in the mixture contained in the vessel; and periodically calibrating the sensor by directing a calibrating gas mixture to the sensor in the chamber, the calibrating gas mixture being breathable by a human being.

2. A method as claimed in claim 1, in which calibration gas mixture comprises a known proportion of a calibration gas.

3. A method as claimed in claim 2, in which the step of calibrating the gas mixture comprises: monitoring a partial pressure value of the calibration gas outputted by the sensor; comparing the partial pressure value of the calibration gas outputted by the sensor to the actual partial pressure value which the sensor should output for said proportion of calibration gas at the pressure in the pressure vessel at which the measurement is made; determining any deviation between said values; and calibrating the sensor to account for any such deviation.

4. A method as claimed in claim 1, in which the step of calibrating the sensor comprises: supplying a low point calibration gas mixture to the sensor comprising a known first proportion of a calibration gas; monitoring a partial pressure value of the calibration gas outputted by the sensor; comparing the partial pressure value of the calibration gas outputted by the sensor to the actual partial pressure value which the sensor should output for said proportion of calibration gas at the pressure in the pressure vessel at which the measurement is made; determining any deviation between said values; and calibrating the sensor to account for any such deviation.

5. A method as claimed in claim 4, in which the step of calibrating the sensor comprises: supplying a high point calibration gas mixture to the sensor comprising a known second proportion of a calibration gas, the second proportion being higher than the first proportion; monitoring the partial pressure value of the calibration gas outputted by the sensor; comparing the partial pressure value of the calibration gas outputted by the sensor to the actual partial pressure value which the sensor should output for said proportion of calibration gas at the pressure in the pressure vessel at which the measurement is made; determining any deviation between said values; and calibrating the sensor to account for any such deviation.

6. A method as claimed in claim 2 in which the calibration gas is O.sub.2.

7. A method as claimed in claim 4, in which the calibration gas is O.sub.2, and in which the first proportion of O.sub.2 in the low point calibration gas mixture is selected so that the partial pressure of the gas in the calibration gas mixture, at the pressure in the vessel, is no less than around 0.15 bar.

8. A method as claimed in claim 7, in which the second proportion of O.sub.2 in the high point calibration gas mixture is selected so that the partial pressure of the gas in the calibration gas mixture, at the pressure in the vessel, is no more than around 0.60 bar.

9. A method as claimed in claim 5, in which the step of calibrating the sensor comprises: progressively varying the proportion of the calibrating gas in the calibrating gas mixture directed to the sensor between the first proportion and the second proportion; monitoring the sensor output during the period that the proportion of said calibrating gas is varied; comparing the partial pressure values of the calibration gas outputted by the sensor to the actual partial pressure values which the sensor should output for said proportion of calibration gas at the pressure in the pressure vessel at which the measurement is made; determining any deviation between said values; and calibrating the sensor to account for any such deviation.

10. A method as claimed in claim 1, in which the step of calibrating the sensor comprises directing a pre-prepared calibration gas mixture to the sensor, said pre-prepared mixture comprising a determined proportion of a calibrating gas.

11. A method as claimed in claim 5, in which two separate sources of pre-prepared mixtures of low point and high point calibrating gas mixtures are directed to the sensor.

12. A method as claimed in claim 11, in which the proportion of calibrating gas in the calibrating gas mixture is varied by controlling the flow of low and high point calibrating gas mixtures to the sensor.

13. A method as claimed in claim 1, in which the step of positioning the sensor within the vessel comprises locating the sensor within a container defining a sensor chamber, and arranging the sensor chamber so that it communicates with the mixture of gasses in the pressure vessel.

14. A method as claimed in claim 13, comprising flooding the sensor chamber with the calibration gas mixture to expel vessel mixture from the sensor chamber.

15. A method as claimed in claim 14, comprising venting the calibrating gas mixture into the pressure vessel.

16. A method as claimed in claim 13, comprising directing the calibrating gas mixture into the vessel along a conduit having an outlet which communicates with the sensor chamber, and arranging the outlet so that it directs the mixture towards an inlet of the sensor.

17. A method as claimed in claim 16, comprising arranging the outlet so that it directs a jet of calibrating gas mixture towards the sensor inlet.

18. A method of calibrating a gas analysis sensor used to measure the partial pressure of at least one gas in a mixture of gasses contained in a pressure vessel, the mixture being pressurised to a level which is above local atmospheric pressure, the method comprising the steps of: positioning the sensor within the pressure vessel; coupling the sensor to a calibrating assembly comprising a source of a calibrating gas mixture which is breathable by a human being, the source being located outside the vessel; and selectively operating the calibrating assembly to supply the calibrating gas mixture from the source to the sensor in the vessel, to calibrate the sensor.

19. A pressure vessel comprising a gas analysis system for determining the partial pressure of at least one gas in a mixture of gasses contained in the pressure vessel, the mixture being pressurised to above local atmospheric pressure, in which the system comprises: a gas analysis sensor positioned within the pressure vessel and exposed to the mixture of gasses at the pressure level found in the pressure vessel, the sensor being operable to measure the actual partial pressure of the at least one gas in the mixture contained in the vessel; and a calibrating assembly for calibrating the sensor, the calibrating assembly comprising a source of a calibrating gas mixture which is breathable by a human being, the source being located outside the vessel, in which the calibrating assembly is operable to selectively supply the calibrating gas mixture to the sensor in the vessel to calibrate the sensor.

20. A pressure vessel as claimed in claim 19, in which the calibrating assembly comprises a source of a low point calibration gas mixture, said mixture comprising a known first proportion of a calibration gas.

21. A pressure vessel as claimed in claim 20, in which the calibrating assembly comprises a source of a high point calibration gas mixture, said mixture comprising a known second proportion of a calibration gas which is higher than the first proportion.

22. A pressure vessel as claimed in claim 19, in which the calibrating assembly is arranged to: monitor a partial pressure value of a calibration gas of the mixture outputted by the sensor; compare the partial pressure value of the calibration gas outputted by the sensor to the actual partial pressure value which the sensor should output for said proportion of calibration gas at the pressure in the pressure vessel at which the measurement is made; determine any deviation between said values; and calibrate the sensor to account for any such deviation.

23. A pressure vessel as claimed in claim 19, in which the calibration assembly comprises a device for comparing the partial pressure values, determining any deviation and calibrating the sensor.

24. A pressure vessel as claimed in claim 21, in which the calibrating assembly is operable to: progressively vary the proportion of the calibrating gas in the calibrating gas mixture directed to the sensor between the first proportion and the second proportion; monitor the sensor output during the period that the proportion of said calibrating gas is varied; compare the partial pressure values of the calibration gas outputted by the sensor to the actual partial pressure values which the sensor should output for said proportion of calibration gas at the pressure in the pressure vessel at which the measurement is made; determine any deviation between said values; and calibrate the sensor to account for any such deviation.

25. A pressure vessel as claimed in claim 19, in which the calibration assembly comprises a pre-prepared calibration gas mixture comprising a determined proportion of a calibrating gas.

26. A pressure vessel as claimed in claim 21, in which the calibrating assembly comprises two separate sources of pre-prepared low and high point gas mixtures.

27. A pressure vessel as claimed in claim 19, in which the calibration assembly is operable to prepare a calibrating gas mixture by mixing a calibrating gas with at least one further gas, and to progressively vary the proportion of the calibrating gas in the mixture.

28. A pressure vessel as claimed in claim 19, comprising a container defining a sensor chamber in which the sensor is located, the sensor chamber arranged so that it is in communication with the mixture of gasses in the pressure vessel.

29. A pressure vessel as claimed in claim 28, in which the calibration assembly is arranged to flood the sensor chamber with the calibration gas mixture to expel vessel mixture from the sensor chamber.

30. A pressure vessel as claimed in claim 28, in which the calibration assembly comprises a conduit for directing the calibrating gas mixture into the vessel, the conduit having an outlet which communicates with the sensor chamber, the outlet being arranged to direct the mixture towards an inlet of the sensor.

31. A pressure vessel as claimed in claim 30, in which the outlet is arranged to direct a jet of calibrating gas mixture towards the sensor inlet.

32. A gas analysis system for determining the partial pressure of at least one gas in a mixture of gasses contained in a pressure vessel, the mixture being pressurised to above local atmospheric pressure, in which the system comprises: a gas analysis sensor adapted to be positioned within the pressure vessel and exposed to the mixture of gasses at the pressure level found in the pressure vessel, the sensor being operable to measure the actual partial pressure of the at least one gas in the mixture contained in the vessel; and a calibrating assembly for calibrating the sensor, the calibrating assembly comprising a source of a calibrating gas mixture which is breathable by a human being, the source adapted to be located outside the vessel, in which the calibrating assembly is operable to selectively supply the calibrating gas mixture to the sensor in the vessel to calibrate the sensor.

Description

[0092] Embodiments of the present invention will now be described, by way of example only, with reference to the accompanying drawings, in which:

[0093] FIG. 1 is a schematic illustration of a pressure chamber with a gas analysis system of a type known in the art and which is employed to determine the partial pressure of a gas in a mixture of gases contained in the pressure chamber;

[0094] FIG. 2 is a schematic illustration of a pressure vessel, in the form of a pressure chamber, comprising a gas analysis system according to an embodiment of the present invention;

[0095] FIG. 3 is a graph illustrating output voltage readings (V.sub.o) of a sensor forming part, of the gas analysis system of FIG. 2, indicative of a measured partial pressure of a target gas, against actual partial pressure of the gas, the graph showing possible deviations and drift from the actual partial pressure indicative of a sensor calibration requirement;

[0096] FIG. 4 is a graph similar to FIG. 3 illustrating voltage output readings (V.sub.o) of the sensor against actual partial pressure of the gas, with safe and comfortable partial pressure ranges (for users of the pressure chamber) superimposed;

[0097] FIG. 5 is a schematic illustration of a pressure vessel, in the form of a gas storage cylinder coupled to a diver's breathing apparatus, and having a gas analysis system according to an embodiment of the present invention; and

[0098] FIG. 6 is a schematic illustration of a pressure vessel, in the form of a gas storage cylinder coupled to a diving bell, and having the gas analysis system of FIG. 5.

[0099] Turning firstly to FIG. 1, there is shown a schematic illustration of a pressure chamber 1 incorporating a gas analysis system 2 of a type which is known in the art. The pressure chamber 1 is a life support chamber, containing a breathable mixture of gases at a pressure which is above local atmospheric pressure. The pressure chamber 1 provides life support for a person or persons who have been exposed to elevated pressures (above local atmospheric pressure) for relatively long periods of time, of the order of hours or days. Suitable examples include divers operating underwater at depths of up to around 200 msw, or even 300 msw with specialised diving equipment. The pressure chamber 1 can also be used as a decompression chamber, during subsequent decompression of a person or persons exposed to such elevated pressures.

[0100] The prior gas analysis system 2 comprises a pressure reducing valve 3, throttle valve 4, flow meter 5, gas analysis sensor 6, flow line 7 and pressure measurement device 8. The gas analysis system 2 is coupled to the pressure chamber 1 so that a portion of the mixture of gases can be exhausted from the chamber through the flow line 7, for direction to the sensor 6 for analysis. The pressure reducing valve 3 reduces the pressure of the mixture of gases directed to the sensor 6 to local atmospheric pressure level. The throttle valve 4 serves for throttling the flow of the mixtures of gases to a flow rate suitable for the gas analysis sensor 6, the flow being metered using the flow meter 5, to verify the flow rate is within a suitable range. The sensor 6 is responsive to partial pressure, and the output of the sensor is proportional to the percentage of the target gas present at atmospheric pressure. The pressure measurement device 8 is for determining the pressure inside the chamber 1, so that partial pressure inside the chamber can be calculated. The system 2 suffers from the significant disadvantages discussed above, in terms of the accuracy of the partial pressure measurement which is taken. This has a consequent impact upon the accuracy of the partial pressure of the gas in question at elevated pressure in the chamber 1, which is determined employing the measured partial pressure, factoring in the chamber pressure, as described in detail above.

[0101] Features of methods, systems and pressure vessels according to aspects of the present invention will now be described with reference to FIGS. 2 to 6 of the accompanying drawings.

[0102] Thus turning now to FIG. 2, there is shown a schematic illustration of a pressure vessel in the form of a pressure chamber 10, having a gas analysis system indicated generally by reference numeral 12, according to an embodiment of the present invention. As with the prior chamber 1 and gas analysis system 2 of FIG. 1, the pressure chamber 10 provides a life support/decompression function, particularly for a diver. The gas analysis system 12 serves for determining the partial pressure of at least one gas in a mixture of gases contained in the pressure chamber 10, where the mixture is pressurised to above local atmospheric pressure.

[0103] The system 12 comprises a gas analysis sensor 14 which is responsive to partial pressure and which is located within the pressure chamber 10. Specifically, the sensor 14 is located within an internal void 16 defined by a wall 18 of the chamber 10. The sensor 14 is exposed to the breathable mixture of gases contained within the chamber 10, in the void 16, which are at the elevated pressure level. Since the sensor 14 is located within the chamber 10, it measures the actual partial pressure of the target gas, at the pressure of the mixture prevailing in the chamber. This overcomes difficulties associated with prior systems, such as the system 2 of FIG. 1, described above. In particular, measuring the partial pressure of the target gas at the elevated pressure level found within the chamber 10 avoids the inaccuracies which occurred in the prior system 2, due to the requirement to position the sensor 6 outside the chamber 1, and to reduce the pressure of the mixture of gases being analysed to local atmospheric pressure prior to measurement at the sensor 6.

[0104] The ability to locate the sensor 14 within the pressure chamber 10, and so to measure the actual partial pressure of the target gas, is facilitated by the gas analysis system 12. This is because the system 12 is arranged to periodically calibrate the sensor 14 by directing a calibrating gas mixture 20, 22 to the sensor 14 within the chamber 10, the calibrating gas mixture being one which is breathable by a human being. Previously and as described in relation to the prior gas analysis system 2 shown in FIG. 1, it was necessary to locate the sensor 6 outside the chamber 1 so that calibration could be effected. This was due to the use of a zero gas which was not breathable, in that it entirely comprised the inert gas that persons inside the chamber 1 were breathing (typically He). Thus by providing a calibrating gas mixture which is breathable, this allows the sensor 14 to be located within the pressure chamber 10.

[0105] It will be understood that the calibrating gas mixture is breathable in that it is capable of supporting the life of persons within the chamber 10. The calibrating gas mixture comprises O.sub.2 and is typically a mixture of O.sub.2 with a suitable inert balance gas such as helium, the mixture thus being “Heliox”, which is well known in the industry. As will be described in more detail below, the proportion of O.sub.2 in the calibrating gas mixture is carefully controlled.

[0106] Whilst reference is made herein to a method of determining the partial pressure of at least one gas in a mixture of gases contained in the pressure vessel 10, and to a corresponding system 12, the present invention also encompasses a method of calibrating a gas analysis sensor and a system for calibrating the sensor 14.

[0107] The systems and methods of the present invention will now be described in more detail.

[0108] As will be understood by person skilled in the art, gas analysis sensors such as the sensor 14 shown in FIG. 2 can provide partial pressure outputs suffering from one or both of fixed deviations (+/−) from the actual partial pressure of the target gas, and a phenomenon known as “drift” (+/−) in which a deviation from the actual partial pressure increases with pressure (resulting in a greater deviation at higher partial pressures than at lower partial pressures of the target gas).

[0109] In order to effectively ascertain both fixed deviations and drift, calibration of the sensor 14 is achieved by separately supplying a low point calibration gas mixture 20 to the sensor 14, and a high point calibration gas mixture 22 to the sensor. The low point calibration gas mixture 20 comprises a known first proportion of a calibration gas, which may be any suitable constituent of the gas mixture, but which may particularly be O.sub.2. The high point calibration gas mixture 22 comprises a known second proportion of the calibration gas, the second proportion being higher than the first proportion.

[0110] The result of this is that the partial pressure of the calibration gas in the low point gas mixture 20, at the pressure prevailing in the chamber 10, is lower than the corresponding partial pressure of that calibration gas in the high point calibration gas mixture 22, Since the first and second proportions of the calibration gas in the low and high point mixtures 20 and 22 is known and indeed carefully controlled, the actual partial pressures of the calibration gas which should be outputted by the sensor 14 are known. Any deviations or drift of the calibration gas partial pressures outputted by the sensor 14 can therefore he determined and the sensor calibrated appropriately.

[0111] For example and turning to FIG. 3, there is shown a graph which is a plot of voltage output of the sensor 14 (V.sub.o) against partial pressure (P) of the calibrating gas, which is suitably O.sub.2 as discussed above (for a static partial pressure of the mixture containing the target gas).

[0112] The voltage output V.sub.o of the sensor 14 directly corresponds to the partial pressure of the gas being measured by the sensor (which under normal circumstances is a constituent of the mixture in the chamber 10, but during calibration is a constituent of the calibration gas mixture). The partial pressure P of the target gas is the actual partial pressure that the sensor should output for a particular, static pressure of the gaseous mixture being analysed. During calibration, the proportion of the target gas in the calibration gas mixture, and so the partial pressure P of the target gas, is known. In the illustrated example, the static pressure is that which would be experienced at 100 msw and so is 10 atm (10.325 bar).

[0113] The line 24 is a plot of the voltage V, Which a correctly functioning sensor 14 should output, as the partial pressure of the target gas (and thus the proportion of gas in the calibration gas mixture) increases from 0 to an upper level P.sub.u. The line 26 is a plot of the voltage V.sub.o outputted by the sensor 14 which is indicative of a deviation ‘d’ from the actual partial pressure of the target gas (and thus from the line 24). The line 28 illustrates a situation in which the sensor 14 is experiencing a drift which increases with partial pressure of the target gas, the slope of the line 28 being different to that of the line 24. As can be seen, at the upper pressure P.sup.u, the drift has resulted in a deviation ‘D’ in the partial pressure outputted by the sensor 14, compared to the actual partial pressure of the target gas (as indicated by the line 24).

[0114] This is best explained as follows, A correctly functioning sensor 14, as indicated by the line 24, outputs a voltage V.sub.1 for a particular partial pressure P.sub.1 of the target gas (say O.sub.2). When the sensor 14 is experiencing a fixed deviation d (line 26), a voltage V.sub.2 is actually outputted by the sensor, which corresponds to a partial pressure P.sub.2 which is higher than the actual partial pressure P.sub.1. In other words, the sensor 14 is providing a false higher indication of the partial pressure of O.sub.2 than is actually the case. When the sensor 14 is experiencing a drift resulting in a deviation 1) (line 28), a voltage V.sub.3 is actually outputted by the sensor, which corresponds to a partial pressure P.sub.3 which is lower than the actual partial pressure P.sub.1. In other words, the sensor 14 is providing a false lower indication of the partial pressure than is actually the case. It will be understood that this can be critical to the safety of persons in the chamber 10, when the sensor 14 is functioning to provide partial pressure measurements of O.sub.2 (or other gasses) in the mixture in the chamber 10.

[0115] Supply of the low and high point calibration gas mixtures 20 and 22 to the sensor 14 is controlled by the gas analysis system 12. To this end, the system 12 comprises sources of low and high point calibrating gas mixtures in the form of high pressure storage cylinders 30 and 32. Valves 34 and 36 are associated with the respective cylinders 30 and 32, to select a gas output from the storage cylinders. Pressure regulators 38 and 40 are coupled to the respective valves 34 and 36 and, in conjunction with a flowmeter 44, control the flow and pressure of calibrating gas mixtures 20, 22 from the cylinders 30, 32. Typically, the pressure of the mixtures 20 and 22 will be controlled to be slightly above the pressure of the gaseous mixture in the chamber 10 (to provide a positive flow into the chamber). The calibrating gas mixture 20, 22 flows to the sensor 14 along a conduit 42, checked by the flowmeter 44.

[0116] Typically, the low point calibration gas mixture 20 will be supplied to the sensor 14 first, followed by the high point calibration gas mixture 22. As described above, suitably the calibration gas forming part of the low and high point calibration gas mixtures 20 and 22 will be O.sub.2. The proportion of O.sub.2 in the low point calibration gas mixture 20 is selected so as to provide a partial pressure of O.sub.2, at the pressure of the mixture in the chamber 10, of around 0.15 bar. This is equivalent to breathing air at an altitude of around 5,500 meters and is a safe low point for supporting human life. This is illustrated in FIG. 4, which is a graph similar to FIG. 3, where the low point level 46 is shown on the line 24. For measurements at 100 msw, the low point partial pressure O.sub.2 of 0.15 bar is equivalent to 0.015 bar at local atmospheric pressure, and so a proportion of 1.5% O.sub.2 by volume in the calibration gas mixture 20 at atmospheric pressure.

[0117] In the high point calibration gas mixture 22, a safe high point level of O.sub.2 is selected to provide a partial pressure O.sub.2 of around 0.60 bar at the pressure of the mixture in the chamber 10. A partial pressure O.sub.2 of 0.60 bar is above a target range of 0.44 to 0.48 bar, but as described above, is acceptable for in-water excursions of a few hours and so represents a safe high point level 48.

[0118] The impact that a deviation d (line 26) or a drift resulting in a deviation D (line 28) in the sensor 14 output would have is illustrated in FIG. 4. In particular, a safe partial pressure range R.sub.1, between the low and high point levels 46 and 48 is shown in the graph. This range R.sub.1 encompasses a narrower pressure range R.sub.2, which represents a more comfortable operating range of partial pressure O.sub.2 for persons in the chamber 10.

[0119] A deviation d in the partial pressure O.sub.2 outputted by the sensor 14, indicated by the line 26, will result in a partial pressure reading being outputted which is a significant magnitude higher than the actual partial pressure of O.sub.2 in the chamber 10. This is critical at the lower end of the range R.sub.1, because the sensor 14 could be indicating a partial pressure of O.sub.2 which is at or above the low point level 46, but which is actually lower and insufficient to support persons in the chamber 10.

[0120] A drift resulting in a deviation D in the partial pressure O.sub.2 measurement outputted by the sensor 14, indicated by the line 28, would in contrast result in a partial pressure reading being outputted which is a significant magnitude lower than the actual partial pressure of O.sub.2 in the chamber 10. This is critical at the upper end of the range R.sub.1, because the sensor 14 could be indicating a partial pressure of O.sub.2 which is at or below the high point level 46, but which is actually higher and toxic to persons in the chamber 10.

[0121] In the illustrated embodiment, the system 12 comprises a container 50 which defines a sensor chamber 52 in which the sensor 14 is mounted. The sensor chamber 52 can communicate with the mixture of gases in the pressure chamber 10, contained in the void 16, suitably by way of apertures 54 in a wall 56 of the container 50. During calibration of the sensor 14, the sensor chamber 52 is flooded with the selected calibration gas mixture 20, 22, thereby expelling all (or a majority) of the gaseous mixture present in the chamber 10 from the sensor chamber 52. In this way, the sensor 14 is entirely or substantially entirely exposed to the selected calibrating gas mixture 20, 22. As the sensor chamber 52 is open to the chamber 10, the selected calibrating gas mixture 20, 22 (which is breathable) simply vents in to the chamber 10 through the apertures 54.

[0122] The conduit 42 along which the calibration gas mixture 20, 22 flows has an outlet 58 which communicates with the sensor chamber 52. The outlet 58 is arranged relative to an inlet 60 of the sensor 14 so that it directs the calibration gas mixture 20, 22 directly towards the sensor inlet 60. This may assist in flooding the sensor inlet 60 with the calibrating gas mixture 20, 22 and may enable the container 50 to be dispensed with.

[0123] The gas analysis system 12 comprises a device for comparing the partial pressure values, determining any deviation and automatically calibrating the sensor, the device indicated generally by reference numeral 62 and taking the form of a suitable processor. The processor 62 includes a display 64 for displaying the partial pressure measurements of the target gas outputted by the sensor 14. Automatic calibration of the sensor 14 is achieved by suitable software carried on the processor 62, of a type which is readily available in the industry.

[0124] Turning now to FIG. 5, there is shown a schematic illustration of a pressure vessel and gas analysis system in accordance with another embodiment of the invention. The pressure vessel is designated by reference numeral 100, and takes the form of a gas storage cylinder or tank, which stores breathing gas for a diver 66. In this embodiment, the gas analysis system is indicated generally by reference numeral 112. Like components of the system 112 with the system 12 of FIG. 2 share the same reference numerals, incremented by 100.

[0125] The system 112 is essentially of like construction and operation to the system 12. The substantial difference between the embodiment of FIG. 5 and that of FIG. 2 is that, instead of monitoring the partial pressure of a target gas or gases within a pressure chamber suitable for receiving e.g. a diver 66, the system 112 monitors the partial pressure of a gas/gases in a mixture contained in the tank 100, which are supplied to breathing apparatus 68 worn by the diver 66 via a hose 70. The system 112 is used to test the breathing gas before it goes to the diver 66 in the water.

[0126] The breathing gas is stored in the tank 100, which is a high pressure cylinder, and the pressure is reduced by a pressure control device 72, in the form of a pressure regulator. This provides overpressure with respect to the hydrostatic pressure at the particular operating depth of the diver 66, matched to the requirement of the breathing apparatus 68 that the diver is using. As can be seen from the drawing, in this instance, a sensor 114 is provide in the tank 100, and the low and high point calibration gas mixtures 120 and 122 stored in respective cylinders 130 and 132. Valves 134 and 136 and regulators 138, 140, together with a flow meter 144, control the flow of the low and high point calibration gas mixtures 120, 122 to the sensor 114 to calibrate the sensor. Monitoring and calibration of the sensor 114 is performed by a processor 162, and partial pressures of target gas output on a display 164.

[0127] FIG. 6 shows a variation of the system 112 shown in FIG. 5, in which the mixture of gases in the tank 100 is supplied to a diving bell 74 (through the hose 70), which provides life support for the diver 66 during deployment underwater. For example, the bell 74 can be used for transferring the diver 66 from a pressure chamber at surface (the pressure of the breathing mixture in the bell 74 being set at the same level as that in the chamber), as well as for providing underwater life support for the diver during times between excursions out of the bell and into the water.

[0128] Typically, the hose 70 is coupled to a gas panel 76 in the bell 74, which controls the supply of breathing gas both to an interior of the bell 74, and through a hose 78 to the breathing apparatus 68 worn by the diver 66. The gas panel 76 is also coupled to emergency breathing gas cylinders (not shown) via hoses 80 and 82, for supplying breathing gas to the interior of the bell 74 and/or to the diver 66 (via the hose 78) in the case of an emergency loss of supply of gas from the tank 100 at surface. From the above, it will be appreciated that the system 112 may effectively serve for monitoring the breathing gas supplied to both. the bell 74 and to the diver's breathing apparatus 68.

[0129] Various modifications may be made to the foregoing without departing from the spirit or scope of the present invention.