Monitoring apparatus for pressure vessels

Abstract

The present invention provides a monitoring apparatus for an outlet of a vessel storing gas under pressure. The monitoring apparatus comprises a flow control valve movable to a position between a fully open position and a fully closed position to adjust a flow of gas from the outlet of the vessel, a valve position detector connected to the flow control valve to detect the position of the flow control valve, an internal pressure sensor to sense an internal pressure P.sub.int(t) of the gas in the vessel at different times, a processor, a memory and an alarm. The processor calculates an actual rate of change in pressure dP.sub.int/dt of the gas in the vessel over time, and compares dP.sub.int/dt with an expected rate of change.

Claims

1. A monitoring apparatus (1, 2) for an outlet (10a) of a vessel (10) storing oxygen gas under pressure and for supplying oxygen gas to a patient, comprising: a flow control valve (21) movable to a position between a fully open position and a fully closed position to adjust a flow of oxygen gas from the outlet (10a) of the vessel (10) to the patient; a valve position detector (22) connected to the flow control valve (21) to detect the position of the flow control valve (21); an internal pressure sensor (14) to sense an internal pressure (Pint(t)) of the oxygen gas in the vessel (10) at different times; a processor (16) connected to the internal pressure sensor (14) to receive from the internal pressure sensor (14) the pressure (Pint(t)) sensed thereby at different times and to calculate an actual rate of change in pressure (dPint/dt) of the oxygen gas in the vessel (10) over time from the pressure (Pint(t)) of the oxygen gas in the vessel (10) sensed at different times; a memory (11) to store a volume of the vessel (10) and for that volume, an expected rate of change in pressure ((dPint/dt)exp) of the oxygen gas in the vessel (10) for each of a plurality of different positions of the flow control valve (21); the processor (16) being connected to the valve position detector (22) to receive from the valve position detector (22) the position of the valve (21) detected thereby, to retrieve from the memory (11) the volume of the vessel (10) and for that volume, the expected rate of change in pressure ((dPint/dt)exp) of the oxygen gas in the vessel (10) for the position of the valve (21) detected by the valve position detector (22), and to compare the actual rate of change in pressure (dPint/dt) with the expected rate of change in pressure ((dPint/dt)exp) for the same position of the valve (21) as detected by the valve position detector (22) and the same volume of the vessel (10) as retrieved from the memory (11); and an alarm (18) connected to the processor (16) to receive from the processor (16) an alarm signal (s) to activate the alarm (18) if the actual rate of change in pressure (dPint/dt) is less than a first threshold ((dP/dt)min) defined in relation to the expected rate of change in pressure ((dPint/dt)exp) which is compared with the actual rate of change in pressure (dPint/dt) and/or is more than a second threshold ((dP/dt)max) defined in relation to the expected rate of change in pressure ((dPint/dt)exp) which is compared with the actual rate of change in pressure (dPint/dt).

2. The monitoring apparatus (1, 2) according to claim 1, wherein the expected rate of change in pressure ((dPint/dt)exp) of the oxygen gas in the vessel (10) stored in the memory (11) for each of a plurality of different positions of the flow control valve (21) and for the volume of the vessel (10) is one of a plurality of expected rates of change in pressure ((dPint/dt)exp) stored in the memory (11) for vessels of different volumes, of the oxygen gas in each respective vessel for each of a plurality of different positions of the flow control valve (21).

3. The monitoring apparatus (1, 2) according to claim 1, further comprising a first user interface (23) whereby a user may manually define at least one of the first and second thresholds ((dPint/dt)min, (dPint/dt)max).

4. The monitoring apparatus (1, 2) according to claim 1, wherein the processor (16) is able to calculate at least one of the first and second thresholds ((dPint/dt)min, (dPint/dt)max) in dependence on the expected rate of change in pressure ((dPint/dt)exp) which is compared with the actual rate of change in pressure (dPint/dt).

5. The monitoring apparatus (1, 2) according to claim 4, wherein the processor (16) calculates a range ((dPint/dt)max-(dPint/dt)min) of acceptable rates of change in pressure between the first and second thresholds ((dPint/dt)min, (dPint/dt)max) in proportion to the expected rate of change in pressure ((dPint/dt)exp) which is compared with the actual rate of change in pressure (dPint/dt).

6. The monitoring apparatus (1, 2) according to claim 1, wherein the processor (16) gives the alarm signal (s) a first characteristic if the actual rate of change in pressure (dPint/dt) is less than the first threshold (dPint/dt)min and a second characteristic different from the first characteristic if the actual rate of change in pressure (dPint/dt) is more than the second threshold (dPint/dt)max.

7. The monitoring apparatus (1, 2) according to claim 1, further comprising a second user interface (24) whereby a user may manually disable the alarm (18).

8. The monitoring apparatus (1, 2) according to claim 1, further comprising an internal temperature sensor (13) to sense a temperature (Tint(t)) of the oxygen gas in the vessel (10) at different times, the processor (16) being connected to the internal temperature sensor (13) to receive from the internal temperature sensor (13) the temperature (Tint(t)) sensed thereby at different times and to calculate at least one of a rate of change in temperature (dTint/dt) of the oxygen gas in the vessel (10) over time and the second derivative (d2Tint/dt2) with respect to time of the temperature of the oxygen gas in the vessel (10) from the temperature (Tint(t)) of the oxygen gas in the vessel (10) sensed at different times, and either to adjust a value of at least one of the first and second thresholds ((dPint/dt)min, (dPint/dt)max) or to disable the alarm (18) on the basis of the rate of change in temperature (dTint/dt) of the oxygen gas in the vessel (10) over time or of the second derivative (d2Tint/dt2) with respect to time of the temperature of the oxygen gas in the vessel (10).

9. The monitoring apparatus (1, 2) according to claim 1, further comprising an external temperature sensor (15) to measure a temperature (Text) of an external environment (20) of the vessel (10), the processor (16) being connected to the external temperature sensor (15) to receive from the external temperature sensor (15) the temperature (Text) of the environment (20) measured thereby and either to adjust a value of at least one of the first and second thresholds ((dPint/dt)min, (dPint/dt)max) or to disable the alarm (18) on the basis of the measured temperature (Text) of the environment (20) or the first derivative (dText/dt) or second derivative (d2Text/dt2) with respect to time of the measured temperature (Text) of the environment (20).

10. The monitoring apparatus (1, 2) according to claim 1, further comprising an external pressure sensor (17) to sense a pressure (Pext) of the external environment (20) of the vessel (10), the processor (16) being connected to the external pressure sensor (17) to receive from the external pressure sensor (17) the pressure (Pext) of the environment (20) sensed thereby and either to adjust a value of at least one of the first and second thresholds ((dPint/dt)min, (dPint/dt)max) or to disable the alarm (18) on the basis of the sensed pressure (Pext) of the environment (20) or the first derivative (dPext/dt) or second derivative (d2Pext/dt2) with respect to time of the sensed pressure (Pext) of the environment (20).

11. The monitoring apparatus (1, 2) according to claim 1, wherein the processor (16) is arranged to poll the internal pressure sensor (14) at a given frequency.

12. The monitoring apparatus (1, 2) according to claim 1, wherein the processor (16) is arranged to log in the memory (11) the internal pressure (Pint(t)) of the oxygen gas in the vessel (10) sensed at different times.

13. The monitoring apparatus (1, 2) according to claim 12, wherein the processor (16) is arranged to log in the memory (11) at least one of the detected position of the flow control valve (21), the temperature (Tint(t)) of the oxygen gas in the vessel (10) measured at different times, the measured temperature (Text) of the external environment (20) of the vessel (10) and the sensed pressure (Pext) of the external environment (20) of the vessel (10).

14. The monitoring apparatus (1, 2) according to claim 1, further comprising a display (19) connected to the processor (16) for visually displaying an alarm condition if the actual rate of change in pressure (dPint/dt) is less than the first threshold (dPint/dt)min and/or more than the second threshold (dPint/dt)max.

15. The monitoring apparatus (1, 2) according to claim 1, further comprising: a first user interface (23) whereby a user may manually define at least one of the first and second thresholds ((dPint/dt)min, (dPint/dt)max), a second user interface (24) whereby a user may manually disable the alarm (18), an internal temperature sensor (13) to sense a temperature (Tint(t)) of the oxygen gas in the vessel (10), an external temperature sensor (15) to measure a temperature (Text) of an external environment (20) of the vessel (10), an external pressure sensor (17) to sense a pressure (Pext) of the external environment (20) of the vessel (10), and a display (19) connected to the processor (16) for visually displaying an alarm condition if the actual rate of change in pressure (dPint/dt) is less than the first threshold (dPint/dt)min and/or more than the second threshold (dPint/dt)max, wherein the flow control valve (21), the valve position detector (22), the internal pressure sensor (14), the processor (16), the memory (11), the alarm (18), the first user interface (23), the second user interface (24), the internal temperature sensor (13), the external temperature sensor (15), the external pressure sensor (17) and the display (19) are integrated into a unit (30) mountable to the outlet (10a) of the vessel (10).

16. A vessel (10) storing oxygen gas under pressure with an outlet (10a) having a monitoring apparatus (1, 2) according to claim 1 mounted thereto.

17. A method of monitoring flow of oxygen gas from an outlet (10a) of a vessel (10) storing oxygen gas under pressure to a patient, comprising: controlling (120) the flow of oxygen gas from the outlet (10a) of the vessel (10) to the patient with a flow control valve (21) movable to a position (x) between a fully open position and a fully closed position; detecting (130) the position (x) of the flow control valve (21); sensing (140) an internal pressure (Pint(t)) of the oxygen gas in the vessel (10) at different times; calculating (150) an actual rate of change in pressure (dPint/dt) of the oxygen gas in the vessel (10) over time from the pressure of the oxygen gas (Pint(t)) in the vessel (10) sensed at different times; storing (110) a volume (V) of the vessel (10) and for that volume, an expected rate of change in pressure ((dPint/dt)exp) of the oxygen gas in the vessel (10) for each of a plurality of different positions of the flow control valve (21); comparing (160) the actual rate of change in pressure (dPint/dt) with the expected rate of change in pressure ((dPint/dt)exp) for the same position (x) of the valve (21) as detected and the same volume (V) of the vessel (10) as stored; defining (170) a first threshold (dPint/dt)min in relation to the expected rate of change in pressure ((dPint/dt)exp) which is compared with the actual rate of change in pressure (dPint/dt); and generating (180) an alarm signal (s) if the actual rate of change in pressure (dPint/dt) is less than the first threshold (dPint/dt)min.

18. The method of monitoring flow of oxygen gas according to claim 17, further comprising: defining (171) a second threshold (dPint/dt)max in relation to the expected rate of change in pressure ((dPint/dt)exp) which is compared with the actual rate of change in pressure (dPint/dt); and generating (181) the alarm signal (s) if the actual rate of change in pressure (dPint/dt) is more than the second threshold (dPint/dt)max.

19. The method of monitoring flow of oxygen gas according to claim 17, comprising manually defining at least one of the first and second thresholds ((dPint/dt)min, (dPint/dt)max).

20. The A method of monitoring flow of oxygen gas according to claim 17, comprising calculating at least one of the first and second thresholds ((dPint/dt)min, (dPint/dt)max) in dependence on the expected rate of change in pressure ((dPint/dt)exp) which is compared with the actual rate of change in pressure (dPint/dt).

21. The method of monitoring flow of oxygen gas according to claim 20, comprising calculating a range ((dPint/dt)max-(dPint/dt)min) between the first and second thresholds ((dPint/dt)min, (dPint/dt)max) in proportion to the expected rate of change in pressure ((dPint/dt)exp) which is compared with the actual rate of change in pressure (dPint/dt).

22. The method of monitoring flow of oxygen gas according to claim 17, further comprising giving the alarm signal (s) a first characteristic if the actual rate of change in pressure (dPint/dt) is less than the first threshold (dPint/dt)min and a second characteristic different from the first characteristic if the actual rate of change in pressure (dPint/dt) is more than the second threshold (dPint/dt)max.

23. The method of monitoring flow of oxygen gas according to claim 17, further comprising manually disabling the alarm signal (s).

24. The method of monitoring flow of oxygen gas according to claim 17, further comprising: measuring (190) a temperature (Tint(t)) of the oxygen gas in the vessel (10) at different times; calculating (191) at least one of a rate of change in temperature (dTint/dt) of the oxygen gas in the vessel (10) over time and the second derivative (d2Tint/dt2) with respect to time of the temperature of the oxygen gas in the vessel (10) from the temperature of the oxygen gas (Tint(t)) in the vessel (10) sensed at different times; and either adjusting a value of at least one of the first and second thresholds ((dPint/dt)min, (dPint/dt)max) or suppressing the alarm signal (s) on the basis of at least one of the rate of change in temperature (dTint/dt) of the oxygen gas in the vessel (10) over time and the second derivative (d2Tint/dt2) with respect to time of the temperature of the oxygen gas in the vessel (10).

25. The method of monitoring flow of oxygen gas according to claim 17, further comprising: measuring a temperature (Text) of an external environment (20) of the vessel (10); and either adjusting a value of at least one of the first and second thresholds ((dPint/dt)min, (dPint/dt)max) or suppressing the alarm signal (s) on the basis of the measured temperature (Text) of the environment (20) or the first derivative (dText/dt) or second derivative (d2Text/dt2) with respect to time of the measured temperature (Text) of the environment (20).

26. The method of monitoring flow of oxygen gas according to claim 17, further comprising: sensing a pressure (Pext) of the external environment (20) of the vessel (10); and either adjusting a value of at least one of the first and second thresholds ((dPint/dt)min, (dPint/dt)max) or suppressing the alarm signal (s) on the basis of the sensed pressure (Pext) of the environment (20) or the first derivative (dPext/dt) or second derivative (d2Pext/dt2) with respect to time of the sensed pressure (Pext) of the environment (20).

27. The method of monitoring flow of oxygen gas according to claim 17, wherein the step of sensing the internal pressure (Pint(t)) of the oxygen gas in the vessel (10) at different times is carried out at a given frequency.

28. The method of monitoring flow of oxygen gas according to claim 27, wherein at least one of the steps of detecting the position (x) of the flow control valve (21), measuring the temperature (Tint(t)) of the oxygen gas in the vessel (10) at different times, measuring the temperature (Text) of the external environment (20) of the vessel (10) and sensing the pressure (Pext) of the external environment (20) of the vessel (10) are carried out at the given frequency.

29. The method of monitoring flow of oxygen gas according to claim 27, wherein the given frequency is between 2 and 0.05 times per second.

30. The method of monitoring flow of oxygen gas according to claim 17, further comprising logging the internal pressure (Pint(t)) of the oxygen gas in the vessel (10) sensed at different times.

31. The method of monitoring flow of oxygen gas according to claim 30, further comprising logging at least one of the detected position (x) of the flow control valve (21), the temperature (Tint(t)) of the oxygen gas in the vessel (10) measured at different times, the measured temperature (Text) of the external environment (20) of the vessel (10) and the sensed pressure (Pext) of the external environment (20) of the vessel (10).

32. The method of monitoring flow of oxygen gas according to claim 30, wherein the step of calculating the actual rate of change in pressure (dPint/dt) of the oxygen gas in the vessel (10) over time is carried out using a moving average over a given period of time of the logged internal pressure (Pint(t)) of the oxygen gas in the vessel (10) sensed at different times.

33. The method of monitoring flow of oxygen gas according to claim 32, further comprising: measuring (190) a temperature (Tint(t)) of the oxygen gas in the vessel (10) at different times; logging the temperature (Tint(t)) of the oxygen gas in the vessel (10) measured at different times, calculating (191) at least one of a rate of change in temperature (dTint/dt) of the gas in the vessel (10) over time and the second derivative (d2Tint/dt2) with respect to time of the temperature of the gas in the vessel (10) from the temperature of the oxygen gas (Tint(t)) in the vessel (10) sensed at different times; and wherein the step of calculating at least one of a rate of change in temperature (dTint/dt) of the oxygen gas in the vessel (10) over time is carried out using a moving average over the same given period of time of the logged temperature (Tint(t)) of the oxygen gas in the vessel (10) measured at different times.

34. The method of monitoring flow of oxygen gas according to claim 32, wherein the given period of time is defined in relation to the expected rate of change in pressure ((dPint/dt)exp) which is compared with the actual rate of change in pressure (dPint/dt).

35. The method of monitoring flow of oxygen gas according to claim 34, wherein the given period of time is between 20 seconds and 10 minutes if the flow control valve (21) is detected to be in an open position and between 10 minutes and 4 hours if the flow control valve (21) is detected to be in the fully closed position.

36. The method of monitoring flow of oxygen gas according to claim 17, further comprising visually displaying an alarm condition if the actual rate of change in pressure (dPint/dt) is less than the first threshold (dPint/dt)min) and/or more than the second threshold ((dPint/dt)max).

37. The method of monitoring flow of oxygen gas according to claim 17, wherein the steps of controlling the flow of oxygen gas from the outlet (10a) of the vessel (10), detecting the position (x) of the flow control valve (21), sensing the internal pressure (Pint(t)) of the oxygen gas in the vessel (10) at different times, calculating the actual rate of change in pressure (dPint/dt) of the oxygen gas in the vessel (10) over time, storing the volume (V) of the vessel (10) and for that volume, an expected rate of change in pressure ((dPint/dt)exp), comparing the actual rate of change in pressure (dPint/dt) with the expected rate of change in pressure ((dPint/dt)exp), measuring the temperature (Tint(t)) of the oxygen gas in the vessel (10) at different times, measuring the temperature (Text) of the external environment (20) of the vessel (10), sensing the pressure (Pext) of the external environment (20) of the vessel (10), defining at least one of the first and second thresholds ((dPint/dt)min, (dPint/dt)max), generating (180) an alarm signal (s) and visually displaying the alarm condition are performed in a unit (30) mounted to the outlet (10a) of the vessel (10).

38. The monitoring apparatus (1, 2) according to claim 37, further comprising an external temperature sensor (15) to measure a temperature (Text) of an external environment (20) of the vessel (10), the processor (16) being connected to the external temperature sensor (15) to receive from the external temperature sensor (15) the temperature (Text) of the environment (20) measured thereby and either to adjust a value of at least one of the first and second thresholds ((dPint/dt)min, (dPint/dt)max) or to disable the alarm (18) on the basis of the measured temperature (Text) of the environment (20) or the first derivative (dText/dt) or second derivative (d2Text/dt2) with respect to time of the measured temperature (Text) of the environment (20).

39. The monitoring apparatus (1, 2) according to claim 38, further comprising an external pressure sensor (17) to sense a pressure (Pext) of the external environment (20) of the vessel (10), the processor (16) being connected to the external pressure sensor (17) to receive from the external pressure sensor (17) the pressure (Pext) of the environment (20) sensed thereby and either to adjust a value of at least one of the first and second thresholds ((dPint/dt)min, (dPint/dt)max) or to disable the alarm (18) on the basis of the sensed pressure (Pext) of the environment (20) or the first derivative (dPext/dt) or second derivative (d2Pext/dt2) with respect to time of the sensed pressure (Pext) of the environment (20).

40. The monitoring apparatus (1, 2) according to claim 39, wherein the processor (16) is arranged to poll the internal pressure sensor (14) at a given frequency.

41. The monitoring apparatus (1, 2) according to claim 40, wherein the processor (16) is arranged to poll at least one of the valve position detector (22), the internal temperature sensor (13), the external temperature sensor (15) and the external pressure sensor (17) at the same given frequency.

42. A monitoring apparatus for an outlet of a vessel storing oxygen gas under pressure and for supplying oxygen gas to a patient, comprising: a flow control valve movable to a position between a fully open position and a fully closed position to adjust a flow of oxygen gas from the outlet of the vessel to the patient; a valve position detector connected to the flow control valve to detect the position of the flow control valve; an internal pressure sensor to sense an internal pressure, Pint(t), of the oxygen gas in the vessel at different times; a processor connected to the internal pressure sensor to receive from the internal pressure sensor the pressure Pint(t) sensed thereby at different times and to calculate an actual rate of change in pressure, dPint/dt, of the oxygen gas in the vessel over time from the pressure Pint(t) of the oxygen gas in the vessel sensed at different times; a memory to store a volume of the vessel and for that volume, an expected rate of change in pressure, (dPint/dt)exp, of the oxygen gas in the vessel for each of a plurality of different positions of the flow control valve; the processor being connected to the valve position detector to receive from the valve position detector the position of the valve detected thereby, to retrieve from the memory the volume of the vessel and for that volume, the expected rate of change in pressure, (dPint/dt)exp, of the oxygen gas in the vessel for the position of the valve detected by the valve position detector, and to compare the actual rate of change in pressure dPint/dt with the expected rate of change in pressure dPint/dt)exp for the same position of the valve as detected by the valve position detector and the same volume of the vessel as retrieved from the memory; and an alarm connected to the processor to receive from the processor: (a) a first alarm signal to activate the alarm if the actual rate of change in pressure dPint/dt is less than a first threshold, (dP/dt)min, defined in relation to the expected rate of change in pressure (dPint/dt)exp which is compared with the actual rate of change in pressure dPint/dt, and (b) a second alarm signal to activate the alarm if the actual rate of change in pressure dPint/dt is more than a second threshold, (dP/dt)max, defined in relation to the expected rate of change in pressure (dPint/dt)exp which is compared with the actual rate of change in pressure dPint/dt.

43. The monitoring apparatus according to claim 42, wherein the first alarm signal has a first characteristic and the second alarm signal has a second characteristic different from the first characteristic.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) Further features and advantages of the present invention will become apparent from the following detailed description, which is given by way of example and in association with the accompanying drawings, in which:

(2) FIG. 1 is a schematic diagram of a first embodiment of a monitoring apparatus according to the invention shown on an outlet of a vessel storing gas under pressure;

(3) FIG. 2 is a graph showing how the pressure of a gas in a vessel storing the gas under pressure and the expected rate of change in pressure of the gas vary over time as the gas is consumed;

(4) FIG. 3 is a schematic diagram of a second embodiment of a monitoring apparatus according to the invention shown on an outlet of a vessel storing gas under pressure;

(5) FIG. 4 is a schematic diagram of an exemplary embodiment of an integrated unit containing a monitoring apparatus according to the invention mounted to the outlet of a vessel storing gas under pressure;

(6) FIG. 5 is a flow diagram of a first embodiment of a method according to the invention of monitoring flow of a gas from an outlet of a vessel storing gas under pressure; and

(7) FIG. 6 is a flow diagram of a second embodiment of a method according to the invention of monitoring flow of a gas from an outlet of a vessel storing gas under pressure.

(8) Referring firstly to FIG. 1, there is schematically shown a first embodiment 1 of a monitoring apparatus according to the invention on an outlet 10a of a vessel 10 storing gas under pressure. The monitoring apparatus 1 comprises an internal pressure sensor 14, a flow control valve 21, a valve position detector 22, a processor 16, a memory 11 and an alarm 18. The internal pressure sensor 14 senses an internal pressure P.sub.int(t) of the gas in the vessel 10 at different times. In this embodiment, the internal pressure sensor 14 senses the pressure of the gas in the vessel 10 on exit of the gas from the vessel through outlet 10a. However, in alternative embodiments, the internal pressure sensor 14 could instead be contained within the vessel 10 and sense the pressure of the gas in the vessel directly. The flow control valve 21 is movable to a position between a fully open position and a fully closed position to adjust a flow of gas from the outlet 10a of the vessel 10, and the valve position detector 22 is connected to the flow control valve 21 to detect the position of the flow control valve. Both the internal pressure sensor 14 and the valve position detector 22 are connected to the processor 16 so that the processor 16 can receive from the internal pressure sensor 14 the pressure P.sub.int(t) of the gas in the vessel 10 sensed thereby at different times and can also receive from the valve position detector 22 the position of the valve 21 detected thereby. The alarm 18 is connected to the processor 16 so that the alarm 18 can receive from the processor 16 an alarm signal s.

(9) In this embodiment, the memory 11 is an internal component of the processor 16. However, in alternative embodiments, the memory 11 could instead be connected to the processor 16 as an external component. Furthermore, the processor 16 could comprise an internal memory 11 in addition to being connected to an external memory. In any event, the memory 11 stores a volume of the vessel 10 and for that volume, an expected rate of change in pressure (dP.sub.int/dt).sub.exp of the gas in the vessel 10 for each of a plurality of different positions of the flow control valve 21. Since the flow control valve 21 is manufactured with high precision, a different expected rate of change in pressure (dP.sub.int/dt).sub.exp can be related to each different position of the flow control valve 21 for a particular volume of the vessel.

(10) In order that the monitoring apparatus 1 may be used with a variety of vessels of different volumes, the memory 11 may store a plurality of expected rates of change in pressure (dP.sub.int/dt).sub.exp for each of a plurality of different positions of the flow control valve 21, each of the plurality of expected rates of change in pressure (dP.sub.int/dt).sub.exp being for a different volume of vessel.

(11) During operation, the processor 16 polls the internal pressure sensor 14 at a given frequency of between 2 and 0.05 times per second and logs in the memory 11 the internal pressure P.sub.int(t) of the gas in the vessel 10 sensed by the internal pressure sensor 14 at different times. The processor 16 then calculates an actual rate of change in pressure dP.sub.int/dt of the gas in the vessel 10 over time from the pressure P.sub.int(t) of the gas in the vessel 10 sensed by the internal pressure sensor 14 at different times. The processor 16 calculates the actual rate of change in pressure dP.sub.int/dt of the gas in the vessel 10 using a moving average over a given period of time of the logged internal pressure P.sub.int(t) of the gas in the vessel 10 sensed at different times. In this embodiment, the given period of time is 2 minutes.

(12) The processor 16 also polls the valve position detector 22 at the same given frequency and logs the position of the valve 21 detected thereby in the memory 11. It then compares the actual rate of change in pressure dP.sub.int/dt with the expected rate of change in pressure (dP.sub.int/dt).sub.exp for the position of the valve 21 detected by the valve position detector 22 and for the volume of the vessel 10 stored in the memory 11. If the processor 16 finds that the actual rate of change in pressure dP.sub.int/dt is less than a first threshold (dP/dt).sub.min defined in relation to the expected rate of change in pressure (dP.sub.int/dt).sub.exp which is compared with the actual rate of change in pressure dP.sub.int/dt and/or is more than a second threshold (dP/dt).sub.max also defined in relation to the expected rate of change in pressure (dP.sub.int/dt).sub.exp which is compared with the actual rate of change in pressure dP.sub.int/dt, then the processor issues an alarm signal s to the alarm 18 to activate the alarm.

(13) Either or both of the first and second thresholds (dP.sub.int/dt).sub.min and (dP.sub.int/dt).sub.max may be manually defined in relation to the expected rate of change in pressure (dP.sub.int/dt).sub.exp by a user of the monitoring apparatus 1, such as a clinician. For example, the user may set the first and second thresholds (dP.sub.int/dt).sub.min and (dP.sub.int/dt).sub.max to be 25% above and below the expected rate of change in pressure (dP.sub.int/dt).sub.exp. For this purpose, the monitoring apparatus 1 may be provided with a first user interface 23, such as a touch screen, as shown in and described below in relation to FIG. 4. Alternatively or additionally, the processor 16 may be able to calculate at least one of the first and second thresholds (dP.sub.int/dt).sub.min, (dP.sub.int/dt).sub.max in dependence on the expected rate of change in pressure (dP.sub.int/dt).sub.exp which is compared to the actual rate of change in pressure dP.sub.int/dt. For example, the processor 16 could also set the first and second thresholds (dP.sub.int/dt).sub.min and (dP.sub.int/dt).sub.max to be 25% above and below the expected rate of change in pressure (dP.sub.int/dt).sub.exp. If so, the processor 16 could calculate a range (dP.sub.int/dt).sub.max(dP.sub.int/dt).sub.min of acceptable rates of change in pressure between the first and second thresholds (dP.sub.int/dt).sub.min, (dP.sub.int/dt).sub.max to be proportional to the expected rate of change in pressure (dP.sub.int/dt).sub.exp which is compared with the actual rate of change in pressure dP.sub.int/dt. Thus, if the expected rate of change in pressure (dP.sub.int/dt).sub.exp is large, the processor would set the range (dP.sub.int/dt).sub.max (dP.sub.int/dt).sub.min of acceptable rates of change in pressure to be proportionally large, whereas if the expected rate of change in pressure (dP.sub.int/dt).sub.exp is small, the processor would set the range (dP.sub.int/dt).sub.max (dP.sub.int/dt).sub.min of acceptable rates of change in pressure to be proportionally small. This is shown in FIG. 2, which is a graph showing how the internal pressure P.sub.int of the gas in the vessel 10 changes over time t as the gas is used up. On the left-hand side of the graph, as the vessel 10 starts to discharge, the expected rate of change in pressure (dP.sub.int/dt).sub.exp is quite large, so the processor can set the range (dP.sub.int/dt).sub.max(dP.sub.int/dt).sub.min of acceptable rates of change in pressure to be proportionally large. On the right-hand side of the graph, as the vessel 10 is nearly fully discharged, so that the internal pressure P.sub.int of the gas in the vessel is approaching atmospheric pressure, the processor can set the range (dP.sub.int/dt).sub.max(dP.sub.int/dt).sub.min of acceptable rates of change in pressure to be correspondingly less.

(14) The processor 16 gives the alarm signal s a first characteristic if the actual rate of change in pressure dP.sub.int/dt is less than the first threshold (dP.sub.int/dt).sub.min and a second characteristic different from the first characteristic if the actual rate of change in pressure dP.sub.int/dt is more than the second threshold (dP.sub.int/dt).sub.max. For example, the first characteristic may be a series of short beeps and the second characteristic may be a series of longer beeps. Thus the alarm signal has a different sound if the actual rate of change in pressure dP.sub.int/dt is too low from if the actual rate of change in pressure dP.sub.int/dt is too high.

(15) A user of the monitoring apparatus 1, such as a clinician, may be able to manually disable the alarm 18. For this purpose, the monitoring apparatus 1 may be provided with a second user interface 24, such as a push button, as shown in and described below in relation to FIG. 4, and/or the first user interface 23 may be provided with additional functionality to allow the user to do so.

(16) Turning next to FIG. 3, there is schematically shown a second embodiment 2 of a monitoring apparatus according to the invention on an outlet 10a of a vessel 10 storing gas under pressure. In addition to the components of the monitoring apparatus 1 shown in FIG. 1 and described above, the monitoring apparatus 2 further comprises an internal temperature sensor 13, an external temperature sensor 15, an external pressure sensor 17 and a display 19. The internal temperature sensor 13 senses a temperature T.sub.int(t) of the gas in the vessel 10 at different times. In this embodiment, the internal temperature sensor 13 senses the temperature of the gas in the vessel 10 on exit of the gas from the vessel through outlet 10a. However, in alternative embodiments, the temperature sensor 13 could instead be contained within the vessel 10 and sense the temperature of the gas in the vessel directly. The external temperature sensor 15 measures a temperature T.sub.ext of an external environment 20 of the vessel 10 and the external pressure sensor 17 senses a pressure P.sub.ext of the external environment 20. The internal temperature sensor 13, the external temperature sensor 15 and the external pressure sensor 17 are all connected to the processor 16 so that the processor 16 can receive from the internal temperature sensor 13 the temperature T.sub.int(t) of the gas in the vessel 10 sensed thereby at different times, and can also receive from the external temperature sensor 15 and the external pressure sensor 17 the temperature T.sub.ext and the pressure P.sub.ext of the external environment 20, respectively, sensed thereby. The display 19 is also connected to the processor 16 so that the display 19 can visually display an alarm condition if the actual rate of change in pressure dP.sub.int/dt is less than the first threshold (dP.sub.int/dt).sub.min and/or more than the second threshold (dP.sub.int/dt).sub.max.

(17) During operation, monitoring apparatus 2 carries out all the same functions in the same way as monitoring apparatus 1 described above. Additionally, however, the processor 16 of monitoring apparatus 2 polls at least one of the internal temperature sensor 13, the external temperature sensor 15 and the external pressure sensor 17 at a given frequency of between 2 and 0.05 times per second and correspondingly logs in the memory 11 at least one of the temperature T.sub.int(t) of the gas in the vessel 10 measured by the internal temperature sensor 13 at different times, the measured temperature T.sub.ext of the external environment 20 of the vessel 10 and the sensed pressure P.sub.ext of the external environment 20. Depending on what information the processor 16 has logged in the memory 11, the processor 16 then calculates one or more of the following quantities. A rate of change in temperature dT.sub.int/dt of the gas in the vessel 10 over time from the temperature T.sub.int(t) of the gas in the vessel 10 sensed by the internal temperature sensor 13 at different times, the second derivative d.sub.2T.sub.int/dt.sup.2 with respect to time of the temperature of the gas in the vessel 10, the first derivative dT.sub.ext/dt or second derivative d.sub.2T.sub.ext/dt.sup.2 with respect to time of the measured temperature T.sub.ext of the environment 20, and the first derivative dP.sub.ext/dt or second derivative d.sub.2P.sub.ext/dt.sup.2 with respect to time of the sensed pressure P.sub.ext of the environment 20. The processor 16 then either adjusts a value of at least one of the first and second thresholds (dP.sub.int/dt).sub.min and (dP.sub.int/dt).sub.max or disables the alarm 18 on the basis of one or more of these quantities. In this way, if the vessel encounters unusual operating conditions, for example, if the vessel is transferred from a cold to a warm environment, or is transferred from low to high altitude, or vice versa, the processor can compensate for changes in the actual rate of change in pressure of the gas in the vessel induced by the unusual operating conditions, in order to avoid a false alarm from being generated by the unusual operating conditions.

(18) If the processor 16 calculates a rate of change in temperature dT.sub.int/dt of the gas in the vessel 10 over time from the temperature T.sub.int(t) of the gas in the vessel 10 sensed by the internal temperature sensor 13 at different times, it performs this calculation using a moving average of the logged temperature T.sub.int(t) of the gas in the vessel 10 measured at different times over the same given period as the processor 16 uses to calculate the actual rate of change in pressure dP.sub.int/dt of the gas in the vessel 10. The given period of time can be defined in relation to the expected rate of change in pressure (dP.sub.int/dt).sub.exp which is compared with the actual rate of change in pressure dP.sub.int/dt. So, for example, the given period of time can be between 20 seconds and 10 minutes if the flow control valve 21 is detected to be in an open position, so that the expected rate of change in pressure (dP.sub.int/dt).sub.exp will be significantly more than if the flow control valve 21 is detected to be in the fully closed position, in which case, the given period of time can be between 10 minutes and 4 hours, since the expected rate of change in pressure (dP.sub.int/dt).sub.exp is then zero.

(19) Turning now to FIG. 4, there is schematically shown an exemplary embodiment of an integrated unit 30 containing a monitoring apparatus according to the invention mounted to the outlet 10a of a vessel 10 storing gas under pressure. The unit 30 contains the flow control valve 21, the valve position detector 22, the internal pressure sensor 14, the processor 16, the memory 11, the alarm 18, the first user interface 23, the second user interface 24, the internal temperature sensor 13, the external temperature sensor 15, the external pressure sensor 17 and the display 19, which are connected to each other and function as described above. In this exemplary embodiment, the first user interface 23 is a touch screen and the second user interface 24 is a push button. The touch screen also functions as a display 19 for visually displaying an alarm condition if the actual rate of change in pressure dP.sub.int/dt is less than the first threshold (dP.sub.int/dt).sub.min and/or more than the second threshold (dP.sub.int/dt).sub.max.

(20) FIG. 5 is a flow diagram of a first embodiment of a method of monitoring flow of a gas from an outlet of a vessel storing gas under pressure. In step 110, a volume V of the vessel and for that volume, an expected rate of change in pressure (dP.sub.int/dt).sub.exp of the gas in the vessel for each of a plurality of different positions of the flow control valve are initially stored. In step 120, the flow of gas from the outlet of the vessel is controlled with a flow control valve movable to a position x between a fully open position and a fully closed position and in step 130, the position x of the flow control valve is detected. In step 140, an internal pressure P.sub.int(t) of the gas in the vessel is sensed at different times. In step 150, an actual rate of change in pressure dP.sub.int/dt of the gas in the vessel over time is calculated from the pressure of the gas P.sub.int(t) in the vessel sensed at different times. In step 160, the actual rate of change in pressure dP.sub.int/dt of the gas in the vessel is then compared with the expected rate of change in pressure (dP.sub.int/dt).sub.exp for the same position x of the valve as was detected in step 130 and the same volume V of the vessel as was stored in step 110. In step 170, a first threshold (dP.sub.int/dt).sub.min is defined in relation to the expected rate of change in pressure (dP.sub.int/dt).sub.exp which is compared with the actual rate of change in pressure dP.sub.int/dt and in step 180, an alarm signal s if generated if the actual rate of change in pressure dP.sub.int/dt is found to be less than the first threshold (dP.sub.int/dt).sub.min.

(21) Finally, FIG. 6 is a flow diagram of a second embodiment of a method of monitoring flow of a gas from an outlet of a vessel storing gas under pressure. The method of FIG. 6 comprises steps 110 to 180 as described in relation to FIG. 5 above. Additionally, however, the method of FIG. 6 further comprises a step 171, in which a second threshold (dP.sub.int/dt).sub.max is defined in relation to the expected rate of change in pressure (dP.sub.int/dt).sub.exp which is compared with the actual rate of change in pressure dP.sub.int/dt, and a step 181, in which the alarm signal s is also generated if the actual rate of change in pressure dP.sub.int/dt is more than the second threshold (dP.sub.int/dt).sub.max. Furthermore, the method of FIG. 6 includes additional steps 190, in which a temperature T.sub.int(t) of the gas in the vessel is measured at different times, and 191, in which at least one of a rate of change in temperature dT.sub.int/dt of the gas in the vessel over time and the second derivative d.sub.2T.sub.int/dt.sup.2 with respect to time of the temperature of the gas in the vessel are calculated from the temperature of the gas T.sub.int(t) in the vessel sensed at different times in step 190. At least one of the rate of change in temperature dT.sub.int/dt of the gas in the vessel over time and the second derivative d.sub.2T.sub.int/dt.sup.2 with respect to time of the temperature of the gas in the vessel are then used to adjust the values of the first and second thresholds (dP.sub.int/dt).sub.min and dP.sub.int/dt).sub.max defined in steps 170 and 171. Steps 190 and 191 are representative of alternative possible embodiments in which the external temperature or pressure of an environment of the vessel may alternatively or additionally be used to adjust at least one of the values of the first and second thresholds (dP.sub.int/dt).sub.min and dP.sub.int/dt).sub.max defined in steps 170 and 171.

(22) Whereas various optional features of the invention have been described above in particular combinations by way of example only, such optional features may be combined in other ways without restriction to the scope of the invention, which is defined by the appended claims.