AU8426991A

AU8426991A - Gas analyser

Info

Publication number: AU8426991A
Application number: AU84269/91A
Authority: AU
Inventors: Robert Geoffrey Tyers
Original assignee: Commonwealth Industrial Gases Ltd
Current assignee: Commonwealth Industrial Gases Ltd
Priority date: 1990-08-17
Filing date: 1991-08-16
Publication date: 1992-03-17
Anticipated expiration: 2011-08-16
Also published as: AU643066B2

Description

Title: GAS ANALYSER

Technical Field

The present invention relates to gas analysers for determining the composition of gas mixtures and, in particular, to a device for use in monitoring mixtures of two gaseous components for an undesirable level of one component. As used herein the term gaseous component is intended to embrace a single gas or a gas mixture of known composition.

The invention finds particular application in the field of medicine for use in monitoring analgesic gas mixtures for low levels of oxygen. Although the invention has application to other fields of use, it will be described herein with reference to this application, that is as a low oxygen monitor. Typically in these instances the two gases are oxygen and nitrous oxide. Background Art

Previous methods of determining oxygen concentration have relied on chemical cells. These devices have the disadvantage of finite life due to cell consumption and, furthermore, have to be constantly recalibrated against inherent drift. Another method of determining the concentration of oxygen is the so-called paramagnetic analyser. Unlike chemical cells, this is a non-consumable device. It is, however, very expensive because of the reguired precision of its mechanical components.

Disclosure of the Invention

It is an object of the present invention to overcome or at least ameliorate the disadvantages of the prior art.

Accordingly, there is provided a gas analyser for determining respective proportions of two gaseous components in a mixture, said analyser including a chamber having an inlet and an outlet for said mixture positioned to provide a flow of said mixture through said chamber, transmitter means to transmit an ultrasonic pulse, receiver means to receive said pulse, - and means to provide a measure of the proportions of said gaseous components from the transit time of a pulse transmitted by said transmitter and received by said receiver means.

Preferably, both the transmitter means and the receiver means are provided by a single ultrasonic transducer positioned at one of the ends of said chamber and the pulse received by the receiver means is an echo of the pulse transmitted by the transmitter means.

Preferably also, the chamber is cylindrical. The means to provide a measure of the composition of the mixture also preferably includes means to compare the composition with predetermined limits for producing alarm signals in the event that the composition falls outside those limits.

The invention allows the speed of sound in a mixture of two known gaseous components to be determined. From the knowledge of the velocity of sound in each of the constituent gas components can be determined the proportion of each in a mixture when the velocity of sound in the mixture is measured. When one or both component gases is itself a mixture, the composition in terms of those component gases can still be determined if the composition of each is known and constant.

It is known that the velocity of sound u, in a gas is proportional to the absolute temperature of the gas, T, and its molecular weight, M, in the following relationship: u α (T/M)⁰'⁵

The following table illustrates typical values of sound velocities at a constant temperature of 293°K: GAS VELOCITY OF SOUND (MEAN) MOLECULAR WEIGHT

AIR 344 m/s 28.8

OXYGEN 326 m/s 32.0

NITROUS OXIDE 278 m/s 44.01

At a constant temperature the velocity of sound in a gas mixture is a function only of the mean molecular weight of the mixture.

Naturally, in practice a method of temperature compensation is required to allow the system to operate accurately over a range of ambient temperatures. Brief Description of the Drawinqs

A preferred embodiment of the invention will now be described, by way of example only, with reference to the accompanying drawings in which:

Figure 1 is a block diagram of a gas analyser according to the invention; and

Figure 2 is a schematic diagram showing the gas analyser of Figure 1 incorporated in an anaesthetic machine.

Best Mode for Carrying Out the Invention

Referring now to Figure 1, the gas analyser 1 includes an elongate cylindrical chamber 2. An ultrasonic transducer 5 is located at one of the ends and the chamber includes an inlet 6 and an outlet 7 for gas mixtures. The inlet and outlet are located at opposite ends of the chamber to ensure that the composition of gas in the entire chamber reflects the composition of the gas flowing into the inlet. The gas analyser is therefore able to monitor the composition of time varying gas mixtures. In addition a temperature sensor 4 provides a measure of temperature of the gas in the chamber.

An oscillator 8 of conventional design generates a 49 kHz signal which is gated by a 1.5 millisecond pulse 9 initiated from a microprocessor 10. The resulting 49 kHz burst is amplified 11 and fed to the transducer. A pulse of sound 12 travels along the chamber through the gas mixture and echoes from the far end, returning to the transducer. The corresponding electrical signal is gated 13 to distinguish it from the original pulse and detected by a receiver 14 which feeds the signal back to the microprocessor.

The microprocessor registers the delay between the original pulse and the echo and therefore, given the dimensions of the chamber, can calculate the speed of sound in the gas mixture and therefore its mean molecular weight. Any compensation for the effects of changes in temperature may be applied at this time by virtue of the measurement from the temperature sensor 4.

The dimensions of the chamber are constrained by the duration of the ultrasonic pulse and the speed of sound. A chamber 300 mm long and 40 mm in diameter allows the leading edge of the transmitted pulse to return to the transducer in approximately 2 ms. This permits the echo pulse to be distinguished from the original pulse. The percentage of either or both constituents may therefore be calculated and displayed, for example, on a digital readout 15. Alternatively, the transit time may be compared with predetermined values and a visual 16 or audible 17 alarm initiated when the transit time falls outside those values. Where an audible alarm is provided a method of silencing the alarm is desirable. Accordingly, a mute switch 18 is provided. Further features of the preferred embodiment, not shown in Figure 1, include a power supply with rechargeable battery and automatic battery management circuitry. In accordance with relevant standards, the gas analyser provides an indication that the battery is adequately charged for standby use.

Industrial Applicability Figure 2 illustrates the application of the gas analyser as a low oxygen monitor in an anaesthetic machine. In such a machine, regulated oxygen 19, nitrous oxide 20 and air 21 are variously mixed for supply to the fresh gas outlet 22. The machine allows combination of either air or nitrous oxide with oxygen and further allows the vapour of an anaesthetic agent to be added to the gas mixture by means of a vapouriser 23. The pressure of the oxygen supply is used to control the supply of the nitrous oxide through valve 24. If no oxygen is available, no nitrous oxide flows. The machine can be switched off by interrupting the oxygen supply at 26 in which case interruption of the air supply can be effected by valve 25.

The anaesthetic machine is augmented by addition of the gas analyser of the invention 27. The cylindrical chamber 28 is connected in line with the gas mixture, before the vapouriser, and the transducer is wired to the gas analyser. Pressure switches 29, 30 and 31 monitor oxygen, nitrous oxide and air pressure respectively and a solenoid/whistle 32 is connected to the oxygen supply so that the gas analyser can initiate an audible signal in the event of error conditions. A safety valve 33 prevents overpressure in the gas mixture and it should be noted that, as an additional safety feature, the gas analyser does not control the operation of the anaesthetic machine and can in fact be added to existing machines with a minimum of wiring and plumbing.

Although the invention has been described with reference to specific embodiments, it will be appreciated by those skilled in the art that the invention may be embodied in many other forms.

Claims

THE CLAIMS DEFINING THE INVENTION ARE AS FOLLOWS:-

1. A gas analyser for determining respective proportions of two gaseous components in a mixture, characterised by the inclusion of a chamber having an inlet and an outlet for said mixture positioned to provide a flow of said mixture through said chamber, transmitter means to transmit an ultrasonic pulse, receiver means to receive said pulse, and means to provide a measure of the respective proportions of said gaseous components from the transit time of a pulse transmitted by said transmitter means and received by said receiver means.

2. A gas analyser as claimed in Claim 1, wherein the chamber is elongate and both transmitter means and receiver means are provided by a single ultrasonic transducer positioned at one of the ends of said chamber, said pulse received by said receiver means being an echo of the pulse transmitted by the transmitter means.

3. A gas analyser as claimed in Claim 1 or Claim 2, wherein the chamber is cylindrical.

4. A gas analyser as claimed in any one of the preceding claims, further comprising means to compare the determined proportions with predetermined limits for producing alarm signals in the event that the proportions fall outside those limits.

5. A gas analyser as claimed in any one of the preceding claims, wherein the longitudinal dimension of the chamber is such that the transit time of the pulse is greater than the duration of the pulse.

6. A gas analyser as claimed in any one of the preceding claims, wherein the duration of the pulse is 1.5 ms.