GB2203247A - Gas analyser - Google Patents

Abstract

A gas analyser for measuring the concentration of one gas in another gas (eg oxygen in nitrogen) comprises first and second axially aligned cylindrical cavities or tubes 40, 48 coupled together by an orifice 49 in a separating plate 41. The gas to be analysed flows through both cavities. A loudspeaker diaphragm 43 forming the end of one of the cavities (40) remote from the separating plate sets up resonant vibration in the form of a standing wave, which is sensed by a microphone 401 at the end of the other cavity 48 remote from the separating plate. The respective lengths of the cavities are chosen such that a particular mode of vibration common to both cavities is preferentially supported, the others being suppressed, the frequency of the resonant vibration being a measure of the concentration of the gas in the mixture. An embodiment with a single cavity is also described, Fig. 1 (not shown). <IMAGE>

Description

The present invention relates to a gas analyser for the analysis of gases and mixtures of gases and in partiazlar to the analysis of the conoentration of a particular gas in a mixture of gases, such as for example the concentration of oxygen in a mixture of oxygen and nitrogen.

The velocity of sound through a gas depends upon the type of gas being considered and its temperature but is almost independent of pressure. Thus at 0 , the velocity of sound in oxygen is approximately 316 mS-1 and the velocity of sound in nitrogen is approximately 334 mS-1.

A method of assessing the velocity of sound in any particular gas might be to introduce the gas into a closed cavity and arrange for vibration of the gas within the cavity to be excited. If the excitation is arranged such that resonant vibration occurs, then the frequency of vibration will be proprotional to the velocity of sound of the entrained gas and represents a measurement that may be used to characterise the gas. If the gas is a pure gas and the experimental conditions can be exactly repeated, then a measurement of resonant vibration frequency may be used to identify which gas is present in the cavity.

If a mixture of gases is introduced into the cavity, the frequency of resonant vibration will again be proportional to the velocity of sound in the mixture. The relationship between velocity of sound and gas mixture composition is not a single one, but it can be-predicted by a theoretical model of the vibratory system. Unfortunately, onoe mixtures of more than two gases are considered a measurement of frequency of resonant vibration can no longer be used to identify the gas sinoe several mixture compositions might yield the same resonant frequency.All that can be said is that the measured frequency either is or is not consistent with what theory might predict. If however the mixture is restricted to two known gases, mixed in unknown proportion, a measurement of frequency of resonant vibrations may again characterise the mixture. Accordingly it has been suggested that such an arrangement could be employed as a gas analyser.

In any practical cavity, there will be a plurality of different vibration regimes that might be excited. In the case of a cylindrical cavity, for example, there will be a longitudinal mode wherein vibration occurs lengthwise back and forth within the tube, a radial mode between site faces of the cavity wall and a third circumferential ncde around the inside of the cavity wall. Ihese latter two modes are referred to in the art as cross modes. Each of these vibration modes exhibits a different frequency of resonance, and moreover each has its overtones.The statements above made concerning characterizing a gas or mixture hold good if and only if the same overture of the same mode is excited.

Unfortunately the resonances of the various cavity present overlapping characteristics. This problem prevents the use of a resonant cavity as a gas analyser since, given a particular frequency of resonance, it is not possible to determine whether it derives from a particular gas vibrating in a particular mode, or an alternative gas vibrating in a different mode. An additional problem is that any resonance maintaining system must be capable of sustaining resonance at any frequency within a range determined by the type of gases or mixtures to be analysed. Generally, for a useful select ion of gases or mixture composition the range of frequencies will be sufficiently broad that more than one possible resonance can be excited.This will result in either superimposed vibrations in more than one mode or frequency hopping between modes. Either way resonant frequency is ambiguous, and gas analysis cannot be achieved.

According to the present invention a gas analyser includes a cavity, adapted for the introduction of a gas or gas mixture for analysis, excitation means for exciting resonant vibration of a gas introduced into the cavity, pick-up means for producing a signal representative of gas vibration frequency, frequency dependent feedback means fran the pick-up means to the excitation arranged such that resonance occurs within a restricted range of frequencies, and frequency selective means in the feed path established by the feedback arranged such that a vibration occurs in a predetermined mode;; the the frequency of the signal characteri8es the gas or gas mixture within the cavity.

Preferably, the restricted range of frequencies is determined by the possible gases or gas mixture compositions for analysis, and is in span less than the fundamental resonance of the cavity.

Again preferably the frequency selective means is a second cavity of different acwEtical resonance to the first and acoustically coupled thereto, the relative resonance characteristics of the two cavities being chosen such that there is a single mode of resonance in the coupled cavities within the restricted range, this mode being the preselected mode.

Alternatively the frequency selective means may comprise an electrical filter operative upon the pidk-up signal, the filter being arranged to track the resonant frequency of the chosen mode.

A preferred arrangement of the present invention includes two or more tube cavities arranged in line coaxially and acoustically coupled one to another. Advantageously the excitation means is arranged to one end of the coupled tubes and the pick-up means to the other.

In order that features and advantages of the present invention may be further appreciated, an embodiment will be described be reference to the acosspanying diagrammatic drawings, of which: Figure 1 represents gas analysis apparatus, Figures 2 and 3 represent cavity resonanses, Figures 4 and 5 represent a first embodiment of a gas analysis sensor, Figure 6 represents further cavity resonanoes, Figure 7 represents a standing wave pattern within coupled cavities, Figures 8 and 9 represent a second embodiment of a gas analysis sensor.

In gas analysis aaaratus (Figure 1), a cavity is formed by a closed cylinder 10. Gas may be introduced into the cylinder (by means not Xn) through a hole 11 and exhausted via a second bole 12. One closure end 14 of the cylinder 10 is arranged as a diaphragm which may be displaced to excite vibration of gas within the cylinder 10.The displacement is produced by feeding an input signal to an electro-magnetic actuator 15, in the form of a coil. Displacement of the diaphragm 14 will produce a wave front within the gas entrained by the cylinder which in turn will cause displacement of posing diaphragm closure 16 which My be picked up by a coil 17 to produce an electrical analogue of the vibration of the gas. The diaphragm 14 and 16 are mounted on the cylinder through the agency of collars 18 and 19 respectively formed of naterial of high acoustical impedance, such as nylon.The purpose of the collars is to ensure that no displacement is transferred fran one diaphragm to the other via the cylinder wall.

An excitation signal is fed to actuator 15 from a sweep oscillator 100, which functions to provide a varying frequency sine wave across a range of frequencies.

Wave fronts excited by diaphragm 14 will process longitudinally within the tube 10, striking diaphragm 16 with consequent reflections. At some frequencies, these reflections will be in sympathy with the wavefronts created by diaphragm 14, and resonances will occur. Impinging wavefronts will cause displacement of the closure diaphragm 16. These will be picked up by pick-up 17, which will in turn produce an input signal on an line 101 representative of vibration of the gas within the tube 10.

For an excitation signal from sweep oscillator 100 rising fram a lower frequency F1 to a higher frequency, the response of pick-up 17 is represented as a curve 20 (Figure 2) plotted a an amplitude of the signal on line 101 against the instantaneous frequency of the oscillator 100. As the frequency is increased from F1, resonance oocurs at a frequency f due to a standing wave being set up within the tube, this being referred to a fundamental resonance or vibration in mode 1. This mode has its overtones, giving rise to resonanoes at frequencies of 2f, 3f, 4f and 5f, being modes 2, 2, 3, 4 and 5 respectively.It will be understood that these modes are longitudinal modes, that is a lengthwise standing compression wave is sustained at each resonant frequency The precise value of the frequency of fundamental vibration of depends upon the length of the tube 10 and the nature of the entrained gas. Given constant tube dimensions, the frequency f will be characteristic of the gas. For example, considering a gas mixture of two gases the frequency f might vary between a value fL for a nixture of predominantly a first gas, and a value FH for a nixture of predominantly a second gas.It will be appreciated that the entire resonant pattern varies between upper and lower limits dependent upon the mixture composition, resonance 2f, for example, varying between 2fL and 2fH.

In addition to the fundamental resonance and its overtones, curve 20 exhibits additional peaks, 21, 22 and 23. These peaks are due to resonances in cross mode vibration. These latter resonances occur by virtue of excitation of radial and circumferential vibration nudes of the gas entrained within the cylinder 10. Since these modes are dependent upon the radius and circumference of the cylinder, for a tube 10 of relatively long length compared with radius, they occur at frequencies comparable with the higher order modes of vibration, such as mode 3 upwards.

For a restricted gas mixture variation, then to characterise the composition of the mixture, it would be necEssary only to determine a frequency of resonance within the range FL to FH. Unfortunately for any usefully practical gas mixture determination the range of FL to FH will be such that other resonant modes My occur within the range, such as resonance at 2fL (mode 2, lowest value) which has a vibration frequency less than fH (mode 1, highest value).

Determination of a full resonance characteristic, that is oeer a substantial part of the band F1 to F2 sufficient for the fundamental frequency to be unambiguously determined, would characterise any mixture composition within the range, but at considerable cost in terms of complexity, possibly approaching the degree of signal processing required in radar.The present invention by contrast, provides a sensor wherein a single frequency measurement may be performed within a range of frequencies substantially of the same width as that determined by the gas mixture variations to be measured, with the inherent advantage that excitation capability outside this range is not Oscillator 100 is arranged as a voltage controlled oscillator providing an output on line 102 in response to a control signal on line 103.With a selection switch 104 set initially to its upper position so that signals on line 105 control the oscillator, actuator 15 provides a signal of changing frequency by virtue of the cutout of a ramp generator 106, which is arranged such that the control signal produces an oscillator frequency starting at fL and rising to Displacement of diaphragm 14 by actuator 15, initially at frequency ft will result in vibration of gas entrained within the tube 10 and be transmitted there through to ditragrn 16 to be picked up by pick-up 17. Signal 101 from pick up 16 is fed to a tracking filter 107, which is a narrow band filter, the centre frequency of which is controlled by the ouput of ramp generator 106 such that its pass band is centred on the frequency of the oscillator 100, initially fL.

As the frequency of excitation is increased, the frequency of fundamental resonance will be reached, at which point the amplitude of the output signal of the pick-up 17 will rise markedly. Since the tracking filter 107 is arranged to have its pass band centred upon the excitation frequency, its output, will also rise. This output is fed via line 108 to one input of a comparator 909, arranged to trigger when the signal representative of the amplitude of vibration exceeds a threshold value VT applied to a second input of the comparator 109. Threshold value VT is chosen such that it is exceeded by the amplitude of the pick-up signal when resonance occurs.When comparator 109 triggers, a signal is produced on line 110, indicative that excitation has reached fundamental resonance frequency. This signal causes a sample and hold circuit 111 to store the control voltage at which resonance occurred and switch 104 to index to its lower position such that the voltage held by circuit 111 is fed to excitation oscillator 100, thereby causing the gas within the tube 10 to be excited at its resonant frequency. An output signal 112 is provided from pickup 17, whidh will be at the resonant frequency.By arranging that this resonance is always the lowest resonant frequency within the range fL to the the frequency of output signal 112 will be at the fundamental resonance frequency f and therefore unambiguously indicative of the composition of the gas mixture entrained by the tube 10.

In use, tube U is arranged such that a gas for analysis is continuously introduced via port 11 and exhausted via port 12. Thus the gas entrained by the tube 10 will be subject to variation as the composition of gas mixture changes.

cme frequency of output signal 112 will be representative of the initial composition of the gas nixture with the tube 10, by the mechanism described above. However, as the composition changes the output of the pick-up 17 will decrease in amplitude until the output of the tracking filter falls below the threshold level VT, whereupon switch 104 indexes back to its lower position. Ramp generator 106 now again produces a control voltage such that frequency of the output of excitation increases from a frequency fL until resonance ocs, and comparator 109 triggers as described above. Since the time taken for resonance to be reached is short compared with the likely rate of change of the composition of the entrained mixture, output 112 is indicative of the present composition of the entrained mixture.Furthermore since the arrangement always acts to acquire and maintain resonance in the fundamental mode, output 112 is characteristic of the mixture composition.

In general in resonant systems it is desirable to operate in a vibration mode with high Q, which yields advantages in terns s of efficiency, signal to noise ratio, etc. A high Q is also important for accuracy. Resonance will be maintained with a fixed phase difference between the excitation and pidk-up signals. Since the Q determines the change of phase as a function of frequency a high Q will give a steeper phase characteristic and hence a better defined resonant frequency. It has been found that Q also increases with pressure.

For the gas entraining tube it has been found that higher vibration modes generally exhibit higher Q than lower vibration modes, for example in terms of Q, mode 4 exhibits a worthwhile increase over mode 1.

For mode 4 vibration (Figure 3), the resonance may lie between 4fL and 4H' for an expected gas mixture. It will be noted that for this resonant frequency, four resonant peaks, that is mode 4 (4f), and cross mode peaks 31, 32 and 33, all fall within the range of possible resonance 4fL to 4fH. Additionally since both 3fH (the highest possible frequency of mode 3 vibration ) and 5fL (the lowest possible frequency of diode 5 vibration) also fall within the range there are potentially six modes of resonance which may be excited within the range.In particular it will be noted that the acquisition/tracking tecdiigpe of the embodiment of the present invention described with reference to Figure 1 cannot be successfully employed, since any of modes 3, cross mode 3 or mode 4 might be acquired, and henoe input 112 not not cnaracteris the composition of the mixture.

An alternative embodiment of gas analyser in accordance with the present invention (Figure 4) includes a cavity formed by a tube 40 which has inlets, such as inlet 41, and outlets, such as outlet 42, for the introduction and exhaust of a gas to be measured. One end of the tube cavity 40 is closed by a diaphragm 43, being the diaphragm of a loud speaker additionally comprising a magnet 44 and a voice coil 45 wound on a former 46. It will be appreciated that a signal may be supplied to the voice coil 46 (by means not shown) to excite vibration of the gas within the tube 40.

The tube and opposite the diaphragm 43 is closed by a plate 47. A second tube 48, coaxial with tube 40, is acostically coupled there to, by virtue of an orifice 49 in plate 41. The tube 48 is provided with inlets and outlets for the introduction and exhaust of gas. Ihe and of tube 48 opposite the original plate 49 is closed by a further plate 400 which is adapted to receive a microphone 401, which acts to pick-up any vibration that may occur of the gas within the acoustically coupled tubes 40, 48.

Tube 40 and 48 are not of the same length, but are selected such that the ratio of their lengths is 4:3 respectively.

The excitation means ( the loudspeaker having diaphragm 43, and associated signal generation means, not shown) and the pick-up means (the microphone 401 and associated signal receiving means, not shown) is arranged to be operative over a range of frequencies defined by the possible frequency of mode 4 vibration within the tube 40, that is between 4fL and 4fH (Figure 3).

The assembly as described thus far is contained within and supported by a manifold 402, which is arranged to provide a path for the flow of gas to be measured into and out of the tubes 40 and 48 via a path 403 by virtue of inlet port 404 and outlet port 405. The manifold 402 is of sustantially rectangular cross section (Figure 5) and may, for example be arranged to aoote flaw in a pipe joined to parts 404 and 405.

The operation of the present embodiment of the invention will now be described.

Considering tube 40 in isolaticn, if it were arranged as a closed cavity and excitation applied between a lower frequency FL and an upper frequency FH, (in a way analogous to the embodiment of Figure 1, for example) resonance would occur at a fundamental frequency and its overtones, as represented by a plot 60 (Figure 6), the equivalence to plot 20 of Figure 2 being readily apparent.

For the sake of clarity in Figure 6, cross mode - resonanoes have been omitted. A similar plot 61 might be obtained for the tube 48, again considered in isolation, and is of identical form, but displaced in frequency by virtue of the shorter length of the tube 48 compared with the length of the tube 40.

Since tubes 40 and 48 are acoustically coupled by orif iced plate 47, a resultant plot 63 may be constructed, representing the resonance that would be abed if the combination were excited over the range FL to FH. Plot 60, 61 and 62 are referred to common axes for ease of comparison, and it will be served that generally the rsnarres of tube 40 (plot 60) are damped by tube 48, and vice versa.The exotions are at a frequency fr (and its overtones) where vibration mode 4 of tube 40 (frequency 4fa) coincides with vibration mode 3 of tube 48 (frequency 3fb) resulting in high Q resonance. Thus it will be appreciated that by providing second tube 48 in the electro-acoustic path from signals in the first tube 40 to the excitation means thereof, frequency selection such that a predetermined mode of vibration occurs (mode 4) has been achieved.

Plots 60, 61 and 62 are attained with entrained gas mixtures of identical composition. As the composition of the mixture is varied, so the frequencies of resonance will change. For example at one extreme of coMposition the resonance of coupled tubes 40 and 48 might be 4fL, whereas at the other extreme resonance might occur at 4fH, It will be observed from plot 60, that since only cne mode of resonance (tube 40 in mode 4 and tube 48 in mode 3) occurs within the range to to 4fH whatever the mixture composition, then a measurement of resonant frequency within this range fully characterises the misture.

Resonant vibration with the coupled tubes results in a standing compression wave pattern. This may be diagrammatically represented by an envelope 70 Figure 7) indicted standing wave nodes and anti-nodes for mode 4 (tube 71) and mode 3 (shorter tube 72).

For a sensor with a longer tube dimension of 40 mn and shorter tube dimension of 30 mm, resonance occurs at 16.55 KHz for a gas mixture having at make up of 20 % oxygen and 80 % nitrogen by pressure. These figures were obtained with an orificed coupling plate having a central orifice of 3.5 mm diameter.

In many applications it is desirable that the dimersions of the sensor are as small as possible. Unfortunately, the Q of a coupled system generally increased with tube length, and so a limiting size is reached belaw which attenuations mean that a practical sensor cannot be realised. Improvements may be obtained by increasing the frequency of the resonance considered, that is sustaining higher vibration modes and by increasing the coupling between tubes, sudi M by employing a plurality of coupling orifices, which may be for example circumferentially disposed.

Unfortunately, both of these potential solutions present the problem of increasing the present of cross mode resonance, which is more likely to occur at higher frequencies and less likely to be daeed as coupling is increased. It has been found that cross mode resonance is less likely, howew if one tube is of reduced diameter compared with the other since the cross modes then occur at different frequencies. To this end a sensor of reduced dimensions (Figure 8) includes a first tube 80 and a second tube 81. The length ratio of the tubes is 4:3 respectively, tube 81 being of reduced diameter coppared with tube 80.Vibration is excited by means of a loud speaker 82 in response to an input signal on line 83 and is picRed up by a microphone 84 producing a signal on line 84. Tubes 80 and 81 are accustically coupled via an orificed plate 85, having circumferentially disposed orifices 87, 88.

The respective lengths of the tubes 80, 81 are chosen such that mode 4 in tube 80 and mode 3 in tube 81 (and overtones thereof) is the only sustainable mode of resonance in the coupled pair; cross modes being attenuated by virtue of the diameter inequality.

The signal on line 84, representative of vibration of gas within the coupled pair, is fed to the non inverting input of a maintaining amplifier 87, via a filter 88. The output of maintaining amplifier 87 provides an input signal on line 83 to loudspeaker 82. Ihis positive feedback loop acts to sustain resonance of the gas within the coupled tubes.

Filter 88 is arranged to have a pass band spanning the range of resonant frequencies in the selected icde (i.e. from 4frL to 4frH (Figure 6) so that resonance in the selected mode is assured, thus the frequency of the signal at the output of maintaining amplifier 87 is characteristic of the present composition of the gas mixture within the sensor.

in order to provide an - accurate to about 2%, it is necessary to measure the frequency of the output signal to about 0.5hz. As stated above, the gas vibration and benoe output frequency is sensitive to temperature, and typically varies by about 0.15#Hz-1. Where temperature variation is possible, compensation nust be made. Th the present embodiment temperature is measured to sufficient accuracy by a diode suitably arranged to be within gas flow 800.

Temperature sensing diode 89 provides an input to a temperature correction circuit 801, which gives a temperature dependent frequency offset such that eventual output 802 is in frequency representative of gas mixture oomposition. As an alternative to temperature measurement, compensation could be by virtue of a signal from a second resonating coupled tube, containing a knawn constant amount of a knawn gas, compensation being achieved for example by a frequency difference reasuzemert.

Coupled tubes 80 and 81 are arranged within and supported by a substantially cylindrical manifold 803, which includes baffles 90 (Figure 9) arranged to limit direct gas flow from inlet port 804 to outlet port 805.

Such a sensor may be arranged to sample a gas mixture, for example by allowing a sample to be loaded to the tube cavities, which are then closed. An alternative sampling sensor might be of the type of the embodiment described and and wherein the baffles 90 permit some bypass flow and may include pressure sensitive valves to divert as much flow as possible through the sensor whilst ensuring that a desired back pressure is not exceeded. Where there is flow through the sensor, it is desirable that it is substantially normal to the direction of longitudinal vibration.Although the dbppler effect of a lengthwise flow does not affect the measurement since waves travel in both directions such a flow may displace the standing wave pattern, resulting in an undesirable amplitude reduction at the point of pick-up.

Generally the notion of the gas and the turbulence of gas entry will be sufficient to initiate vibration, which may then be maintained in resonance as described above.

Given a tube cavity, it will be apparent that a plurality of different second tube might be inserted in the electrn/acoustic feedback path to plate a signal resonance, and that the choice of tube length deperds on the selection of a suitable pdetermned mode For two tube sensors mode 4 (as described) and mode 3 (with the second tube two-thirds the length of the first tube) have been found advantageazs. It will further be appreciated that for other p - - predeterrined modes, more than one tube, each with its own resonance/damping characteristic My be employed in the feedback path.

Claims (5)

1. A gas analyser including

a cavity, adSpked for the introduction of a gas or gas mixture for analysis, excitation means for exciting resonant vibration of a gas introduced into the cavity, pick-up means for producing a signal representative of gas vibration frequency, frequency dependent feedback means from the pick-up means to the excitation arranged such that resonance occurs within a restricted range of freqpencies, and frequency selective means in the feed path established by the feedback arranged such that a vibration occurs in a predetermined mode; whereby the frequency of the signal characterises the gas or gas mixture within the cavity.

2. A gas analyser as claieed in claim 1 and wherein the restricted range of frequencies is detennined by the possible gases or gas mixture compositions for analysis, and is in span less than the fundamental resonance of the cavity.

3. A gas analyser as claimed in claim 1 and claim 2 and Wherein the frequency selective means is a second cavity of difference acotstical resonance to the first and acoustically coupled thereto, the relative resonance characteristics of the two cavities being chosen sucfi that there is a single mode of resonance in the coupled cavities within the restricted range, this mode being the preselected mode.

4. A gas analyser as claimed in any preceding claim and including two or more tube cavities arranged in line coaxially and acoustically coopted one to another.

5. A gas analyser substantially as bereindescribed with reference to the accoupanying drawings.