GB2262337A

GB2262337A - Apparatus for sensing a gas by pressure modulation spectroscopy

Info

Publication number: GB2262337A
Application number: GB9126093A
Authority: GB
Inventors: John Philip Dakin
Original assignee: Sensor Dynamics Ltd
Current assignee: Sensor Dynamics Ltd
Priority date: 1991-12-09
Filing date: 1991-12-09
Publication date: 1993-06-16
Anticipated expiration: 2011-12-09
Also published as: GB2262337B; GB9126093D0

Abstract

Apparatus for sensing a gas by pressure modulation spectroscopy, which apparatus comprises a reference gas cell 8, an acoustic resonant cavity (20, Fig 3, not shown) for providing varying pressure in the reference gas cell, an electrically driven acoustic transducer (22) for driving the cavity (20), and monitoring means for monitoring the state of acoustic resonance of the gas in the acoustic resonant cavity (20) relative to the state of an alternating electrical drive signal which is applied to the acoustic driving transducer (22). The apparatus is such that the information derived on the state of the actual acoustic resonant cavity, relative to the applied alternating electrical drive signal to the acoustic driving transducer, is used to control the frequency of the alternating electrical drive signal applied to the acoustic driving transducer driving the acoustic resonant cavity and hence to lock the frequency of operation of the apparatus to a point at or close to the peak resonance condition of the acoustic resonant cavity. <IMAGE>

Description

APPARATUS FOR SESTb G A GAS BY PRESSURE MODULATION SPECTROSCOPY This invention relates to apparatus for sensing a gas by pressure modulation spectroscopy.

It is known to detect gas, or measure gas concentration, by a method known as pressure modulation spectroscopy. The known method involves passing light, from a suitable source, sequentially (in either order) through a measurement region, where it is desired to detect gas and also through a reference cell region, containing a known sample of the gas, before then allowing the light to impinge on an optical detector. The gas in the reference cell is subjected to a varying pressure in order to modulate the absorption of the gas, and hence to modulate the signal on the optical detector to an extent dependent on the concentration of the gas in the measurement region.

The optical system may include a wide variety of other elements (for example lenses, prisms, mirrors or optical wave guides) in the optical path between the light source and the optical detector, in order to direct, more efficiently, the light from the light source to the optical detector via the cells, and, if desired to cause more than one passage of the light either through one or both of the cells, to increase their gaseous absorption). In addition, both the light source and the gas to be detected may Jointly be composed of a region of a hot gas, or a flame, or a gas plasma containing the gas to be detected.

In accordance with the present invention there is provided apparatus for sensing a gas by pressure modulation spectroscopy, which apparatus comprises a reference gas cell, an acoustic resonant cavity for providing varying pressure in the reference gas cell, an electrically driven acoustic driving transducer for driving the cavity, and monitoring means for monitoring the state of acoustic resonance of the gas in the acoustic resonant cavity relative to the state of an alternating electrical drive signal which is applied to the acoustic driving transducer, and the apparatus being such that the information derived on the state of the actual acoustic resonant cavity, relative to the applied alternating electrical drive signal to the acoustic driving transducer, is used to control the frequency of the alternating electrical drive signal applied to the acoustic driving transducer driving the acoustic resonant cavity and hence to lock the frequency of operation of the apparatus to a point at or close to the peak resonance condition of the acoustic resonant cavity.

Preferably, the monitoring means is a gas pressure, gas velocity or acoustic transducer. Other types of monitoring means may however be employed.

The apparatus may be one in which the monitoring means is provided within the acoustic resonant cavity, or is acoustically coupled to the gas in the acoustic resonant cavity, and in which the monitoring means provides an electrical signal in response to pressure changes in the gas within the acoustic resonant cavity, Alternatively, the apparatus may be one in which the monitoring means is a sensor monitoring a cyclic component of molecular displacements of the gas in the acoustic resonant cavity, such displacements being characteristic of acoustic conditions in the acoustic resonant cavity.

The apparatus may alternatively b one in which the monitoring means is a sensor monitoring a cyclic variation in gas velocity in the acoustic resonant cavity, the gas velocity changes being characteristic of acoustic conditions in the acoustic resonant cavity.

The apparatus may alternatively be one in which the monitoring means comprises an optical probe which monitors changes in the optical absorption of the acoustic resonant cavity in response to changes in gas pressure which absorption changes are substantially synchronous with pressure changes in the acoustic resonant cavity.

The optical probe preferably comprises a pressure modulation system in which the changes in the gas in the acoustic resonant cavity are monitored without any need for any separate light source, transducer or detector to be provided for the purpose of the gas pressure sensing.

The monitoring means may provide an electrical signal output which is electrically band-pass . filtered to improve the signal/noise ratio, before comparison of the signal with the alternating electrical drive signal to the acoustic driving transducer.

The apparatus may be one in which the monitoring means provides an electrical signal output, and in which the phase of the electrical signal output is compared to the phase of the alternating electrical drive signal which is applied to the acoustic driving transducer, in order to ascertain the condition relative to that which should be present at resonance and to provide a correction signal to feed back in order to control the frequency of the alternating electrical drive signal to the acoustic driving transducer.

The apparatus may include a phase-locked-loop oscillator circuit for providing the alternating electrical drive signal to the acoustic driving transducer, the apparatus then being one in which the signal from the monitoring means is inputted to the phase-lockedloop in order to provide a phase reference relative to the alternating electrical drive signals The apparatus of the invention may alternatively be one in which the frequency of the alternating electrical drive signal to the acoustic driving transducer is subjected to a small controlled cyclical fluctuation, and n which the polarity of the resulting cyclical amplitude changes in the signal from the monitoring means is monitored to determine the state of the acoustic oscillation of the acoustic resonant cavity relative to a desired resonant condition and to provide a correction signal to control the mean frequency of the alternating electrical drive signal to the acoustic driving transducer, whereby the acoustic resonant cavity is maintained at or close to resonance.

The acoustic driving transducer may be an electrcal-to-Fressure transducer or an electrical-todisplacement transducer, The cavity may be in a body such for example as a tube which is open at one end or which is open at both ends. The body may be regarded as a pressure modulation cell. The cavity forms a resonant acoustic cavity in order to enhance the pressure modulation of the gas. The acoustic resonant cavity is driven into resonance by driving the acoustic driving transducer at ,a suitable frequency corresponding to an acoustic resonance. However, in the absence of the control apparatus of the present invention, it has the possible disadvantage that it will not maintain the resonant condition accurately if the temperature or conditions of the gas, or the dimensions of the acoustic resonant cavity, change. A drift from resonance causes a significant reduction in the cyclic pressure variation of the gas in the acoustic resonant cavity and causes the sensitivity of the monitoring means to change.

Embodiments of the invention will now be described solely by way of example and with reference to the accompanying drawings in which: Figure 1 shows gas sensor apparatus based on optical absorption in gas; Figure 2 shows gas sensor apparatus based on optical emission from hot gas; Figure 3 shows an acoustic resonator pressure modulation cell; and Figure 4 is an illustration of pressure modulation spectroscopy.

Referring to Figure 1, there is shown apparatus 2 for sensing a gas. The apparatus 2 comprises a light source 4, a gas measurement cell 6, a gas reference cell 8, an optional broad band filter 10 and a detector 12. An arrow 14 indicates a varying pressure influence to modulate gas cell absorption lines in the gas reference cell 8.

Referring now to Figure 2, similar parts as in Figure 1 have ben given the same reference numerals for ease of comparison and understanding. In Figure 2, it will be seen that the light source 4 has been replaced by a hot gas 4. Also, the gas measurement cell 6 has been omitted. The apparatus 2 shown in Figure 2 is based on optical emission from the hot gas 4.

The apparatus 2 shown in Figures 1 and 2 uses real time correlation spectroscopy, with the gas reference cell 8 modulated by an applied pressure influence as indicated by the arrow 14.

Referring now to Figure 3, there is shown an acoustic resonator modulation cell 16 which is capable of providing the applied pressure influence indicated by arrow 14 in Figures 1 and 2. The pressure modulation cell 16 comprises a resonator body 48, a resonant cavity 20 which contains gas, and a unimorph PZT transducer 22. Also shown in Figure 3 is a cross section 24 of a light beam travelling in a path through the pressure modulation cell 16.

As can be seen from Figure 3, the acoustic resonant cavity 20 is a hole in the resonator body 18.

The resonator body 18 is a metal cylinder which is driven at one end by the acoustic pressure transducer shown as the unimorph PZT transducer 22. The light beam 24 passes transversely through the acoustic resonant cavity 20, via small optical windows (not shown) set in opposite walls of the resonator body 18.

Referring now to Figure 4, there is shown an illustration of pressure modulation spectroscopy, where the normal absorption spectrum of the gas is shown as a solid line. This normal absorption spectrum of the gas is changed when the pressure is increased, the individual spectral lines being increased and broadened, as shown by the dotted line.

It is to be appreciated that the embodiments of the invention described above with reference to the accompanying drawings have been given by way of example only and that modifications may be effected. Thus, for example, the unimorph PZT transducer 22 shown in Figure 3 may be replaced by another acoustic pressure transducer.

Claims

CLAIIG

Apparatus for sensing a gas by pressure modulation spectroscopy, which apparatus comprises a reference gas cell, an acoustic resonant cavity for providing varying pressure in the reference gas cell.

an electrically driven acoustic driving transducer for driving the cavity, and monitoring means for monitoring the state of acoustic resonance of the gas in the acoustic resonant cavity relative to the state of an alternating electrical drive signal which is applied to the acoustic driving transducer, and the apparatus being such that information derived on the state of the actual acoustic resonant cavity, relative to the applied alternating electrical drive signal to the acoustic driving transducer, is used to control the frequency of the alternating electrical drive signal applied to the acoustic driving transducer driving the acoustic resonant cavity and hence to lock the frequency of operation of the apparatus to a point at or close to the peak resonance condition of the acoustic resonant cavity.
2. Apparatus according to claim 1 in which the monitoring means is a gas pressure, gas velocity or acoustic transducer.
Apparatus according to claim 1 or claim 2 in which the monitoring means is provided within the acoustic resonant cavity, or is acoustically coupled ,to the gas in the acoustic resonant cavity, and in which the monitoring means provides an electrical signal in response to pressure changes in the gas within the acoustic resonant cavity.
4. Apparatus according to claim 1 or claim 2 in which the monitoring means is a sensor monitoring a cyclic component of molecular displacements of the gas in the acoustic resonant cavity, such displacements being characteristic of acoustic conditions in the acoustic resonant cavity.
5. Apparatus according to claim 1 or claim 2 in which the monitoring means is a sensor monitoring a cyclic variation in gas velocity in the acoustic resonant cavity, the gas velocity changes being characteristic of acoustic conditions in the acoustic resonant cavity.
6. Apparatus according to claim 1 or claim 2 in which the monitoring means comprises an optical probe which monitors changes in the optical absorption of the acoustic resonant cavity in response to changes in pressure, which absorption changes are substantially synchronous with pressure changes in the acoustic resonant cavity.
7. Apparatus according to claim 6 in which the optical probe comprises a pressure modulation system in which the changes in the gas in the acoustic resonant cavity are monitored without any need for any separate light source, transducer or detector to be provided for the purpose of the gas pressure sensing,
8. Apparatus according to any one of the preceding claims in which the monitoring means provide an electrical signal output which is electrically band-pass filtered to improve the signal/noise ratio, before comparison of the signal with the alternating electrical drive signal to the acoustic driving transducer.
9. Apparatus according to any one of the preceding claims in which the monitoring means provide an electrical signal output, and in which the phase of the electrical signal output is compared to the phase of the alternating electrical drive signal which is applied to the acoustic driving transducer, in order to ascertain the condition relative to that which should be present at resonance to provide a correction signal to feed back in order to control the frequency of the alternating electrical drive signal to the acoustic driving transducer.
10. Apparatus according to claim 9 and including a phase-locked-loop oscillator circuit for providing the alternating electrical drive signal to the acoustic driving transducer, and in which the signal from the monitoring means is inputted to the phase-locked-loop in order to provide a phase reference relative to the alternating electrical drive signal.
11. Apparatus according to any one of claims 1 to 8 in which the frequency of the alternating electrical drive signal to the acoustic driving transducer is subjected to a small controlled cyclical fluctuation, and in which the polarity of the resulting cyclical amplitude changes in the signal from the monitoring means is monitored to determine the state of the acoustic oscillation of the acoustic resonant cavity relative to a desired resonant condition and to provide a correction signal to control the mean frequency of the alternating electrical drive signal to the acoustic driving transducer, whereby the acoustic resonant cavity is maintained at or close to resonance.
12. Apparatus according to any one of the preceding claims in which the acoustic driving transducer is an electrical-to-pressure transducer or an electrical-to-displacement transducer.
13. Apparatus for sensing a gas by pressure modulation spectroscopy, substantially as herein described with reference to the accompanying drawings.