GB2367360A

GB2367360A - Microwave acoustic gas analyser

Info

Publication number: GB2367360A
Application number: GB0109036A
Authority: GB
Inventors: Ching Chu
Original assignee: Datex Ohmeda Inc
Current assignee: Datex Ohmeda Inc
Priority date: 2000-05-01
Filing date: 2001-04-10
Publication date: 2002-04-03
Also published as: GB0109036D0; DE10120568A1; ITRM20010232A0; FR2808884A3; FR2808884A1

Abstract

Apparatus 10 for analysing the concentration of a test gas in a gas sample by analysing the acoustic signal generated by the test gas when subject to a microwave test signal 16. The apparatus 10 includes a microwave source 14 that emits a microwave signal 16 which is modulated by a modulator 18 and emitted in a resonant cavity 24. The modulated test signal 22 is emitted through the gas sample contained within the gas chamber 12. The test gas generates an acoustic signal at specific microwave frequencies which is measured by an acoustic transducer 28. The acoustic transducer 28 generates an output signal having properties directly related to the concentration of the test gas in the gas chamber 12. The output signal from the acoustic transducer 28 is processed to determine the concentration of the test gas in the gas chamber 12.

Description

2367360 MICROWAVE-ACOUSTIC GAS ANALYZER

BACKGROUND OF THE INVENTION

The present invention generally relates to an apparatus and method utilizing the microwave-acoustic effect to determine the concentration of a specified 5 gas in a gas sample. More specifically, the present invention relates to a microwaveacoustic gas analyzer that is useful to'dete rmine the concentration of an anesthetic vapor in a gas sample, including the vapor alone or in a mixture with other gases.

Anesthetic agents, as well as other types of gases, have spectral absorption peaks in the visible and/or infrared light bands. As a result of these 10 absorption peaks, a photo-acoustic detection method can be used to determine the amount of a specified gas or vapor in a gas mixture. Photo- acoustic measurement is based on the tendency of the molecules within a gas, when exposed to certain frequencies of radiant energy (infrared radiant energy), to absorb the energy and reach higher levels of molecular vibration and rotation status, thereby reaching a 15 higher temperature and pressure. When the radiant energy is amplitude modulated, the resulting fluctuations in energy available for absorption produce corresponding temperature and pressure fluctuations.

A sensitive microphone can be used to generate an electrical output representing the pressure fluctuations. The amplitude of the acoustic signal and 20 resulting electrical output from the microphone are proportional to the intensity of the radiation and the concentration value of the absorbing gas or vapor. Accordingly, given a constant amplitude of radiant energy illumination, the electrical output can be detected at the modulating frequency to provide a concentration value proportional to an absorbing amount of the gas. Further, the relationship with light source intensity 25 allows the user to increase sensitivity by increasing light source intensity. Thus, the devices are well suited for measuring small concentrations of a gas or vapor, such as the concentration of an anesthetic agent in a patient breathing system.

One disadvantage of these types of photo-acoustic detection systems is the need to provide radiant energy in an extremely narrow band. Typical infrared 30 emitters provide radiant energy that includes a relatively broad spectrum. One solution to this broad spectrum of radiated energy is to use optical filters to filter the emitted energy to a more narrow bandwidth. Alternatively, expensive lasers or other types of optical filter devices can be used to provide radiant energy with a narrow bandwidth.

1 A further disadvantage of these types of systems is the relative difficulty of modulating the. radiant infrared energy at a high frequency. This is due, in part, to the limitations on the frequency at which a low cost incandescent infrared source can be turned on and off quickly. One method of solving this problem is to use a rotating 5 perforated disk for modulating the wavelength or amplitude, as. illustrated in the Nexo U.S. Patent No. 4,818,882.

In addition to having spectral absorption peaks in the visible and/or infrared light bands, anesthetic drugs also have spectral absorption peaks in the microwave range. The spectral absorption peaks in the microwave range are due to 10 anesthetic drug vapor molecular rotational energy level transitions.

Therefore, it is an object of the present invention to utilize modulated microwave energy to detect the presence and concentration of a test gas in a sample gas or gas mixture, such as the concentration of anesthetic vapors in a gas sample, using the acoustic phenomena resulting from the application of microwave energy to 15 the gas sample. Further, it is an object of the present invention to provide a microwave source that can be modulated in various different manners and passed through a gas sample including anesthetic agent.

SUMMARY OF THE INVENTION

The present invention is a microwave-acoustic gas analyzer that utilizes 20 a microwave frequency test signal and the molecular rotational energy level transitions of a test gas to generate an acoustic signal having properties directly related to the concentration of the test gas in a gas sample. The microwave-acoustic gas analyzer of the present invention can thus be used to determine the concentration of a test gas, such as an anesthetic agent, in a gas sample.

25 The gas analyzer of the present invention includes a microwave source that generates a microwav e signal generally in the range of 500 MHz to 500 GHz.

The microwave signal from the microwave source is selected based upon predetermined absorption frequencies for the test gas being analyzed. The microwave source Gan be selected from various types of conventional microwave 30 generators, each of which emit a microwave signal at one or more than one frequency.

The microwave signal from the microwave source is modulated prior to its emission through the test gas sample. In the preferred embodiment of the invention, the microwave signal is frequency modulated prior to its emission through 2 the test gas sample. The frequency modulator of the present invention is able to modulate the microwave signal in the kHz to MHz frequency range to generate the test signal. In addition to frequency modulation, various other methods of modulation, such as amplitude, phase, pulse and code modulation are contemplated.

5 After modulation, the microwave test signal is emitted or excited within a resonant cavity and passes through the gas sample, including the. test gas, contained within a test gas chamber or cell. The test gas, such as an anesthetic agent, absorbs the microwave energy from the test signal and generates an acoustic signal within the test gas chamber or cell. Specifically, the test gas, such as an anesthetic agent, in the 10 gas sample absorbs the microwave energy from the test signal and the molecular rotational energy level transitions of the test gas create pressure variations within the test gas chamber or cell.

An acoustic transducer is positioned to detect the pressure transitions within the test gas chamber. The acoustic transducer, which is typically a sensitive 15 microphone, generates an output signal based on the acoustic signal sensed within the test gas chamber. The output signal from the acoustic transducer is received by an output signal processor that processes the output signal in relation to the test signal. The processed output signal from the acoustic transducer bears a direct relation to the concentration of the test gas within the gas sample.

20 Various alternate configurations for the test gas chamber and resonant cavity for the microwave-acoustic gas analyzer of the present invention are possible.

In addition, the pressure transducer is contemplated as being a differential acoustic transducer contained within a continuous tubular gas chamber.

BRIEF DESCRIPTION OF THE DRAWINGS

25 The drawings illustrate the best mode presently contemplated of carrying out the invention.

In the drawings:

Fig. 1 is a schematic illustration of the microwave-acoustic gas analyzer of the present invention; 30 Fig. 2 is a graphical illustration of the period of the microwave test signal and the detected response of a test gas as well as the response of pure oxygen; Fig. 3 is a graph illustrating a test signal pulse as well as the response of an acoustic transducer when the concentration of the test gas is varied; 3 Fig. 4a is a graph illustrating the microwave-acoustic vector of a test gas at a selected frequency with concentration decay; Fig. 4b is a graph of the output of an acoustic transducer illustrating the response of a test gas to microwave energy over a first frequency range; 5 Fig. 4c is a graph of the output of an acoustic transducer illustrating the response of a test gas to microwave -energy over a second frequency range; , Fig. 5 is a first embodiment of the test gas chamber and microwave source embodying the present invention; Fig. 6 is a second embodiment of the test gas chamber; 10 Fig. 7 is a third embodiment of the test gas chamber; Fig. 8 is a fourth embodiment of the test gas chamber; Fig. 9 is a fifth embodiment of the test gas chamber and resonant cavity for the microwave source; Fig. 10 is a sixth embodiment of the test gas chamber and resonant 15 cavity; Fig. 11 is a seventh embodiment of the test gas chamber and microwave source embodying the present invention; Fig. 12a is a front view of a eighth embodiment of the test gas chamber and resonant cavity for the microwave source:

20 Fig. 12b is a side view illustrating the test gas chamber and resonant cavity of Fig. 12a; Fig. 13 is a ninth embodiment of a test gas chamber and microwave source embodying the present invention; Fig. 14 is a graph illustrating the wave form generated by the modulation 25 frequency oscillator of the ninth embodiment of the invention illustrated in Fig. 13; Fig. 15 is a graph illustrating the microwave energy signal generated by the microwave source of Fig. 13; and Fig. 16 is a graph illustrating an output signal generated by the acoustic transducer of the embodiment of the invention illustrated in Fig. 13.

30 DETAILED DESCRIPTION OF THE INVENTION

Referring first to Fig. 1, thereshown is a microwave-acoustic gas analyzer 10 of the present invention. The gas analyzer 10 of the present invention is used to analyze the concentration of a desired gas or vapor contained within a test gas chamber 12. The desired gas or vapor contained within the test gas chamber 12 4 can be any type of gas or vapor that has absorption peaks in the microwave range due to the molecular rotational energy level transitions within the gas or vapor. In the preferred embodiment of the invention, the desired gas contained within the test gas chamber is a vapor including an anesthesia drug. For example, the vapor contained 5 within the test gas chamber 12 to be tested can include anesthesia drugs such as Desfiurane, sevoflurahe and various.other common commercially available anesthetic agents.

The microwave-acoustic gas analyzer 10 shown in Fig. 1 includes a microwave source 14 for generating a signal in the microwave frequency. In the 10 preferred embodiment of the invention, the microwave source 14 generates a microwave signal 16 having a frequency between 500 MHz and 500 GHz, such that the microwave source 14 actually can produce either microwave or millimeter wave signals. For purposes of the following explanation, the microwave signal range will be defined as between 1000 MHz and 30 GHz, while the millimeter range will be between 15 30 GHz and 300 GHz in accordance with accepted classification guidelines. In the preferred embodiment of the invention, the microwave source 14 can be selected from a group including a low cost Gunn diode, IMPATT diode, tunnel diode or microwave semiconductor transistors such as Si or GaAs FETs. In general, the microwave source 14 generates a microwave signal that is specifically centered around the 20 desired absorption frequency for the gas being analyzed within the test gas chamber 12. The microwave source 14 may comprise one or more components whose frequency or frequencies and/or harmonics are at multiple peak absorption frequencies of the test gas to be detected within the test gas chamber 12.

The microwave signal 16 generated by the microwave source 14 is 25 preferably modulated by a modulator 18. The modulator 18 modulates the microwave signals by'either amplitude-modulati6n (AM), frequency modulation -(FM), phasemodulation (PSK), pulse modulation (PM) or code modulation. In the embodiment of the invention shown in Fig. 1, the modulator 18 frequency modulates the microwave signal 16 by a modulation frequency generated by a modulation frequency oscillator 30 20. An advantage of the microwave-acoustic gas analyzer 10 of the present invention as compared to a photo-acoustic gas analyzer is the ability to modulate the microwave signal 16 at a higher frequency as compared to the infrared signal used in a photoacoustic analyzer. For example, the frequency modulator 18 shown in the illustrated embodiment of the invention can modulate the microwave signal 16 in the kilohertz range. In the photo-acoustic analyzer of the prior art, the infrared signal could typically only be modulated in the Hertz range. In the embodiment of the invention illustrated in Fig. 2, the test signal 22 is a 2 KHz modulation waveform of 10 GHz.

After the microwave signal 16 passes through the modulator 18, the test 5 signal 22 having a modulation frequency f leaves the resonant cavity 24 and enters into the test gas chamber 12. The test gas chamber 12 includes an inlet 26 that allows the test gas to enter into the generally hollow test gas chamber 12.

In accordance with the invention, the frequency of the microwave source 14, and thus the microwave signal 16, is specifically selected for the type of gas being 10 analyzed within the test gas chamber 12. For example, if the test gas to be analyzed within the test gas chamber 12 is Desflurane, the frequency of the microwave signal 16 will be selected at approximately 11. 54 GHz since it has been experimentally determined that Desflurane has a microwave energy absorption peak at approximately this frequency.

15 Although the frequency of the microwave source 14 is discussed as being at a single, determined frequency, it is contemplated by the inventor that the frequency range of the microwave source 14 can be swept through a frequency range.

By sweeping the frequency of the microwave source 14 through a range, the microwave-acoustic gas analyzer 10 can perform microwave acoustic gas 20 spectroscopy such that the output signal from the acoustic transducer is generated over a frequency range. By utilizing the microwave source 14 to generate frequencies over a range, the microwave-acoustic gas analyzer can analyze the gas sample for the various different frequencies, as is common in a spectroscopy system.

At the testing frequency, the test gas or vapor being analyzed absorbs 25 the radiated microwave energy and the molecular rotational energy level transitions within the test gas result in pressure or acoustic fluctuations within the test gas chamber 12. The acoustic fluctuations within the test gas chamber 12 can be detected by an acoustic transducer 28 connected to the test gas chamber 12. In the preferred embodiment of the invention, the acoustic transducer 28 can be any type of 30 microphone or sensor, such as an electracondensor microphone or capacitive, electret, electrostatic, electromagnetic-dynamic, piezoelectric, piezoresistive, optoelectronic, fiber optic, or laser- interferometer type microphone. In any case, the acoustic transducer 28 senses the pressure waves created within the test gas chamber 12.

6 The acoustic wave generated within the test gas chamber by the absorption of the microwave energy from the test signal 22 bears a direct proportional relationship to the concentration of the test gas within the test gas chamber. Thus, based on the output signal generated by the acoustic transducer 28, a determination 5 can be made based on the output signal of the concentration of the test gas within the test gas chamber. The output signal from. the pressure transducer 28 can be processed in various manners to extract information concerning the acoustic signal generated within the test gas chamber 12.

In the embodiment of the invention illustrated in Fig. 1, the output signal 10 from the acoustic transducer 28 is fed into an amplifier 30. The signal from the amplifier 30 is fed into a synchronization detector 32, along with the signal from the modulation frequency oscillator 20. From the synchronization detector 32, the output signal is then fed into an output signal processor 34. The output signal processor 34 can perform a variety of processing steps on the output signal to analyze the output 15 signal and determine the amount of test gas in the test gas chamber 28. The method of signal processing performed by the output signal processor 34 can be as simple as measuring the amplitude of the output signal at the frequency of the modulation.. Alternatively, the output signal processor 34 can measure the phase difference between the modulated test signal 22 and the output signal generated by the acoustic 20 transducer 28. This type of analysis is a better method and utilizes a quadrature amplifier 34, illustrated in Figs. 5-13, as will be discussed in greater detail below. Additionally, direct acoustic waveform analysis is also one of the best methods for analyzing the output signal from the acoustic transducer 28.

Referring now to Fig. 2, thereshown is the test signal 22 applied to the 25 test gas within the test gas chamber 12. In the preferred embodiment of the invention illustrated in Fig. 2, the test signal 22 is applied to a Desfiurane vapor contained within the test gas chamber 12. The test signal in Fig. 2 is modulated at 1 KHz, as indicated by the ON state 36 and OFF state 38 of the test signal 22. The trace 40 shown in Fig. 2 illustrates the output of the acoustic transducer 28 upon application of the test signal 30 22 to the test gas chamber including the Desfiurane vapor. As can be seen, the trace 40 is generally sinusoidal in nature and directly reflects the application of the microwave test signal 22 to the test vapor.

Positioned below trace 40 in Fig, 2 is a trace 42 that illustrates the response of 100% 02 to the same microwave test signal 22. As trace 42 indicates, 7 100% 02 has little to no response to the microwave test signal 22. In Fig. 2, the test signal has a 10 GHz carder frequency modulated by a 1 KHz signal. Thus, it can be appreciated that different gases have different responses to a microwave test signal and thus generate an acoustic waveform that can be detected by a pressure 5 transducer.

Referring now to Fig. 3, thereshown is the effect of a single microwave pulse 44 applied to a test gas sample having various concentrations of a test gas or vapor. Specifically, the test gas illustrated in Fig. 3 is Desflurane. Trace 46 illustrates the response of the acoustic transducer 28 when the amount of the test gas in the gas 10 sample is 0. As expected, the trace 46 is a generally flat line and does not illustrate any response to the applied pulse 44. Trace 48 shows the response when 5 cc of Desflurane is in a 20 cc test chamber. As can be seen in trace 48, the output signal from the acoustic transducer 28 responds with a maximum amplitude A when the concentration of Desflurane is.25%.

15 Trace 50 in Fig. 3 illustrates the response from the acoustic transducer 28 when 10 cc of Desfiurane is in the 20 cc test chamber. As can be seen by trace 50, the amplitude of the response signal is illustrated by B. Thus, when the concentration of Desfiurane is increased from 25% to 50%, the amplitude of the signal from the acoustic transducer 28 increases.

20 Finally, trace 52 in Fig. 3 illustrates the case in which 20 cc of Desflurane are placed in the 20 cc test chamber. The amplitude of the signal from the acoustic transducer 28, as illustrated by C, when the test gas chamber 12 includes 100% of the test gas Desfiurane is clearly much greater than the amplitude when the test chamber included 50% Desflurane, As discussed previously, comparing the 25. amplitude of the output signal from the acoustic transducer 28 is only one method to determine the amount of the test gas in the test gas chamber 12.

Referring now to Fig. 4a, thereshown are various traces for the test gas Desflurane at 17.1 GHz. Each of the traces in Fig. 4a illustrates the concentration decay at the frequency of 17.1 GHz. In each of the graphs of Fig. 4a, the 30 concentration of the test gas Desflurane is the highest in graph (i) and the concentration of the test gas decreases to its lowest level in trace (xv). Each of the traces display the real and imaginary components of the acoustic response, which indicate the time delay of the signal as it is passed through the test gas sample. Specifically, the greater the delay of the test signal indicates a higher concentration of 8 the test gas Desfiurane in the test gas sample. Thus, as the concentration of the test gas Desflurane decreases, the trace shortens in length and rotates in the clockwise direction.

Fig. 4b illustrates the magnitude of the microwave-acoustic (MWA) 5 output. over a frequency range centered around approximately 6.5 G Hz. Likewise, Fig. 4c illustrates the magnitude of the MWA output over a. frequency range centered..

around a frequency of approximately 17.1 GHz.

As can be seen in Fig. 1, the output signal processor 34 outputs a detector signal 68. This detector signal is used to determine the concentration of the 10 test gas within the gas sample.

Referring now to Figs. 5-13, thereshown are various alternate embodiments for the construction of the microwave resonance cavity and the test gas chamber, each of which operate generally in the manner described previously with reference to Figs. 1-4. In each of the alternate embodiments illustrated in Figs. 5-13, 15 the output signal from the acoustic transducer 28 is fed into a quadrature amplifier 34 for signal processing. As illustrated in Fig. 5, the output signal is initially fed to amplifier 30. The signal from the amplifier 30 is fed to two separate modulator switches 70 which compare the signal to a synchronization signal from the microwave source and modulator 72 as well as a signal that is.90" out of phase with the signal 20 from the microwave source and modulator 72. The signal from the quadrature amplifier 34 is sent to a digital scope 74 which acts as the output signal processor to display the response signal from the acoustic transducer 28. In general, the quadrature amplifier 34 and digital scope 74 are identical in each of the embodiments of the invention illustrated in Figs. 5-13 and the details of which will only be discussed 25 with reference to Fig. 5.

In Fig. 5, the test gas chamber is a spherical chamber 76 in a dielectric material 77 and includes an access port 78 for the acoustic transducer 28. The shape of the spherical test gas chamber 76 aids in determining the frequency emitted by the microwave source.

30 Referring now to Fig. 6, thereshown is a first alternate embodiment of the resonant cavity and test gas chamber of the present invention. In the embodiment shown in Fig. 6, a center conductor 80 transmits the test signal into a tubular test gas chamber 82 defined by a coaxial cylindrical metal wall 84. The gas sample is contained within the open interior of the tubular gas chamber 82 or within an acoustic 9 window 83 such that the microwave signal passes through the gas sample and the acoustic pressure wave is detected by the acoustic transducer 28. A plastic tubular member 86 surrounds a portion of the gas chamber 82 and aids in directing the acoustic signal to the acoustic transducer 28.

5 Referring now to Fig. 7, in the second alternate embodiment, the gas.

sample is contained within an annular test gas chamber 88 that surrounds a central conductor 90. The microwave signal is radiated from the central conductor 90 and passes through the test gas contained within the annular gas chamber 88. The acoustic signal generated by the test gas is received by the acoustic transducer 28 10 that is positioned within a neck portion 92. The gas chamber 88 terminates with a 50 ohm load 91, or alternatively, is shorted or open.

Referring now to Fig. 8, in the third embodiment of the invention, the test gas is contained within a continuous tube 94 that includes a differential acoustic transducer 96. As can be seen in Fig. 8, a testing portion 98 of the continuous tube 15 extends into the test gas chamber 100 and is subjected to the microwave test signal.

The microwave test signal passes through the portion of the gas sample in the testing portion 98. and creates an acoustic signal that is received by both sides of the differential acoustic transducer 96. The continuous tube 94 is selected such that the length of the short leg 102 and the long leg 104 leading to the differential acoustic 20 transducer 96 are various multiples of the wavelength of the test signal. The differential acoustic transducer 96 thus compares the difference between the two legs 102 and 104 of the continuous tube 94. In the preferred embodiment of the invention, the continuous tube 94 is formed form Teflon and has an inner diameter of one millimeter. The end of the test gas chamber terminates with a waveguide terminator 25 101.

Referring now to Fig. 9, the fourth alternate embodiment includes a cylindrical resonant cavity 106 that includes the microwave signal emitter 108. The microwave test signal enters into the resonant cavity 106 and passes through the gas sample contained within a testing portion 110 of a continuous tube 112 containing the 30 gas sample. As illustrated in Fig. 9. the test gas enters into the tube 112 through an inlet 114 and exits the tube through an outlet 116. The differential acoustic transducer 96 is positioned to detect the acoustic signal generated by the test gas within the gas sample.

Fig. 10 illustrates a fifth alternate embodiment in which the resonant cavity 118 is spherical and includes the microwave emitter 120. The microwave test signal emitted by the emitter 120 passes through the gas sample including the test gas contained within tube 122. The acoustic transducer 28 senses the acoustic signal 5 generated by the test gas and transfers the output signal to quadrature amplifier 34.

Fig. 11 illustrates a sixth alternate embodiment that includes a microstripe emitter 126 that emits the microwave signal through the gas sample included within tube 128 that passes through an outer housing 130.

Fig. 12a illustrates a Gunn diode 130 positioned within a microwave 10 resonator cavity 132 to generate the microwave test signal. The microwave test signal from the Gunn diode 130 passes through the gas sample included within the fixed volume tube 134 including the acoustic transducer 28. As can be seen in Fig. 12b, the device also includes a mixer diode 136 positioned within a separate enclosure adjacent to the microwave resonator cavity 132.

15 Referring now to the final embodiment illustrated in Fig. 13, a modulating signal is generated by the modulator 132 and directed to a GaAs FET 134. From,the GaAs FET 134, a microstripe line 136 on a printed circuit board generates the microwave test signal that is transmitted by a dielectric resonator 138. The microwave test signal passes through the test gas contained within a tube 140. In the preferred 20 embodiment of the invention, the tube 146 is formed from Teflon and has an inner diameter of 1.2 millimeters. The acoustic transducer 28 detects the acoustic signal generated by the test gas and outputs the signal to a similar quadrature amplifier (not shown).

Referring now to Figs. 14-16, thereshown are the various waveforms 25 measured at separate locations within the specific embodiments of the microwave acoustic gas analyzer illustrated in Figs. 5-13. Specifically, the graphs of Figs. 14-16 are derived from the embodiment of the invention illustrated in Fig. 13. Referring first to Fig. 14, thereshown is the modulation square wave generated by the modulator 132 of Fig. 13. The modulation square wave illustrated in Fig. 14 has a frequency of 1.66 30 kHz. Fig. 15 illustrates the microwave signal generated by the dielectric resonator 138 of Fig. 13. As can be seen in Fig. 15, the microwave signal is centered around the frequency of 9.77 GHz.

Fig. 16 illustrates the output signal from the acoustic transducer 28 of Fig. 13. As can be seen by comparing Fig. 16 and Fig. 14, the output signal from the acoustic transducer 28 responds directly to the modulated microwave test signal.

Although numerous alternate embodiments for the microwave-acoustic 5 gas analyzer are illustrated in Figs. 5-13, it is contemplated by the inventor that various other configurations for both the test gas chamber and the resonant cavity, as well as the conductor for the microwave test signal, could be utilized while operating within the scope of the invention. Various alternatives and embodiments are contemplated as being within 10

the scope of the following claims particularly pointing out and distinctly claiming the subject matter regarded as the invention.

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Claims

1. An apparatus for analyzing the concentration of a test gas in a gas sample, the apparatus comprising:.

a microwave source for generating a test signal having a frequency between 500 MHz and 300 GHz; 5 a test gas chamber containing the gas sample to be analyzed, the test gas chamber being positioned relative to the microwave source such that the test signal from the microwave source passes through the gas sample in a test chamber, wherein the test gas in the gas sample absorbs the test signal and converts the test signal into an acoustic signal; 10 an acoustic transducer positioned to detect the acoustic signal generated in the test gas chamber and generates an output signal, the output signal being correlated to the amount of the test gas in the gas sample; and a signal processor positioned to receive the output signal from the acoustic transducer, the signal processor being operable to determine the amount of.

15 test gas in the gas sample based upo n the output signal.

2. The apparatus of claim I wherein the test gas in the gas sample converts the microwave test signal into the acoustic signal due to the molecular rotational energy level transitions in the test gas.

3. The apparatus of claim 1 further comprising a modulator positioned to modulate the test signal prior to the transmission of the test signal through the gas sample.

4. The apparatus of claim 3 wherein the modulator is a frequency modulator.

5. The apparatus of claim 4 wherein the frequency modulator modulates the test signal between 1 kHz and I MHz.

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6. The apparatus of claim 1 wherein the signal processor is a quadrature amplifier that detects the phase difference between the test signal and the output signal.

7. The apparatus of claim 1 wherein the test gas is an anesthetic vapor.

8. The apparatus of claim I wherein the output signal of the acoustic transducer is an electric signal.

9. The apparatus of claim I wherein the test gas chamber is formed from Teflon. kTrA

10. The apparatus of claim 5 wherein the frequency modulator modulates the test signal over a predetermined frequency range such that the signal processor receives the output signal at varying modulating frequencies to determine the, peak absorption frequency in the gas sample being tested.

11. The apparatus of claim 1 further comprising a resonant cavity containing the microwave source, wherein at least a portion of the test gas chamber is contained within the resonant cavity.

12. The apparatus of claim 11 wherein only a portion of the test gas chamber is contained within the resonant cavity and the acoustic transducer is a differential acoustic transducer that detects the acoustic signal generated in the test gas chamber.

13. The apparatus of claim I wherein the test signal has a frequency in the microwave range between 500 MHz and 500 GHz.

14. An apparatus for analyzing the amount of an anesthetic agent in a gas sample, the apparatus comprising:

a microwave source positioned in a resonant cavity for generating a test signal having a test frequency between 500 MHz and 500 GHz; 14 5 a test gas chamber containing the gas sample to be analyzed, the test gas chamber being positioned adjacent to the resonant cavity such that the test signal from the microwave source passes through the gas sample in the test chamber, wherein the anesthetic agent in the gas sample absorbs the energy of the test signal and the molecular rotational energy level transitions of the anesthetic agent convert 10 the test signal into an acoustic signal; an acoustic transducer positioned to detect the acoustic signal generated in the test gas chamber and generate an output signal, the output signal being proportional to the amount of the anesthetic agent in the gas sample; and a signal processor positioned to receive the output signal from the 15 acoustic transducer to determine the amount of anesthetic agent in the gas sample.

15. The apparatus of claim 14 further comprising a modulator positioned to modulate the test signal prior to transmission of the test signal through the gas sample.

1.6. The apparatus of claim 15 wherein the modulator is a frequency modulator that modulates the test signal between I kHz and 1 MHz.

17. The apparatus of claim 14 wherein the signal processor includes a quadrature amplifier that detects the phase difference between the modulated test signal and the output signal from the acoustic transducer, the phase difference being proportional to the amount of the anesthetic agent in the gas sample.

18. The apparatus of claim 14 wherein the test gas chamber is a continuous tube, a portion of which Is contained within the resonant cavity.

19. The apparatus of claim 18 wherein the acoustic transducer is positioned within the continuous tube and is a differential microphone.

20. The apparatus of claim 14 wherein the test signal has a test frequency in the microwave range between 500 MHz and 50 GHz.

21. An apparatus for analyzing the concentration of a test gas in a gas sample substantially as hereinbefore described with reference to any one of the accompanying drawings.

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