- Email: [email protected]
Tunable gas sensors
265
Abstract This paper proposes a new sensing principle for gas detection. The wurking principle is based on the ultrasonic resonance within a cavity. Such sensors are working in how-through conditions at, or near, room t~~rature. A quartz crystal resonator generates ultrasonic waves within a cavity. These are reelected by the cavity wail. If the gap between the quartz. crystal surface and the parallel. reflecting wall is an integer of if-waveien~~, then resonantr! of the gas within the cavity occurs, and the whole vibm~ona~ energy of the quartz resonator is absorbed by thegas. By rn~ban~~~y tuning the gap to a certain length, the resonance of whatever gas, either pure, or a mixture of constant imposition is obtained. When a small amount of another gas, di&iug in Atwood velocity, is present in the flowing gas, the resonance condition is changed, and a signal is detected. The adverse effect of the ambient temperature variation is eliminated by both thermocompensation and temperature control. The response of these sensors to hydrogen or methane in air is less than one second, w~~aut using a catalyst. Their use as gas chromato~aphy detectors is proved by revealing 10.8 ppm of oxygen in a nitrogen matrix, when hydrogen is used as eluent gas.
In recent years interest in gas detection has been increasing. This interest comes from two di~erent viewpoints. One refers to fiammable gas detection at concentration levels lower than the explosion limits. These concentrations are in the percent by volume range. The other refers to toxic gas detection at ~~~nt~tion levels lower than the limits of toxicity. These concentrations are in the ppm and sub-ppm by volume range. A reliable gas sensor should be sensitive, selective and stable. Significant efforts are made to fulfil these r~ujr~ents. Both physical and chemical sensors compete in this field. Different classes of gas sensors are now available: ~~~~~h~rn~~~~ se~~nd~tor structured, po~e~c, metal oxide, optical and ultrasonic. A very sensitive gas sensor has been developed as a gas chromatography detector [If. Here the ultrasound waves are propagated at one transducer and received at another. A phase meter monitor the signal received, which is sensitive to any change in the gas composition. A disadvantage of this sensor is the high cost and sophistication of the electronics necessary for good performances. Mecca and Bucnr [2] proposed a new ultrasonic method for gas detection which uses only one quartz resonator as a transducer. Based on this idea, Mecca and Tatar ]3] developed an ultrasonic hydrogen detector, which was further improved by I&O-S and Ghete PI.
This paper presents further developments of these achievements, allowmg the detection of whatever noncorrosive gas differing in ~tr~o~d vek~&ty from the reference gas,
The working principle of the tunable gas sensors is based on the Energy Transfer Model [5]. This model states that whenever a medium is in contact with the vibrating surface of a quartz resonator, they form a compound resonator, the frequency and vibration amplitude of which are greatly r&&d by the characteristics of the medium vibrating synchronously with the quartz resonator surface It is well known that shear waves do not propagate in gases. However, if compressional waves are generated by an X-cut quartz resonator, the ultrasonic longitudinal waves will propagate in the grounding gas. If this gas is induced to resonate at the same frequency as that of the quartz resonatar, then an important amount of vibration energy of the quartz resonator will be absorbed by the resonating gas. The resonance conditiott can easily be a~om~lish~ when standing waves are generated between the surface of the quartz ~sonator and the surfam of a parallel retlecting wall, Standing waves are generated whenever the dir&n= l, between the two parallel surfaces, is an integer of balf-wavelengths
@ 1993-- ElsevierSequoia.All rk&ts rawed
266 TABLE
I Ultrasound velocity in some common gases at 0 “C
Gas
“g (m/s)
Hydrogen Helium
1269.5 972
Methane Carbon monoxide Nitrogen Air Nitric oxide Oxygen Argon Nitrous oxide Carbon dioxide Sulfur dioxide Carbon disulfide
I= nA/2 = nv,&
432 337.1 331 331.36 325 317.2 307 261.8 258 216.2 189
(1)
where ug is the ultrasound velocity in the gas filling the resonator cavity and f4 is the frequency of the quartz resonator vibrating in a compressional mode. The vibration energy absorbed by the resonating gas can be of a signi~cant amount, inducing the complete damping of the quartz resonator vibrations. The condition for gas resonance, indicated by eqn. (l), reveals that, for the same quartz resonator vibrating in a compressional mode, the resonance depends on the nature of the gas, through the ultrasound velocity. This is given for some common gases in Table 1 [6]. There is a si~ificant difference in ultrasound velocity between various gases. This feature can be used for gas detection. Whenever the distance I between the surface of the quartz resonator and the surface of the reflecting wall is so adjusted to fulfil the resonance condition for a certain reference gas, the presence of a small amount of another gas, differing in ultrasound velocity, will alter the resonance, and a change in the vibration amplitude of the quartz resonator will be recorded. This vibration amplitude can be measured by rectifying the output r.f. voltage of the transistorized oscillator driven by the quartz resonator [7,8]. The effect of a small mount of hydrogen added to air flowing through the cavity between the quartz resonator surface and the reflecting wall is shown in Fig. 1. Here is illustrated the case of a resonant cavity so adjusted that the distance between the surface of a 1 MHz quartz resonator and the surface of the reflecting wall is I,, when air is flowing through the cavity. The recorded signal, related to the vibration amplitude of the quartz resonator is U,, smaller than the maximum signal U,, corresponding to the out-of-resonance condition, When air flowing through this cavity contains a small amount of hydrogen, the ultrasound velocity in such a mixture is higher than in pure air, and
GOP lmml
Fig. I. Hydrogen detection with a tunable gas sensor using a 1MHzqpartz resonator.
a significant change in the recorded signal will be revealed, the resulting voltage being U,. The gap between the quartz resonator surface and the reflecting wall surface can be adjusted to whatever length, close to the resonance condition, for whatever gas. Thus we can call such a sensor a ‘tunable gas sensor’. Whenever a gas, differing in ultrasound velocity is present in the gas on which the cavity was tuned, a measurable change in the rectified r.f. voltage, corresponding to the vibration amp~tude, will be recorded.
3. Thermocompensationof a tunable gas sensor The resonance condition (I) is affected by the temperature changes through cavity dilatation, according to the law I=&(1 +ar)
(2)
and through ultrasound velocity change, according to the law t’ = V*(1 + t/273.15)‘”
(3)
These two major effects can alter the reliability of a tunable gas sensor. A good temperature control of the detection cell is very important. However, the adverse effect of the temperature changes can be greatly diminished by thermocompensation. This is accomplished when the diIatation of the gap /,, follows the change of the resonance condition (1) through the temperature dependence of the ultrasound velocity. In order to match the dilatation of the gap to the change of the ultrasound velocity, a detection cell was designed, which is schematically shown in Fig. 2. The quartz resonator, with a nominal frequency of 1 MHz and a diameter of 2Omm (K~st~l-~erar~it~g Neckarbischofsheim GmbH) is accommodated within the body of the brass cell. To avoid mechanical energy losses, the quartz resonator is fixed in its nodal vibrational plane. The electrical connections to the evaporated Au/Q electrodes touch the quartz resonator in its nodal plane
261
Fig. 2. Schematic drawing of a ~erm~om~nsat~ sensor.
tunable gas
t PC1 Fig. 3. Effect of temperature on the apparent hydrogen concentration change when three lengths of ceramic cylinders (A, B and C) are used for a ~e~ocorn~nsa~d tunable gas sensor.
too. The gas enters the cell through a pipe penetrating the lid and exits through a pipe emerging from the body of the detector. A rubber gasket hermetically closes the cell. It also allows the lid to move, follo~ng the dilatation of the alurn~~ screws, which rest on the lid surface by means of some ceramic cylinders with very small dilatation. Their length is so calculated to match the dilatation of the gap to the ultrasound velocity change with a temperature change. As eqn. (3) is not a linear-like one, like the dilatation law, the best thermo~om~nsation is obtained in a narrow tem~ra~re range, as shown in Fig. 3 for the case of hydrogen detection in air.
The increasing use of hydrogen gas should not be considered as one without disadvantages. A hydrogen leak in the atmosphere should be avoided, because hydrogen when mixed with air in the ratio of 4.0074.2 vol.% is explosive [9]. For this reason, among others, it has become very irn~~ant to develop highly sensitive hydrogen sensors to prevent accidents due to hydrogen gas leakage, thus saving lives and equipment. Such sensors should allow continuous monitoring of the concentration of gases in the environment in a quantitative and qualitative way. Several classes of hydrogen sensors have been developed: ~miconductor, pyroel~t~c, piezoeleetric and el~tr~hemical devices. The colon feature of all these sensors is that they use a catalyst, mainly palladium, to selectively detect hydrogen. The main drawback of these sensors is the possibility of catalyst poisoning by sulfur dioxide, hydrogen sulfide, carbon monoxide, sulfur and others. Con~qu~tly they can work properly only in controlled atmospheres. It has already been shown that the working principle of a tunable gas sensor can be used for hydrogen detection [3,4]. Here new experimental results are given. Tuning the gap & to get the near-resonance condition for air, like in Fig. 1, it was possible to record the response of the sensor to various ~on~~ra~ions of hydrogen in air. The results are shown in Fig. 4, where the resonance deep is revealed in a rather large hydrogen concentration range, from 0.75 to 3.1 vol.% hydrogen in air. This is because of the strong vibration energy absorption by the resonating gas. A similar result was
5 UN1
2
1
4. Hydrogen and metbane detection in air In recent years, hydrog~ has become one of the most useful gases. Many industries such as chemical, metallurgical, food and others use hydrogen as a raw material.
Oo
12
3
L
5
6 COl-d%t
Fig. 4. Response of the tunable gas sensor to various concentrations of hydrogen in ait at 37 “CT.
268
.’
.’
/ i
---__A O 0
0.4
ae
1.2 1.6 conc.Wd
Fig. 7. Response of a tunable gas sensor to various concentrations of hydrogen and methane in air at 37 “C.
1
2
3
4
5
b
cont.P/PI
Fig. 5. Response of the tunable gas sensor to various concentrations of hydrogen in argon at 37 “C.
obtained for hydrogen-argon mixtures, when the gap 1, was tuned to get the near-resonance condition for ar-
gon. The sensor response for different concentrations of hydrogen in argon is shown in Fig. 5. Here the resonance deep is broader than in the case of air because of higher molecular weight and viscosity of the argon gas. The time response of such a sensor is very fast, depending on the necessary time to fill the detection volume of the cavity with the new concentration of the gas. This is revealed in Fig. 6 for various concentrations of hydrogen in air, at 37 “C.
The interest in methane detection comes from its wide domestic use. When mixed with air in the ratio of 5.00- 15.00 vol.%, it is also explosive [9]. As shown in Table 1, the ultrasound velocity in methane is different from that in air, allowing its detection with a tunable gas sensor. The expected sensitivity of a tunable gas sensor for methane detection in air will be smaller than that for hydrogen detection because the ultrasound velocity in methane is closer to that in air, than in the case of hydrogen. This is shown in Fig. 7, where the response of the tunable gas sensor to various concentrations of hydrogen and methane in air at 37 “C is revealed. The experimental results shown in Fig. 7 prove that the sensitivity of the tunable gas sensor for each of these gases is very high (0.01 vol.% for hydrogen and 0.018 vol.% for methane), allowing their detection in the environment at concentration levels much lower than the explosion limits.
5. Gas chromatography detector 1.6
1”
LU
t hid
Fig.6. Time response of the tunable gas Sensor to various con-
centrations of hydrogen in air at 37 “C.
A tunable gas sensor might also be used as a universal gas chromatography detector. The detection of various gases is based on the existing difference in ultrasound velocity between the eluent gas and the separated components of the gas sample. In order to check the performances of a tunable gas sensor as a detector for gas chromatography, the gap between the quartz resonator surface and the reflecting wall was mechanically tuned to near resonance condition for hydrogen, which was the eluent gas of the gas chromatograph. A gas sample containing 10.8 ppm oxygen in a nitrogen matrix was used (can mix No. 48 from Scott Analysed Gases). As shown in Fig. 8, the
269
It can detect inflammable gases, like hydrogen and methane in air, without using a catalyst, at concentration levels lower than the explosion limits. The time response of such a sensor is less than one second. When used as a universal gas chromatography detector, this sensor can be tuned for the eluent gas, revealing the separated gas components with a sensitivity higher than that of a thermal conductivity detector.
UWI 20 1( 16
12
6
References 4
I M. J. O’Brien,in R. L. Grob (ed.), Modern Pracrice of Gas
d 4
6
8
10
t ImInI
..
Fig. 8. Response of a tunable gas sensor when used as a gas chromatography detector at 37 “C.
separated oxygen peak is sensitively detected by a tunable gas sensor. The nitrogen peak is very large and discontinuous. This is because of the very large concentration of nitrogen, which causes the sweep of the whole resonance deep, including the situation of complete damping of the quartz resonator vibrations. When using a conventional thermal conductivity detector, for the same gas sample, the oxygen was not detected and the nurogen peak was much narrower. This proves the very high sensitivity of a tunable gas sensor when used as a gas chromatography detector.
6. Conclusions The resonance of a gas within a cavity can be used for the detection of whatever non-corrosive gas differing in ultrasound velocity from the reference gas on which the cavity was tuned. This sensor can be mechanically tuned to a near-resonance condition for whatever reference gas.
Chromatography, Wiley, New York, 1977, pp. 281-282. 2 V. Mecca and R. V. Bucur, A new method for detection of a dangerous gas in the atmosphere, Romanian Pafen! No. 76465 (1978). 3 V. Mecca and E. Tatar, Piemelectric hydrogen detector, Romanian Patent No. 76466 (1978). 4 V. Mecca and P. Ghete, Thermocompcnsated ultrasonic hydrogen detector, In?. .I. Hydrogen Energy, 9 (1984) 861 -864. 5 V. Mecca and R. V. Bucur, The mechanism of the interaction of thin films with resonating quartz crystal substrates: the Energy Transfer Model, Thin Solid Films, 60 (1979) 73-84. 6 C. D. Hodgman (ed.), Handbook of Chemistry and Physics, Chemical Rubber Publishing Co., Cleveland, OH, 1959, p. 2502. 7 V. Mecca and R. V. Bucur, The use of RF-voltage in quartz crystal microbalance measurements: application to non metallic films, J. Phys. E. Sci. Instrum., 7 (1974) 348-349. 8 V. M. Mecca, A new method of measuring the mass sensitive areas of quartz crystal resonators, .r. Phys. E. Sci. Instrum., 22 (1989) 59-61. 9 C. D. Hodgrnan (ed.), Handbook of Chemi&y and Physics, Chemical Rubber Co., Cleveland, OH, 1959, pp. 1927-1929.
Biography Vasile-Mihai Meceu was born in 1948 in Brasov. He graduated from the Physics Department of the ‘BabesBolyai’ University of Cluj-Napoca in 1972. He received his Ph.D. degree in 1979 on a subject concerning the use of the quartz crystal microbalance for hydrogen and deuterium interaction with palladium films. He is currently working in vibrating quartz sensors and thin film physics. He is also involved in hydrogen storage and detection.
Abstract This paper proposes a new sensing principle for gas detection. The wurking principle is based on the ultrasonic resonance within a cavity. Such sensors are working in how-through conditions at, or near, room t~~rature. A quartz crystal resonator generates ultrasonic waves within a cavity. These are reelected by the cavity wail. If the gap between the quartz. crystal surface and the parallel. reflecting wall is an integer of if-waveien~~, then resonantr! of the gas within the cavity occurs, and the whole vibm~ona~ energy of the quartz resonator is absorbed by thegas. By rn~ban~~~y tuning the gap to a certain length, the resonance of whatever gas, either pure, or a mixture of constant imposition is obtained. When a small amount of another gas, di&iug in Atwood velocity, is present in the flowing gas, the resonance condition is changed, and a signal is detected. The adverse effect of the ambient temperature variation is eliminated by both thermocompensation and temperature control. The response of these sensors to hydrogen or methane in air is less than one second, w~~aut using a catalyst. Their use as gas chromato~aphy detectors is proved by revealing 10.8 ppm of oxygen in a nitrogen matrix, when hydrogen is used as eluent gas.
In recent years interest in gas detection has been increasing. This interest comes from two di~erent viewpoints. One refers to fiammable gas detection at concentration levels lower than the explosion limits. These concentrations are in the percent by volume range. The other refers to toxic gas detection at ~~~nt~tion levels lower than the limits of toxicity. These concentrations are in the ppm and sub-ppm by volume range. A reliable gas sensor should be sensitive, selective and stable. Significant efforts are made to fulfil these r~ujr~ents. Both physical and chemical sensors compete in this field. Different classes of gas sensors are now available: ~~~~~h~rn~~~~ se~~nd~tor structured, po~e~c, metal oxide, optical and ultrasonic. A very sensitive gas sensor has been developed as a gas chromatography detector [If. Here the ultrasound waves are propagated at one transducer and received at another. A phase meter monitor the signal received, which is sensitive to any change in the gas composition. A disadvantage of this sensor is the high cost and sophistication of the electronics necessary for good performances. Mecca and Bucnr [2] proposed a new ultrasonic method for gas detection which uses only one quartz resonator as a transducer. Based on this idea, Mecca and Tatar ]3] developed an ultrasonic hydrogen detector, which was further improved by I&O-S and Ghete PI.
This paper presents further developments of these achievements, allowmg the detection of whatever noncorrosive gas differing in ~tr~o~d vek~&ty from the reference gas,
The working principle of the tunable gas sensors is based on the Energy Transfer Model [5]. This model states that whenever a medium is in contact with the vibrating surface of a quartz resonator, they form a compound resonator, the frequency and vibration amplitude of which are greatly r&&d by the characteristics of the medium vibrating synchronously with the quartz resonator surface It is well known that shear waves do not propagate in gases. However, if compressional waves are generated by an X-cut quartz resonator, the ultrasonic longitudinal waves will propagate in the grounding gas. If this gas is induced to resonate at the same frequency as that of the quartz resonatar, then an important amount of vibration energy of the quartz resonator will be absorbed by the resonating gas. The resonance conditiott can easily be a~om~lish~ when standing waves are generated between the surface of the quartz ~sonator and the surfam of a parallel retlecting wall, Standing waves are generated whenever the dir&n= l, between the two parallel surfaces, is an integer of balf-wavelengths
@ 1993-- ElsevierSequoia.All rk&ts rawed
266 TABLE
I Ultrasound velocity in some common gases at 0 “C
Gas
“g (m/s)
Hydrogen Helium
1269.5 972
Methane Carbon monoxide Nitrogen Air Nitric oxide Oxygen Argon Nitrous oxide Carbon dioxide Sulfur dioxide Carbon disulfide
I= nA/2 = nv,&
432 337.1 331 331.36 325 317.2 307 261.8 258 216.2 189
(1)
where ug is the ultrasound velocity in the gas filling the resonator cavity and f4 is the frequency of the quartz resonator vibrating in a compressional mode. The vibration energy absorbed by the resonating gas can be of a signi~cant amount, inducing the complete damping of the quartz resonator vibrations. The condition for gas resonance, indicated by eqn. (l), reveals that, for the same quartz resonator vibrating in a compressional mode, the resonance depends on the nature of the gas, through the ultrasound velocity. This is given for some common gases in Table 1 [6]. There is a si~ificant difference in ultrasound velocity between various gases. This feature can be used for gas detection. Whenever the distance I between the surface of the quartz resonator and the surface of the reflecting wall is so adjusted to fulfil the resonance condition for a certain reference gas, the presence of a small amount of another gas, differing in ultrasound velocity, will alter the resonance, and a change in the vibration amplitude of the quartz resonator will be recorded. This vibration amplitude can be measured by rectifying the output r.f. voltage of the transistorized oscillator driven by the quartz resonator [7,8]. The effect of a small mount of hydrogen added to air flowing through the cavity between the quartz resonator surface and the reflecting wall is shown in Fig. 1. Here is illustrated the case of a resonant cavity so adjusted that the distance between the surface of a 1 MHz quartz resonator and the surface of the reflecting wall is I,, when air is flowing through the cavity. The recorded signal, related to the vibration amplitude of the quartz resonator is U,, smaller than the maximum signal U,, corresponding to the out-of-resonance condition, When air flowing through this cavity contains a small amount of hydrogen, the ultrasound velocity in such a mixture is higher than in pure air, and
GOP lmml
Fig. I. Hydrogen detection with a tunable gas sensor using a 1MHzqpartz resonator.
a significant change in the recorded signal will be revealed, the resulting voltage being U,. The gap between the quartz resonator surface and the reflecting wall surface can be adjusted to whatever length, close to the resonance condition, for whatever gas. Thus we can call such a sensor a ‘tunable gas sensor’. Whenever a gas, differing in ultrasound velocity is present in the gas on which the cavity was tuned, a measurable change in the rectified r.f. voltage, corresponding to the vibration amp~tude, will be recorded.
3. Thermocompensationof a tunable gas sensor The resonance condition (I) is affected by the temperature changes through cavity dilatation, according to the law I=&(1 +ar)
(2)
and through ultrasound velocity change, according to the law t’ = V*(1 + t/273.15)‘”
(3)
These two major effects can alter the reliability of a tunable gas sensor. A good temperature control of the detection cell is very important. However, the adverse effect of the temperature changes can be greatly diminished by thermocompensation. This is accomplished when the diIatation of the gap /,, follows the change of the resonance condition (1) through the temperature dependence of the ultrasound velocity. In order to match the dilatation of the gap to the change of the ultrasound velocity, a detection cell was designed, which is schematically shown in Fig. 2. The quartz resonator, with a nominal frequency of 1 MHz and a diameter of 2Omm (K~st~l-~erar~it~g Neckarbischofsheim GmbH) is accommodated within the body of the brass cell. To avoid mechanical energy losses, the quartz resonator is fixed in its nodal vibrational plane. The electrical connections to the evaporated Au/Q electrodes touch the quartz resonator in its nodal plane
261
Fig. 2. Schematic drawing of a ~erm~om~nsat~ sensor.
tunable gas
t PC1 Fig. 3. Effect of temperature on the apparent hydrogen concentration change when three lengths of ceramic cylinders (A, B and C) are used for a ~e~ocorn~nsa~d tunable gas sensor.
too. The gas enters the cell through a pipe penetrating the lid and exits through a pipe emerging from the body of the detector. A rubber gasket hermetically closes the cell. It also allows the lid to move, follo~ng the dilatation of the alurn~~ screws, which rest on the lid surface by means of some ceramic cylinders with very small dilatation. Their length is so calculated to match the dilatation of the gap to the ultrasound velocity change with a temperature change. As eqn. (3) is not a linear-like one, like the dilatation law, the best thermo~om~nsation is obtained in a narrow tem~ra~re range, as shown in Fig. 3 for the case of hydrogen detection in air.
The increasing use of hydrogen gas should not be considered as one without disadvantages. A hydrogen leak in the atmosphere should be avoided, because hydrogen when mixed with air in the ratio of 4.0074.2 vol.% is explosive [9]. For this reason, among others, it has become very irn~~ant to develop highly sensitive hydrogen sensors to prevent accidents due to hydrogen gas leakage, thus saving lives and equipment. Such sensors should allow continuous monitoring of the concentration of gases in the environment in a quantitative and qualitative way. Several classes of hydrogen sensors have been developed: ~miconductor, pyroel~t~c, piezoeleetric and el~tr~hemical devices. The colon feature of all these sensors is that they use a catalyst, mainly palladium, to selectively detect hydrogen. The main drawback of these sensors is the possibility of catalyst poisoning by sulfur dioxide, hydrogen sulfide, carbon monoxide, sulfur and others. Con~qu~tly they can work properly only in controlled atmospheres. It has already been shown that the working principle of a tunable gas sensor can be used for hydrogen detection [3,4]. Here new experimental results are given. Tuning the gap & to get the near-resonance condition for air, like in Fig. 1, it was possible to record the response of the sensor to various ~on~~ra~ions of hydrogen in air. The results are shown in Fig. 4, where the resonance deep is revealed in a rather large hydrogen concentration range, from 0.75 to 3.1 vol.% hydrogen in air. This is because of the strong vibration energy absorption by the resonating gas. A similar result was
5 UN1
2
1
4. Hydrogen and metbane detection in air In recent years, hydrog~ has become one of the most useful gases. Many industries such as chemical, metallurgical, food and others use hydrogen as a raw material.
Oo
12
3
L
5
6 COl-d%t
Fig. 4. Response of the tunable gas sensor to various concentrations of hydrogen in ait at 37 “CT.
268
.’
.’
/ i
---__A O 0
0.4
ae
1.2 1.6 conc.Wd
Fig. 7. Response of a tunable gas sensor to various concentrations of hydrogen and methane in air at 37 “C.
1
2
3
4
5
b
cont.P/PI
Fig. 5. Response of the tunable gas sensor to various concentrations of hydrogen in argon at 37 “C.
obtained for hydrogen-argon mixtures, when the gap 1, was tuned to get the near-resonance condition for ar-
gon. The sensor response for different concentrations of hydrogen in argon is shown in Fig. 5. Here the resonance deep is broader than in the case of air because of higher molecular weight and viscosity of the argon gas. The time response of such a sensor is very fast, depending on the necessary time to fill the detection volume of the cavity with the new concentration of the gas. This is revealed in Fig. 6 for various concentrations of hydrogen in air, at 37 “C.
The interest in methane detection comes from its wide domestic use. When mixed with air in the ratio of 5.00- 15.00 vol.%, it is also explosive [9]. As shown in Table 1, the ultrasound velocity in methane is different from that in air, allowing its detection with a tunable gas sensor. The expected sensitivity of a tunable gas sensor for methane detection in air will be smaller than that for hydrogen detection because the ultrasound velocity in methane is closer to that in air, than in the case of hydrogen. This is shown in Fig. 7, where the response of the tunable gas sensor to various concentrations of hydrogen and methane in air at 37 “C is revealed. The experimental results shown in Fig. 7 prove that the sensitivity of the tunable gas sensor for each of these gases is very high (0.01 vol.% for hydrogen and 0.018 vol.% for methane), allowing their detection in the environment at concentration levels much lower than the explosion limits.
5. Gas chromatography detector 1.6
1”
LU
t hid
Fig.6. Time response of the tunable gas Sensor to various con-
centrations of hydrogen in air at 37 “C.
A tunable gas sensor might also be used as a universal gas chromatography detector. The detection of various gases is based on the existing difference in ultrasound velocity between the eluent gas and the separated components of the gas sample. In order to check the performances of a tunable gas sensor as a detector for gas chromatography, the gap between the quartz resonator surface and the reflecting wall was mechanically tuned to near resonance condition for hydrogen, which was the eluent gas of the gas chromatograph. A gas sample containing 10.8 ppm oxygen in a nitrogen matrix was used (can mix No. 48 from Scott Analysed Gases). As shown in Fig. 8, the
269
It can detect inflammable gases, like hydrogen and methane in air, without using a catalyst, at concentration levels lower than the explosion limits. The time response of such a sensor is less than one second. When used as a universal gas chromatography detector, this sensor can be tuned for the eluent gas, revealing the separated gas components with a sensitivity higher than that of a thermal conductivity detector.
UWI 20 1( 16
12
6
References 4
I M. J. O’Brien,in R. L. Grob (ed.), Modern Pracrice of Gas
d 4
6
8
10
t ImInI
..
Fig. 8. Response of a tunable gas sensor when used as a gas chromatography detector at 37 “C.
separated oxygen peak is sensitively detected by a tunable gas sensor. The nitrogen peak is very large and discontinuous. This is because of the very large concentration of nitrogen, which causes the sweep of the whole resonance deep, including the situation of complete damping of the quartz resonator vibrations. When using a conventional thermal conductivity detector, for the same gas sample, the oxygen was not detected and the nurogen peak was much narrower. This proves the very high sensitivity of a tunable gas sensor when used as a gas chromatography detector.
6. Conclusions The resonance of a gas within a cavity can be used for the detection of whatever non-corrosive gas differing in ultrasound velocity from the reference gas on which the cavity was tuned. This sensor can be mechanically tuned to a near-resonance condition for whatever reference gas.
Chromatography, Wiley, New York, 1977, pp. 281-282. 2 V. Mecca and R. V. Bucur, A new method for detection of a dangerous gas in the atmosphere, Romanian Pafen! No. 76465 (1978). 3 V. Mecca and E. Tatar, Piemelectric hydrogen detector, Romanian Patent No. 76466 (1978). 4 V. Mecca and P. Ghete, Thermocompcnsated ultrasonic hydrogen detector, In?. .I. Hydrogen Energy, 9 (1984) 861 -864. 5 V. Mecca and R. V. Bucur, The mechanism of the interaction of thin films with resonating quartz crystal substrates: the Energy Transfer Model, Thin Solid Films, 60 (1979) 73-84. 6 C. D. Hodgman (ed.), Handbook of Chemistry and Physics, Chemical Rubber Publishing Co., Cleveland, OH, 1959, p. 2502. 7 V. Mecca and R. V. Bucur, The use of RF-voltage in quartz crystal microbalance measurements: application to non metallic films, J. Phys. E. Sci. Instrum., 7 (1974) 348-349. 8 V. M. Mecca, A new method of measuring the mass sensitive areas of quartz crystal resonators, .r. Phys. E. Sci. Instrum., 22 (1989) 59-61. 9 C. D. Hodgrnan (ed.), Handbook of Chemi&y and Physics, Chemical Rubber Co., Cleveland, OH, 1959, pp. 1927-1929.
Biography Vasile-Mihai Meceu was born in 1948 in Brasov. He graduated from the Physics Department of the ‘BabesBolyai’ University of Cluj-Napoca in 1972. He received his Ph.D. degree in 1979 on a subject concerning the use of the quartz crystal microbalance for hydrogen and deuterium interaction with palladium films. He is currently working in vibrating quartz sensors and thin film physics. He is also involved in hydrogen storage and detection.