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Ultrasonic techniques for detecting helium leaks
Sensors and Actuators B 71 (2000) 197±202
Ultrasonic techniques for detecting helium leaks Shuh-Haw Sheen*, Hual-Te Chien, Apostolos C. Raptis Energy Technology Division, Argonne National Laboratory, Argonne, IL, USA Received 21 September 1999; received in revised form 13 June 2000; accepted 17 July 2000
Abstract We have examined two ultrasonic techniques for their applications in helium leak detectors. In one technique, a surface acoustic wave (SAW) device is used to detect helium gas through changes of thermal conductivity in helium/air mixtures. The second technique measures variations of sound speed (SS) in such mixtures. Sensitivities and detection limits of both techniques were predicted with simple models and veri®ed by measurements. The SAW sensor and the SS techniques can detect helium leaks as small as 10ÿ4 and 10ÿ5 cm3/s, respectively. Published by Elsevier Science B.V. Keywords: Ultrasonics; SAW; Helium leak; Leak detection
1. Introduction The mass spectrometer (MS) has been the standard helium leak detector in common use for checking vacuum systems or ®nding component leaks. It provides high detection sensitivity and few false alarms. But an MS helium leak detector itself requires a high-vacuum system, which sometimes limits its applications. In particular, some industrial processes (such as leak-testing of automotive component) must detect and locate leaks in a production line. Thus, portable or a remote detection system becomes desirable. Photoacoustic techniques [1] may be used for detecting/ locating leaks remotely if an environmentally acceptable seed gas is used. Portable helium leak detectors are also available, based mostly on either mass spectrometry or hotwire thermal conductivity. Their disadvantages are long response time, poor stability, and high cost. Therefore, there is a need to develop advanced technologies for locating and quantifying leaks as small as 10ÿ6 standard cubic centimeters per second (scc/s) in a short detection time (a few seconds). This paper describes two novel techniques for detecting helium gas in air. One is a surface acoustic wave (SAW) technique and the other is a simple speed-of-sound method. A SAW gas sensor typically uses an active surface coating that is sensitive to the target gas. Detection sensitivity therefore depends on the interaction mechanism between the gas and the coating. For example, dipole/dipole inter* Corresponding author. Tel.: 1-630-252-7502; fax: 1-630-252-3250. E-mail address: [email protected] (S.-H. Sheen).
0925-4005/00/$ ± see front matter Published by Elsevier Science B.V. PII: S 0 9 2 5 - 4 0 0 5 ( 0 0 ) 0 0 6 1 6 - X
action is the main sensing mechanism for a SAW humidity sensor. But the noble gases, such as helium, do not interact chemically with most coating materials. To make an SAW helium detector, one must rely on other physical properties such as the thermal conductivity difference between air and helium; the thermal conductivity of helium is 5.7 times greater than that of air. Another distinct physical property is the speed of sound in helium, which is much higher than in air (1015 versus 346 m/s). In principle, one can quantify the volume percent of helium in air by measuring the speed of sound in a helium/air mixture. 2. SAW technique Conventional SAW chemical sensors [2] rely on chemically selective surface coatings to detect the targeted chemicals. During detection, changes in SAW phase velocity or resonance frequency are measured. These changes can be used to quantify the chemicals bonded to the coatings either chemically or physically. However, this sensing mechanism cannot be applied to the detection of helium, which is chemically inert and physically small. However, a SAWbased helium sensor can be designed to detect helium through a surface temperature change because helium gas has a much higher thermal conductivity than that of air. SAW devices that are sensitive to thermal effects have been applied to measurements of temperature [3,4], gas ¯ow rate [5], and concentrations of gases such as helium and hydrogen [6] in air. Typically, such a SAW thermal sensor uses an LiNbO3 piezoelectric substrate because of its linear tem-
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perature dependence and high temperature coef®cient (ÿ90 ppm/8C). During sensor operation, the substrate must be held at a higher temperature than the ambient or test medium temperature so that when the thermal conductivity of the medium changes, the substrate surface temperature (and therefore the resonant SAW frequency) changes proportionally. 2.1. Model prediction A heat-transfer model can be used to predict the theoretical sensitivity of the SAW technique when used to detect helium in air. The sensor is generally used under a constant ¯ow rate to minimize the ¯ow effect. In principle, three heattransfer processes occur: conduction, radiation, and convection. Because the temperature difference between the sensor surface and the air ¯ow is relatively low, only the convection process needs to be considered. Heat loss due to convection qc is attributed to natural and forced convection, as shown in Eq. (1): qc hn ÿ hf A Ts ÿ Ta
(1)
where A is the heated sensor surface area, Ts ÿ Ta the temperature difference between the sensor surface and the gas flow, and hn and hf are the natural and forced convection coefficients, respectively. For a dynamic process, the temperature change (DT) of the SAW sensor can be approximated by 4:187PH drs Cs ÿ drs Cs =hn hf t ÿ e DT hn hf A hn hf
(2)
where PH is the input power to the heater in watts, rs the density of LiNbO3 (4.64 g/cm3), Cs the specific heat capacity of LiNbO3 (0.1542 cal/8C g), d the thickness of LiNbO3, and where A is the surface area of LiNbO3. The sensor designed for this work has a thickness of 0.05 cm and a surface area of 0.16 cm2. If we assume that the thermal properties of an air/helium mixture are linearly proportional to the volume fractions of each gas, we have hn 0:54ka 1 ÿ x kH x3=4 1=4 980 Ts ÿ Ta r2a Ca 1 ÿ x r2H CH x Ta Lma 1 ÿ x mH x
(3)
and
the specific heat capacity of air (0.24 cal/8C g), CH the specific heat capacity of helium (1.242 cal/8C g), L the length of LiNbO3 (0.8 cm), x the volume fraction of helium, and where Vf is the flow velocity. From Eqs. (2)±(4), we can predict the performance of the SAW thermal sensor. Fig. 1 shows the estimated surface temperature changes at two ¯ow rates and under various helium concentrations. It is clear that higher helium concentrations give lower surface temperatures, and that an increase in ¯ow rate improves the response time of the sensor and also reduces the temperature change or detection sensitivity. Outputs from the SAW thermal sensor are the resonant frequency shifts that depend on the temperature coef®cient of LiNbO3. In Fig. 2, we show the model predictions of the frequency shifts after the sensor is exposed to air/helium mixtures for 10 s versus helium concentration at three ¯ow rates. Based on the model predictions, the SAW sensor may be able to detect helium leaks of >5 10ÿ4 scc/s because the signal-to-noise ratio limits the sensitivity of the frequency shift measurement to >500 Hz. 2.2. SAW sensor design The SAW sensor is a resonator design of a delay-line. The sensor was fabricated by SRD Corp. (Orono, ME) and its basic parameters are as follows: Substrate Dimensions Resonant frequency Heater Sensitivity
1288 Y-cut LiNbO3 2 mm 8 mm 0:5 mm 243 MHz On top surface, constant current, estimated surface temperature 90 C 76 ppm/8C
Fig. 3 shows a SAW sensor assembly that uses two SAW sensors of similar resonant frequencies. The dual-sensor design is used to eliminate the ¯ow rate effect. One sensor is exposed to air/helium ¯ow, the other to ambient air ¯ow. Thus, the difference in temperature changes of the two sensors can be directly correlated with helium concentration. 3. Results and discussion
hf 0:6795ka 1 ÿ x kH x2=3
1=2 Vf ra 1
1=2
ÿ x rH x L1=2 ma 1
Ca 1 ÿ x CH x
ÿ x mH x1=6
1=3
(4)
where ka is the thermal conductivity of air (0:0576 10ÿ3 cal/cm s 8C), kH the thermal conductivity of helium (0:343 10ÿ3 cal/cm s 8C), ma the viscosity of air (3:89 10ÿ4 P), mH the viscosity of helium (3:81 10ÿ4 P), ra the air density (1:186 10ÿ3 g/cm3 at 258C), rH the helium density (1:636 10ÿ4 g/cm3 at 258C), Ca
Both single-sensor and differential sensor heads were tested. The differential sensor head has two separate ¯ow channels, one of which is used as the reference. The sensor was controlled by a SAW instrument (SAW PRO-250 manufactured by Microsensor Systems Inc., Bowling Green, KY), and data were collected and analyzed by a PC computer. Helium gas was mixed with air, and the helium volume concentration was controlled by two mass ¯ow controllers (MKS instrument Inc., Andover, MA) that provide an accuracy of 5%. The surface temperature of the SAW device
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199
Fig. 1. Estimated temperature changes of SAW thermal sensor at two flow rates for helium concentrations up to 0.1 vol.%.
was not measured but was controlled by the heater voltage. Fig. 4 shows sensor responses to pure helium under a constant ¯ow rate (0.03 cm3/s) at two heater voltages. Frequency shifts of 7 kHz were detected at this low ¯ow rate. The higher heater voltage (2 V) gives a larger output but requires a longer time to reach steady-state temperature. Fig. 5 shows the sensor response to two helium concentrations at constant total ¯ow rate (0.03 cm3/s). At this ¯ow
rate, the sensor showed a reasonable response time (<1 min) to variation of helium concentration in air, but a relatively longer time was required to recover the baseline. The baseline drift was signi®cant, suggesting that frequent calibration will be required. Fig. 6 shows measured frequency shifts for various helium concentrations; diminishing sensitivity at low concentration is observed, which causes deviation from linearity.
Fig. 2. Predicted resonant frequency shifts of SAW sensor vs. helium concentration up to 0.1 vol.% for three flow rates after the sensor is exposed to flow for 10 s.
Fig. 3. Helium leak detector with dual SAW sensors.
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Fig. 4. Sensor response curves with pure helium for two input heater voltages.
Fig. 5. Sensor response curves at two helium concentrations.
In principle, the approach of using dual SAW sensors and measuring the differential output should provide higher sensitivity. A small air pump was added to the differential sensor head so that the ¯ow rate would be the same in both ¯ow channels. This arrangement is particularly useful when the sensor is used as a portable helium leak detector. The reference sensor responds to temperature and composition of the ambient air, while the other sensor detects the gas
Fig. 6. Frequency shift vs. helium concentration.
Fig. 7. Sensor response under scanning arrangement at a high leak rate of 6 cm3/min.
mixture drawn in from a sniffer. To locate a leak, one can scan the sniffer across the leak. Figs. 7 and 8 show the sensor's responses, given in frequency difference, when it scans across simulated helium leaks of 6 and 0.5 cm3/min, respectively. Both tests show that leak location can be promptly identi®ed by the scanning method and that the location is better de®ned for the higher leak rate (Fig. 7). The two peaks shown in Fig. 7 were obtained from opposite scan directions. Reproducibility is clearly demonstrated. The polarity of the change of the difference frequency depends on the choice of reference sensor. Although we have demonstrated that a SAW microsensor can be used to measure helium concentration in air based on the thermal conductivity difference between helium and air, the technique has certain limitations and problems: Poor detection limit Ð smallest detectable leak is 5 10ÿ4 scc/s. SAW sensor fabrication problems Ð the primary problem is in the heater arrangement. Connecting wires cannot sustain high flow rates. Sensor-matching problems Ð each SAW sensor has its own resonant frequency, so a matched pair cannot be
Fig. 8. Sensor response under scanning arrangement at a leak rate of 0.5 cm3/min.
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201
obtained; thus, it is difficult to control the baseline drift in the differential sensor head approach. 4. Speed-of-sound technique This approach is based on measuring the isentropic speed of sound and has been applied to binary [7] and ternary [8] gas analyses. The isentropic speed of sound in a gas is given by r gRT (5) V M where g is the heat capacity ratio, R the gas constant, T the absolute temperature, and where M is the molecular weight of the gas. Based on Eq. (5), the speed of sound in helium is about three times faster than that in air. This difference determines the sensitivity of detecting helium in air. Either the pulse-echo or pitch-catch technique is used to measure the speed of sound. Typically, a train of echoes or reflections is detected if a narrow acoustic cavity is used. To enhance detection sensitivity, one can monitor the higher-order reflections, which in effect increases the path length. For example, if one monitors the time-of-flight (TOF) of the fifth reflection in a 0.25 in.-diameter cavity, the detection sensitivity of helium gas in air is 2.2 ms/1% of He in the air. In principle, sensitivity can be further increased if a higherorder reflection is chosen. In practice, however, because of the attenuation of high-frequency sound in a gas, the effective path length is limited by the signal-to-noise ratio. To estimate the detection sensitivity of helium in air, we derived a simple linear model that assumes that the speed of sound in a helium/air mixture is a linear combination of the volume fractions of two gases. The assumption can be expressed by Eq. (6), in which x represents the volume fraction of helium in the mixture: VM xVHe 1 ÿ xVair
(6)
where VM, VHe, and Vair are sound speeds (SS) in the mixture, helium, and air, respectively. From Eq. (6), we obtain x in terms of the TOF difference (Dt): x
Dt VHe =Vair ÿ 1 d=Vair ÿ Dt
(7)
If the electronics can attain a time resolution of 10 ns, the estimated detection sensitivity of helium in air, with d 11:43 cm, is 1:5 10ÿ5 ppm. This sensitivity does not directly represent the helium leakage rate. To estimate the detectable leakage rate, one must consider the effective sensing volume and the time that the sensor is over the leak region. 4.1. Sensor design Fig. 9 shows a laboratory prototype of the SS sensor. It consists of two 0.5 MHz transducers, a gas pump, and a
Fig. 9. Prototype sound speed sensor for detecting He in air.
cavity in which sound waves propagate. The critical design parameters of the sensor are cavity size and ¯ow rate; the former determines the sensitivity and the latter controls the response time. The prototype contains a cylindrical cavity 0.25 in. in diameter and 0.5 in. long; hence, the cavity volume is 0.4 cm3. The small cavity was chosen for higher sensitivity and rapid response time. The effective path length is determined by the re¯ection peak selected from the re¯ection pulse train for the TOF analysis. The accuracy of the TOF measurement is not affected by the pulse energy loss at the transducer surface. 5. Results and discussion The transducers used for the SS sensor are 0.5 MHz air transducers (Ultran Laboratories Inc.). The sensor can also operate in pulse-echo mode with one transducer. The present data were obtained by using two transducers that operate in pitch-catch mode. For higher sensitivity, we monitor the ®fth re¯ection for the TOF measurement. TOF was measured by setting a ®xed trigger level and detecting the cross-over time of the ®fth re¯ection. Fig. 10 shows the TOF curves detected by the SS sensor for helium and SF6 leaking at 0.5 and 0.1 scc/min, respectively. Note that the TOFs change for helium opposite that for SF6; this is because the isentropic SS in SF6 is lower than that of the reference SS in air. As indicated in ¯uid acoustic work [7], it is critical in gas sensing to maintain a constant ¯ow rate. This is achieved by controlling the pump speed so that, for a given helium concentration, an equilibrium TOF can be reached quickly. Fig. 11 shows the TOF shifts over a range of helium leak
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Fig. 10. TOF curves obtained with the SS sensor for helium and SF6 leaking at 0.5 and 0.1 scc/min, respectively.
by using a simple pulse-echo or pitch-catch technique. The SAW device uses a resonator on an LiNbO3 substrate. A dual-sensor arrangement was used for the helium leak detector. Fast response and the capability to detect helium leaks of >10ÿ4 cm3/s were demonstrated for the detector using the thermal-effect SAW technology. The SS technique can also be applied to helium leak detection. Sensitivity of the SS technique can be maximized by monitoring higherorder re¯ections (or echoes). By using the dual-cavity design and measuring the differential output from the dual sensors, one can reduce the uncertainty due to ambient air composition. The SS helium leak detector can achieve a detection sensitivity of 10ÿ5 ppm and a leak rate, into open air, of 10ÿ3 scc/s. Acknowledgements The work described in this paper was supported by the National Center for Manufacturing Science (NCMS), Ann Arbor, MI, USA. References
Fig. 11. TOF changes as a function of leakage rate for detecting helium in air.
rates. The SS sensor shows a rather linear response to low helium leakage rates and leakage rates of >10ÿ5 scc/s can be detected. 6. Conclusions Two ultrasonic techniques were examined for use in a low-cost, fast-response portable helium leak detector. In one technique, the thermal conductivity change of the helium/air mixture is measured with a heated SAW device. The other monitors the isentropic speed of sound change in the mixture
[1] M. Sigrist, Environmental and chemical trace gas analysis by photoacoustic methods, in: A. Mandelis (Ed.), Principles and Perspectives of Photothermal and Photoacoustic Phenomena, Elsevier, Amsterdam, 1992. [2] C.G. Fox, J.F. Alder, Surface acoustic wave sensors for atmospheric gas monitoring, in: P.T. Moseley, J. Norris, D. Williams (Eds.), Techniques and Mechanisms in Gas Sensing, Adam Hilger, Bristol, 1991. [3] F. Moller, J. Kuhn, SAW resonator temperature sensor, Sens. Actuators A 30 (1002) 73±75. [4] M. Hoummady, D. Hauden, Acoustic wave thermal sensitivity: temperature sensors and temperature compensation in microsensors, Sens. Actuators A 44 (1994) 177±182. [5] S.G. Joshi, Surface-acoustic-wave (SAW) flow sensor, IEEE Trans. Ultrasonics, Ferroelectrics, Frequency Control 38 (2) (1991) 148± 154. [6] V.I. Anisimkin, M. Penza, V.A. Osipenko, L. Vasanelli, Gas thermal conductivity sensor based on SAW, IEEE Trans. Ultrasonics, Ferroelectrics, Frequency Control 42 (6) (1995) 978±980. [7] L. Zipser, Fluidic-acoustic gas sensors, Sens. Actuators B 7 (1992) 592±595. [8] L. Zipser, F. Wachter, Acoustic sensor for ternary gas analysis, Sens. Actuators B 26/27 (1995) 195±198.
Ultrasonic techniques for detecting helium leaks Shuh-Haw Sheen*, Hual-Te Chien, Apostolos C. Raptis Energy Technology Division, Argonne National Laboratory, Argonne, IL, USA Received 21 September 1999; received in revised form 13 June 2000; accepted 17 July 2000
Abstract We have examined two ultrasonic techniques for their applications in helium leak detectors. In one technique, a surface acoustic wave (SAW) device is used to detect helium gas through changes of thermal conductivity in helium/air mixtures. The second technique measures variations of sound speed (SS) in such mixtures. Sensitivities and detection limits of both techniques were predicted with simple models and veri®ed by measurements. The SAW sensor and the SS techniques can detect helium leaks as small as 10ÿ4 and 10ÿ5 cm3/s, respectively. Published by Elsevier Science B.V. Keywords: Ultrasonics; SAW; Helium leak; Leak detection
1. Introduction The mass spectrometer (MS) has been the standard helium leak detector in common use for checking vacuum systems or ®nding component leaks. It provides high detection sensitivity and few false alarms. But an MS helium leak detector itself requires a high-vacuum system, which sometimes limits its applications. In particular, some industrial processes (such as leak-testing of automotive component) must detect and locate leaks in a production line. Thus, portable or a remote detection system becomes desirable. Photoacoustic techniques [1] may be used for detecting/ locating leaks remotely if an environmentally acceptable seed gas is used. Portable helium leak detectors are also available, based mostly on either mass spectrometry or hotwire thermal conductivity. Their disadvantages are long response time, poor stability, and high cost. Therefore, there is a need to develop advanced technologies for locating and quantifying leaks as small as 10ÿ6 standard cubic centimeters per second (scc/s) in a short detection time (a few seconds). This paper describes two novel techniques for detecting helium gas in air. One is a surface acoustic wave (SAW) technique and the other is a simple speed-of-sound method. A SAW gas sensor typically uses an active surface coating that is sensitive to the target gas. Detection sensitivity therefore depends on the interaction mechanism between the gas and the coating. For example, dipole/dipole inter* Corresponding author. Tel.: 1-630-252-7502; fax: 1-630-252-3250. E-mail address: [email protected] (S.-H. Sheen).
0925-4005/00/$ ± see front matter Published by Elsevier Science B.V. PII: S 0 9 2 5 - 4 0 0 5 ( 0 0 ) 0 0 6 1 6 - X
action is the main sensing mechanism for a SAW humidity sensor. But the noble gases, such as helium, do not interact chemically with most coating materials. To make an SAW helium detector, one must rely on other physical properties such as the thermal conductivity difference between air and helium; the thermal conductivity of helium is 5.7 times greater than that of air. Another distinct physical property is the speed of sound in helium, which is much higher than in air (1015 versus 346 m/s). In principle, one can quantify the volume percent of helium in air by measuring the speed of sound in a helium/air mixture. 2. SAW technique Conventional SAW chemical sensors [2] rely on chemically selective surface coatings to detect the targeted chemicals. During detection, changes in SAW phase velocity or resonance frequency are measured. These changes can be used to quantify the chemicals bonded to the coatings either chemically or physically. However, this sensing mechanism cannot be applied to the detection of helium, which is chemically inert and physically small. However, a SAWbased helium sensor can be designed to detect helium through a surface temperature change because helium gas has a much higher thermal conductivity than that of air. SAW devices that are sensitive to thermal effects have been applied to measurements of temperature [3,4], gas ¯ow rate [5], and concentrations of gases such as helium and hydrogen [6] in air. Typically, such a SAW thermal sensor uses an LiNbO3 piezoelectric substrate because of its linear tem-
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perature dependence and high temperature coef®cient (ÿ90 ppm/8C). During sensor operation, the substrate must be held at a higher temperature than the ambient or test medium temperature so that when the thermal conductivity of the medium changes, the substrate surface temperature (and therefore the resonant SAW frequency) changes proportionally. 2.1. Model prediction A heat-transfer model can be used to predict the theoretical sensitivity of the SAW technique when used to detect helium in air. The sensor is generally used under a constant ¯ow rate to minimize the ¯ow effect. In principle, three heattransfer processes occur: conduction, radiation, and convection. Because the temperature difference between the sensor surface and the air ¯ow is relatively low, only the convection process needs to be considered. Heat loss due to convection qc is attributed to natural and forced convection, as shown in Eq. (1): qc hn ÿ hf A Ts ÿ Ta
(1)
where A is the heated sensor surface area, Ts ÿ Ta the temperature difference between the sensor surface and the gas flow, and hn and hf are the natural and forced convection coefficients, respectively. For a dynamic process, the temperature change (DT) of the SAW sensor can be approximated by 4:187PH drs Cs ÿ drs Cs =hn hf t ÿ e DT hn hf A hn hf
(2)
where PH is the input power to the heater in watts, rs the density of LiNbO3 (4.64 g/cm3), Cs the specific heat capacity of LiNbO3 (0.1542 cal/8C g), d the thickness of LiNbO3, and where A is the surface area of LiNbO3. The sensor designed for this work has a thickness of 0.05 cm and a surface area of 0.16 cm2. If we assume that the thermal properties of an air/helium mixture are linearly proportional to the volume fractions of each gas, we have hn 0:54ka 1 ÿ x kH x3=4 1=4 980 Ts ÿ Ta r2a Ca 1 ÿ x r2H CH x Ta Lma 1 ÿ x mH x
(3)
and
the specific heat capacity of air (0.24 cal/8C g), CH the specific heat capacity of helium (1.242 cal/8C g), L the length of LiNbO3 (0.8 cm), x the volume fraction of helium, and where Vf is the flow velocity. From Eqs. (2)±(4), we can predict the performance of the SAW thermal sensor. Fig. 1 shows the estimated surface temperature changes at two ¯ow rates and under various helium concentrations. It is clear that higher helium concentrations give lower surface temperatures, and that an increase in ¯ow rate improves the response time of the sensor and also reduces the temperature change or detection sensitivity. Outputs from the SAW thermal sensor are the resonant frequency shifts that depend on the temperature coef®cient of LiNbO3. In Fig. 2, we show the model predictions of the frequency shifts after the sensor is exposed to air/helium mixtures for 10 s versus helium concentration at three ¯ow rates. Based on the model predictions, the SAW sensor may be able to detect helium leaks of >5 10ÿ4 scc/s because the signal-to-noise ratio limits the sensitivity of the frequency shift measurement to >500 Hz. 2.2. SAW sensor design The SAW sensor is a resonator design of a delay-line. The sensor was fabricated by SRD Corp. (Orono, ME) and its basic parameters are as follows: Substrate Dimensions Resonant frequency Heater Sensitivity
1288 Y-cut LiNbO3 2 mm 8 mm 0:5 mm 243 MHz On top surface, constant current, estimated surface temperature 90 C 76 ppm/8C
Fig. 3 shows a SAW sensor assembly that uses two SAW sensors of similar resonant frequencies. The dual-sensor design is used to eliminate the ¯ow rate effect. One sensor is exposed to air/helium ¯ow, the other to ambient air ¯ow. Thus, the difference in temperature changes of the two sensors can be directly correlated with helium concentration. 3. Results and discussion
hf 0:6795ka 1 ÿ x kH x2=3
1=2 Vf ra 1
1=2
ÿ x rH x L1=2 ma 1
Ca 1 ÿ x CH x
ÿ x mH x1=6
1=3
(4)
where ka is the thermal conductivity of air (0:0576 10ÿ3 cal/cm s 8C), kH the thermal conductivity of helium (0:343 10ÿ3 cal/cm s 8C), ma the viscosity of air (3:89 10ÿ4 P), mH the viscosity of helium (3:81 10ÿ4 P), ra the air density (1:186 10ÿ3 g/cm3 at 258C), rH the helium density (1:636 10ÿ4 g/cm3 at 258C), Ca
Both single-sensor and differential sensor heads were tested. The differential sensor head has two separate ¯ow channels, one of which is used as the reference. The sensor was controlled by a SAW instrument (SAW PRO-250 manufactured by Microsensor Systems Inc., Bowling Green, KY), and data were collected and analyzed by a PC computer. Helium gas was mixed with air, and the helium volume concentration was controlled by two mass ¯ow controllers (MKS instrument Inc., Andover, MA) that provide an accuracy of 5%. The surface temperature of the SAW device
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199
Fig. 1. Estimated temperature changes of SAW thermal sensor at two flow rates for helium concentrations up to 0.1 vol.%.
was not measured but was controlled by the heater voltage. Fig. 4 shows sensor responses to pure helium under a constant ¯ow rate (0.03 cm3/s) at two heater voltages. Frequency shifts of 7 kHz were detected at this low ¯ow rate. The higher heater voltage (2 V) gives a larger output but requires a longer time to reach steady-state temperature. Fig. 5 shows the sensor response to two helium concentrations at constant total ¯ow rate (0.03 cm3/s). At this ¯ow
rate, the sensor showed a reasonable response time (<1 min) to variation of helium concentration in air, but a relatively longer time was required to recover the baseline. The baseline drift was signi®cant, suggesting that frequent calibration will be required. Fig. 6 shows measured frequency shifts for various helium concentrations; diminishing sensitivity at low concentration is observed, which causes deviation from linearity.
Fig. 2. Predicted resonant frequency shifts of SAW sensor vs. helium concentration up to 0.1 vol.% for three flow rates after the sensor is exposed to flow for 10 s.
Fig. 3. Helium leak detector with dual SAW sensors.
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Fig. 4. Sensor response curves with pure helium for two input heater voltages.
Fig. 5. Sensor response curves at two helium concentrations.
In principle, the approach of using dual SAW sensors and measuring the differential output should provide higher sensitivity. A small air pump was added to the differential sensor head so that the ¯ow rate would be the same in both ¯ow channels. This arrangement is particularly useful when the sensor is used as a portable helium leak detector. The reference sensor responds to temperature and composition of the ambient air, while the other sensor detects the gas
Fig. 6. Frequency shift vs. helium concentration.
Fig. 7. Sensor response under scanning arrangement at a high leak rate of 6 cm3/min.
mixture drawn in from a sniffer. To locate a leak, one can scan the sniffer across the leak. Figs. 7 and 8 show the sensor's responses, given in frequency difference, when it scans across simulated helium leaks of 6 and 0.5 cm3/min, respectively. Both tests show that leak location can be promptly identi®ed by the scanning method and that the location is better de®ned for the higher leak rate (Fig. 7). The two peaks shown in Fig. 7 were obtained from opposite scan directions. Reproducibility is clearly demonstrated. The polarity of the change of the difference frequency depends on the choice of reference sensor. Although we have demonstrated that a SAW microsensor can be used to measure helium concentration in air based on the thermal conductivity difference between helium and air, the technique has certain limitations and problems: Poor detection limit Ð smallest detectable leak is 5 10ÿ4 scc/s. SAW sensor fabrication problems Ð the primary problem is in the heater arrangement. Connecting wires cannot sustain high flow rates. Sensor-matching problems Ð each SAW sensor has its own resonant frequency, so a matched pair cannot be
Fig. 8. Sensor response under scanning arrangement at a leak rate of 0.5 cm3/min.
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201
obtained; thus, it is difficult to control the baseline drift in the differential sensor head approach. 4. Speed-of-sound technique This approach is based on measuring the isentropic speed of sound and has been applied to binary [7] and ternary [8] gas analyses. The isentropic speed of sound in a gas is given by r gRT (5) V M where g is the heat capacity ratio, R the gas constant, T the absolute temperature, and where M is the molecular weight of the gas. Based on Eq. (5), the speed of sound in helium is about three times faster than that in air. This difference determines the sensitivity of detecting helium in air. Either the pulse-echo or pitch-catch technique is used to measure the speed of sound. Typically, a train of echoes or reflections is detected if a narrow acoustic cavity is used. To enhance detection sensitivity, one can monitor the higher-order reflections, which in effect increases the path length. For example, if one monitors the time-of-flight (TOF) of the fifth reflection in a 0.25 in.-diameter cavity, the detection sensitivity of helium gas in air is 2.2 ms/1% of He in the air. In principle, sensitivity can be further increased if a higherorder reflection is chosen. In practice, however, because of the attenuation of high-frequency sound in a gas, the effective path length is limited by the signal-to-noise ratio. To estimate the detection sensitivity of helium in air, we derived a simple linear model that assumes that the speed of sound in a helium/air mixture is a linear combination of the volume fractions of two gases. The assumption can be expressed by Eq. (6), in which x represents the volume fraction of helium in the mixture: VM xVHe 1 ÿ xVair
(6)
where VM, VHe, and Vair are sound speeds (SS) in the mixture, helium, and air, respectively. From Eq. (6), we obtain x in terms of the TOF difference (Dt): x
Dt VHe =Vair ÿ 1 d=Vair ÿ Dt
(7)
If the electronics can attain a time resolution of 10 ns, the estimated detection sensitivity of helium in air, with d 11:43 cm, is 1:5 10ÿ5 ppm. This sensitivity does not directly represent the helium leakage rate. To estimate the detectable leakage rate, one must consider the effective sensing volume and the time that the sensor is over the leak region. 4.1. Sensor design Fig. 9 shows a laboratory prototype of the SS sensor. It consists of two 0.5 MHz transducers, a gas pump, and a
Fig. 9. Prototype sound speed sensor for detecting He in air.
cavity in which sound waves propagate. The critical design parameters of the sensor are cavity size and ¯ow rate; the former determines the sensitivity and the latter controls the response time. The prototype contains a cylindrical cavity 0.25 in. in diameter and 0.5 in. long; hence, the cavity volume is 0.4 cm3. The small cavity was chosen for higher sensitivity and rapid response time. The effective path length is determined by the re¯ection peak selected from the re¯ection pulse train for the TOF analysis. The accuracy of the TOF measurement is not affected by the pulse energy loss at the transducer surface. 5. Results and discussion The transducers used for the SS sensor are 0.5 MHz air transducers (Ultran Laboratories Inc.). The sensor can also operate in pulse-echo mode with one transducer. The present data were obtained by using two transducers that operate in pitch-catch mode. For higher sensitivity, we monitor the ®fth re¯ection for the TOF measurement. TOF was measured by setting a ®xed trigger level and detecting the cross-over time of the ®fth re¯ection. Fig. 10 shows the TOF curves detected by the SS sensor for helium and SF6 leaking at 0.5 and 0.1 scc/min, respectively. Note that the TOFs change for helium opposite that for SF6; this is because the isentropic SS in SF6 is lower than that of the reference SS in air. As indicated in ¯uid acoustic work [7], it is critical in gas sensing to maintain a constant ¯ow rate. This is achieved by controlling the pump speed so that, for a given helium concentration, an equilibrium TOF can be reached quickly. Fig. 11 shows the TOF shifts over a range of helium leak
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Fig. 10. TOF curves obtained with the SS sensor for helium and SF6 leaking at 0.5 and 0.1 scc/min, respectively.
by using a simple pulse-echo or pitch-catch technique. The SAW device uses a resonator on an LiNbO3 substrate. A dual-sensor arrangement was used for the helium leak detector. Fast response and the capability to detect helium leaks of >10ÿ4 cm3/s were demonstrated for the detector using the thermal-effect SAW technology. The SS technique can also be applied to helium leak detection. Sensitivity of the SS technique can be maximized by monitoring higherorder re¯ections (or echoes). By using the dual-cavity design and measuring the differential output from the dual sensors, one can reduce the uncertainty due to ambient air composition. The SS helium leak detector can achieve a detection sensitivity of 10ÿ5 ppm and a leak rate, into open air, of 10ÿ3 scc/s. Acknowledgements The work described in this paper was supported by the National Center for Manufacturing Science (NCMS), Ann Arbor, MI, USA. References
Fig. 11. TOF changes as a function of leakage rate for detecting helium in air.
rates. The SS sensor shows a rather linear response to low helium leakage rates and leakage rates of >10ÿ5 scc/s can be detected. 6. Conclusions Two ultrasonic techniques were examined for use in a low-cost, fast-response portable helium leak detector. In one technique, the thermal conductivity change of the helium/air mixture is measured with a heated SAW device. The other monitors the isentropic speed of sound change in the mixture
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