2012 Sixth International Conference on Sensing Technology (ICST)
Gas sensing using acoustic attenuation with improved resolution Ajit Singh
Prof. Maringanti Radhakrishna
Electronic Division Indian Institute of Information Technology Allahabad, India Email:
[email protected]
Electronic Division Indian Institute of Information Technology Allahabad, India Email:
[email protected]
Abstract—Sensors being used for gas sensing in underground coal mine require frequent recalibration due to highly humid and toxic environment prevalent in UG coal mines. If we intend to deploy a WSN which can continuously monitor the UG mine environment then we require sensors which are able to withstand the environment of UG coal mine. Acoustics gas sensors are more rugged and would be able to work for prolonged period in UG coal mine environment. Acoustic sensors work on the principle that acoustic signal velocity and amplitude are affected by changing composition of air. Attenuation of acoustics waves in presence of a target gas can be used to detect its presence and give its concentration level. We present an acoustic signal processing approach which can measure attenuation of acoustic signal with improved resolution. Proposed approach takes advantage of the fact that when triggered with a transmitted pulse of acoustic sensor resonance frequency, the receiver output is a damped ringing waveform. We measure the area under envelop of the received waveform to provide increased resolution.
I. I NTRODUCTION Prolonged and continuous gas sensing in UG coal mines throws a unique set of challenges. Currently used gas sensors are not suitable for above as highly humid and toxic environment prevalent in UG coal mines makes them inoperable after a certain period of time. To be useful the sensors therefore have to be brought to surface for restoration of their sensing ability and recalibrated. In UG coal mine galleries two sensing scenarios are visualized for gas sensors. In the first scenario they will be used to detect a leak of target gas from a point source. In second scenario they will measure the concentration level of target gas, where leaked gas source is not known, it could be slowly leaking from multiple points and therefore it has diffused(assumed evenly) in the environment air. Gas sensor need to detect the gas leakage in the first case, whereas it has to detect slow build up gas concentration level in the second case. Currently used electrochemical, pellistors and Infra Red gas sensors suffers from many limitation, for prolonged and continuous gas sensing in UG coal mines [10], [11]. They can be said to be point sensor i.e. they are sensitive to the gas concentration level in their close vicinity. For the first scenario gas sensor need to be properly placed to be able to sense the leakage. In second scenario, it is expected that leaking gas diffuse in the tunnel air evenly and quickly and therefore measurement made at any location in neighbourhood is correct
978-1-4673-2248-5/12/$31.00 ©2012 IEEE
reflection of current gas concentration level. Characteristic of gas leakage in coal mine is different from these assumptions. Leaking methane being lighter than air will diffuse towards the roof forming a thin layer [1]. Carbon dioxide being heavier than air will diffuse towards the floor. Assumption that leaked gas diffuses quickly is also not appropriate. Rate of diffusion will depend on volume of the gas being leaked. So a sensing node placed at fixed position would not be able to detect a gas leak or detect it only after high concentration of leaked gas have been developed at places. Therefore a sensing system is desired which should have a wider area of coverage. In contrast to existing sensors, an acoustic sensor is sensitive to gas composition change in a wider area as acoustic beam travel from transmitter to receiver traversing through air. It is an area sensor. It can be therefore used to monitor gas leakage in large area, the distance between transmitting and receiving sensors. Acoustics based gas sensor have been used in the past, some for specifically sensing gas concentration level in underground coal mines like methane whistle, used in German coal mines. Acoustic phenomena that are used for gas sensing are acoustic resonance [2], surface acoustic waves [9], acoustic attenuation [6], [12] and speed of sound [14]. Gases in a mixture can be identified using acoustic attenuation [3], [6], [13]. Prototypes for sensing acoustic attenuation for gas mixture were evaluated [3], [7] establishing that using acoustic attenuation for identifying change in gas composition is a viable option. For analysis a controlled environment with stable temperature and humidity condition was designed. Acoustic resonance and SAW based sensors are even more accurate [2], [9], but there working is strongly affected by stability of the environment in the enclosing they are placed into. Measuring acoustic attenuation with sufficient accuracy is a challenge and this challenge increases if measurement are not being done in controlled environment, which would be required if acoustic sensors are to be used in real life situations. Our objective is to evaluate suitability of acoustic attenuation parameter for gas sensing when transceiver pairs are placed larger distance apart, of the order of few meters. We are aware that there won’t be tight control over temperature and humidity parameters, correspondingly measurements being made will have larger errors. This paper analyses whether
543
acoustic attenuation can give sufficient information that it can be used in improving the accuracy of acoustically sensing leak or presence of gases. Beside attenuation, other acoustic parameter will also be used, but in this paper only acoustic attenuation change is being evaluated. Another significant way in which our analysis is different from existing work is that, the concentration levels of gases which are used in existing analysis are an order of magnitude more than what will be required to be detected in UG coal mines. In first scenario the change in concentration level is going to vary depending on distance of sensing unit and the leak rate. Still it would be in the range of 5-10% by volume averaged over the distance between sensing units. In Second scenario, the change in concentration level that has to be detected in UG coal mine would be even smaller, e.g. 2% (lower flammable limit) by volume change in concentration level for methane with a minimal resolution of 0.25% by volume. Therefore there is a need to measure change in acoustic parameters with improved resolution. Acoustic attenuation in itself, for these very low concentration level changes would not be a reliable parameter for identifying and predicting gas leaks, but they can be used to increase the confidence in the prediction made using other acoustic parameters like time of flight. Acoustic sensing can only be used for gases whose affect on acoustic signal is significant like methane,Carbon dioxide,ammonia etc in comparison to air. In this paper acoustic signal attenuation in methane and carbon dioxide is measured. Methane is the prime gas whose concentration level has to be monitored in UG coal mine. To improve the resolution instead of measuring peak to peak deviation in amplitude of received signal, received signal intensity is measured by measuring area under curve of the received signal waveform. Affect of environmental parameters like temperature and humidity has to be considered in the analysis during measurements of acoustic attenuation. Change due to these parameters can be used to improve confidence in predictions. Fortunately in the UG coal mine environment which are highly humid, humidity level more or less remains constant which is close to 100%, so effect of humidity can be readily pre calculated and included in prediction being made. Also in Underground environment temperature is also stable in a narrow range or fluctuate very slowly. II. T EST S ETUP A test set-up consisting of a ultrasonic 40 kHz transmitter and receiver sensor placed fix distance apart facing each other is designed for measurement. To evaluate for first scenario a syringe is filled with methane or Carbon dioxide and gas is injected in the path between receiver and transmitter to mimic behaviour of a gas leak. For second scenario after injection of measured amount of gas, a fan is used to mix the gas evenly in the chamber. After being thoroughly mixed with the air, methane or Carbon dioxide does not quickly segregate and form a layer at the ceiling or the floor of test chamber [1].
Fig. 1.
Fig. 2.
Received Signal processing
Squared received Signal(Blue) and its envelop(Red)
For making a measurement, transmitting unit is excited with 10 square pulse of 40 kHz frequency. The signal traverse through the air and then is sensed by the receiver. Received signal is filtered using 500 kHz anti aliasing filter and digitized. After digitization acquired signal is passed through a digital 35 kHz high pass filter (figure 1). For real time measurement of area under curve of received signal a FPGA based parallel implementation of signal processing algorithm is used. Objective of using an FPGA based set-up is to improve the resolution of the sensing system. Further signal processing based on other aspect of the acoustic signal will be integrated with the current system. An FPGA based system will help in adding parallelism and reducing the signal processing time thereby enabling real time and fast sensing. Thereafter received signal envelop is generated using decimation and low pass filtering. Area under curve is then calculated using trapezoidal rule (figure 2). III. DATA ANALYSIS AND R ESULTS The absorption of sound in fluids is due to the inability of molecular degree of freedom to follow acoustic fluctuation [4], [5] i.e. acoustic signal attenuation is related to molecular structure of the gases through which it travels. In comparison to air which largely consist of nitrogen and oxygen which are diatomic gases, acoustic signal attenuate more in carbon dioxide and methane which are polyatomic gases [3], [6], [7], [13]. When target gas is introduced in the environment received acoustic signal characteristic changes. This change can be measured using received signal amplitude variation or we can measure signal intensity by taking integral of the square of the signal. We have analysed advantage of measuring received signal area under curve and using this parameters to detect change in gas concentration level. For our analysis we have measured and compared deviation
544
in peak amplitude of received acoustic signal and change in signal intensity measured by taking area under curve of the received damped ringing signal waveform, for the corresponding change in gas composition. In first scenario received signal maximum amplitude when gas is not leaking is compared with maximum amplitude of signal when gas(methane and CO2) is leaking to identify effect of leaking gas on the acoustic signal amplitude. For a small change in gas concentration the corresponding change in amplitude is negligible. If we take maximum, minimum or average amplitude of received signal result would be similar to (figure 3) and would be inconclusive for low concentration change in gas composition. In second scenario variation is even more inconclusive as concentration changes are even lower. This is improved by taking into consideration the fact that the received signal is a damped ringing waveform not a single pulse. The waveform peak and the ringing duration is going to be dependent on the received acoustic signal intensity which in turn is dependent on the gas composition of the medium through which it travels. Area under the envelop of the received waveform is used to measure the received signal intensity and correspondingly change in gas composition. For first scenario, as evident from the (figure 4), relative changes in area under curve for gas leak is much more compared to relative change in the peak amplitude of the received signal waveform. Variation in the measurements is due to limitation of the test set-up to provide constant gas release. For second scenario, an improved test set-up has to be used to quantify change in area under curve with corresponding precise change in target gas concentration level. The lower flammable limit for methane in UG coal mine is 5% by volume but most of the commercially available methane sensors use 2% by volume as the lower limit, i.e. the limit at which the alarm is generated. Proposed approach would be useful for situations where concentration level change of 5-10% by volume are involved, but its utility for lower concentration changes seems limited. Further experimentation with more precise test set-up is required to quantify the lower limit of concentration level change where acoustic attenuation change, measured by taking area under curve of the received signal can be reliably used to make a prediction. Acoustic attenuation measurement even if it is not reliably able to measure change in gas concentration upto required resolution, will be able to assist in improving the confidence in other acoustic measurements. For e.g., Acoustic signal attenuation is greatly affected by change in humidity levels [8] but has negligible affected due temperature change. This feature can be used to negate the error in time of flight measurements due to temperature change. In this analysis all measurement are taken in dry condition but measurements will be severely affected if the environment have high humidity levels. Humidity sensors can be used to measure current humidity level and humidity level measurements can be included in the analysis. Temperature also can be measured using a temperature sensor and its effect can be included in the analysis.
Fig. 3. Max Amplitude in Methane(Red),Carbon Dioxide(Green) and Air(Blue)
Fig. 4. Received signal Area Under Curve in Methane(Red),Carbon Dioxide(Green) and Air(Blue)
IV. C ONCLUSION Acoustic measurement of gas leak require that we are able to measure attenuation of acoustic signal in gas with high degree of accuracy. Measuring area under curve of received signal enable us to measure change in signal strength with improved resolution. Further work is need to evaluate and quantify change in area under curve with exact change in gas concentration level for the second scenario. Also effect of high humidity is to be evaluated and quantified. Proposed approach can still be used to detect gas leakage not only in UG coal mines but in other places as well where possibility of a gas leak is there, like petrochemical plants. Advantage of acoustic sensing approach is that a single sensing system can be used to sense many different gases. Using acoustic sensors for gas sensing in UG coal mines will enable real time, fast and more accurate sensing of gas leakage.
545
R EFERENCES [1] Kissell, Fred. N., Information Circular 9486,CDC NIOSH, Handbook For Methane Control in Mining, (2006). [2] Garrett,S , Sonic Gas analyzer for Hydrogen and Methane , Proceedings of the Acoustics, 2008, Paris, France, 29 June - 04 July, (2008). [3] Petculescu, Andi, Future trends in acoustic gas monitoring and sensing, Journal of Optoelectronics and advanced materials, 8 (1), 217–221, (2006). [4] Henderson, Malcom C., Sound In Air: Absorption and Dispersion , SOUND , 2 (6), 28–36, (1963). [5] Evans, L.B., Bass, H.E. and Sutherland, L.C., Atmospheric Absorption of sound: Theoretical predictions, Journal of Acoustical society of America, 51 (5) part(2), 1565–1575, (1972). [6] Ejakov, Sally G., Phillips, Scott, Dain, Yefim, Lueptow, Richard, M. and Visser, Jacobus H., Acoustic attenuation in gas mixtures with nitrogen : Experimental Data and Calculations, Journal of Acoustical society of America, 113 (4) part(1), 1871–1879, April (2003).
[7] Petculescu, Andi, Hall, B., Fraenzle, R.,Phillips, S. and Leuptow, R. M., A Prototype acoustic gas sensor based on attenuation(L), Journal of Acoustical society of America, 120 (4), 1779–1782, (2006). [8] Bohn,Dennis A. , Environmental Effects on the speed of sound , 83rd convention of the audio engineering society, 1987, New York, USA, 16 - 19 October, (1987). [9] Shuh-Haw Sheen, Hual-Te Chien, Apostolos C. Raptis, Ultrasonic techniques for detecting helium leaks, Sensors and Actuators ,Elsevier Science, 71, 197-202, (2000). [10] Taylor,C.D., Chilton,J.E. and Martikainen, A.L., Use of infrared sensors for monitoring methane in underground mines , 12th US/North American Mine Ventilation Symposium, 2008, Wallace, USA, 9 - 11 June, (2008). [11] hester, Colin, Emerging Technology in gas monitoring equipment , QCO/DME Coal Industry Safety Conference in conjunction with Hazcoal Management, 1996, Queensland, Australia,(1996). [12] Dain, Y. and Leuptow, Richard M., Acoustic attenuation in threecomponent gas mixtures Theory, Journal of Acoustical society of America, 109 (5), 1955–1964,May (2001). [13] Dain, Yefim and Leuptow, Richard M., Acoustic attenuation in threegas mixtures Results, Journal of Acoustical society of America, 110 (6), 2974–2979,December (2001). [14] Tobias, Peter, Apparatus and method for using the speed of sound in photo acoustic gas sensor measurement, Honeywell International Inc., Morristown NJ(US),US Patent Pub No: 0147051 A1, Dec. 11 2008, Jun 17 2010.
546
