design of a portable optical sensor for methane gas detection

Sensors and Actuators B 113 (2006) 830–836

Design of a portable optical sensor for methane gas detection

Crawford Massiea, George Stewarta,∗, George McGregorb, John R. Gilchristb

a Department of Electronic and Electrical Engineering, The University of Strathclyde, 204 George Street, Glasgow G1 1XW, UKb Gas Measurement Instruments Ltd., Inchinnan Business Park, Renfrew PA4 9RG, UK

Received 18 January 2005; received in revised form 11 March 2005; accepted 14 March 2005Available online 23 May 2005

Abstract

A detailed investigation has been carried out on the design of a low-cost portable optical sensor for methane detection with a sensitivityof ∼1% of the Lower Explosive Level (LEL) for methane (500 ppm) and able to operate in harsh environments with temperature variationbetween−20 and 50◦C. The sensor design is based on the use of near-IR LEDs operating around the overtone absorption lines of methane at1660 nm using a stainless steel tube to direct the light through the gas to the detectors. Various configurations of source/detector layout havebeen examined to provide appropriate reference and signal paths in order to achieve reliable methane detection at LEL levels in the presenceo mperatures sensitivity of0©

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riwamtowmhiced

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dvan-ifice topipection, andoughd inthane,ion,theceasesn, so

ons,rathervelynower

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f temperature variation. An optimum design has been identified using two detectors with appropriate optical filtering and with tetabilisation of the source and detectors. Based on this design, a prototype instrument has been demonstrated with an ultimate.2% LEL methane (100 ppm).2005 Elsevier B.V. All rights reserved.

eywords: Methane sensor; Optical gas sensor; Portable gas sensor; Hydrocarbon gas sensor

. Introduction

Detection of methane gas is extremely important for safetyeasons in the oil and gas industries, in water treatment plants,n landfill sites and in commercial or domestic environments,here methane gas may filter up through the ground and cre-te an explosion hazard (the lower explosive limit, LEL, forethane is 5% by volume methane gas). Portable gas detec-

ors for locating gas leaks are clearly important in a numberf these applications, but must be of a low-cost design foridespread use and availability. The established method forethane gas detection is the catalytic sensor[1–3] which,owever, is not methane-specific, as any gas whose ignition

s catalysed by the pellistor will be detected. Additionally,atalytic sensors do not operate correctly in low-oxygennvironments and can be poisoned. Consequently, suchetector systems may require frequent functional checks.

It is therefore of commercial interest to develop portableas sensors, based on optical techniques[4]. Optical sensors,

∗ Corresponding author. Tel.: +44 141 548 2887; fax: +44 141 548 2926.

based on semiconductor sources, have the potential atages of: (i) intrinsically safe, (ii) ability to detect a specgas by selection of appropriate wavelengths, (iii) abloperate in zero-oxygen environment (e.g. for purging oflines), and (iv) low cost of ownership, since the gas-deteprinciple is a physical process (not a chemical reaction)therefore, poisoning of the sensor is not an issue (althdirt/contamination on the optics needs to be considerethe sensor design). For hydrocarbon gases, such as methe strongest optical absorption occurs in the mid-IR regaround 3.3�m, but the use of a broadband source inmid-IR to detect methane[5] has cost and performanlimitations. For example, a number of hydrocarbon ghave absorption bands/lines in the same spectral regioa mid-IR system will often respond to other hydrocarband hence, they are generic hydrocarbon detectorsthan methane-specific. Mid-IR detectors are relatiexpensive and are often cooled[6] in order to achieve aenhanced signal-to-noise ratio, which increases the pconsumption—a disadvantage for portable sensors.

One way to overcome some of these problems is toerate the sensor at overtone absorption lines in the ne
E-mail address: [email protected] (G. Stewart).
925-4005/$ – see front matter © 2005 Elsevier B.V. All rights reserved.oi:10.1016/j.snb.2005.03.105

C. Massie et al. / Sensors and Actuators B 113 (2006) 830–836 831

around 1665 nm for methane[7–9]. In this spectral region,inexpensive LEDs and detectors are widely available, anddetectors operate with low noise at room temperature (DFBlaser sources are preferred[7] but are expensive for portablesensors). The chief drawback is that the absorption strengthof second harmonic lines in the near-IR is at least two ordersof magnitude weaker than fundamental mid-IR absorptionlines. This presents a challenge in sensor design, especiallywhen a considerable range of environmental temperature isinvolved.

The objective of this work was therefore designing anddevelopment of an inexpensive, rugged, portable, near-IRmethane sensor, with sufficient sensitivity for commercialapplication (<1%LEL or equivalently <500 ppm1), and ableto operate over an environmental temperature range of−20and +50◦C. Hence, although the basic scientific principlesare well-known, the practical engineering design requires anumber of factors to be considered in the design includinginstrument size, cost, power consumption and immunity toenvironmental conditions.

2. Theoretical description of absorption fromnear-IR LED sources

We assume that the LED has an intensity distributionI(ν),w lia

I

w

bingg n isw utputi

I

w :

α

h e oft iccco asm e

ntageo

Fig. 1. (a) General intensity distribution of LED against wavenumber; (b)simple “block” approximation for the intensity distribution of the LED alongwith multiple absorption lines; (c) effect of strong absorption on LED sourcedistribution.

overlap integral between the source distribution and the ab-sorption cross-section profile.

Absorption data and spectral line positions in the near-IR are available on the Hitran database[10,11] for methane(CH4), but not for other hydrocarbon gases, such as, ethane(C2H6), propane (C3H8) and butane (C4H10), which are im-portant to consider in the design of a methane-specific sen-sor, as discussed later. Spectral line positions for these gaseswere, therefore, measured experimentally on a spectrome-ter, and are shown inFig. 2. For methane,Fig. 2 showsthe absorption lines around 1660 nm, with the P, Q and Rbranches. The Q branch contains a number of individual lineswhich are not resolved in this diagram; the strongest is theQ6 line at 1665.5 nm with an absorption ofα = 0.25 cm−1

at the line centre[7]. The Hitran database[10,11] providesinformation on line strengths, and gives an integrated linestrength2 of S = 5.172× 10−20 cm2 molecule−1 cm−1 for allthe methane lines between 1637 nm (ν2 = 6107 cm−1) and1697 nm (ν1 = 5891 cm−1).

2 The line strength is defined byS =∞∫0

σ(ν)dν.

ith wavenumberν, as illustrated inFig. 1(a). If the totantensity emitted by the LED is designated byI0, then defining

normalisedf (v) = I(v/I0), we can write:

0 =∞∫

0

I(ν)dν = I0

∞∫

0

f (ν)dν (1)

here∞∫0

f (ν)dν = 1.

If the output of the LED is passed through an absoras in a cell of lengthl, and assuming that the absorptioeak and does not change the source distribution, the o

ntensity from the cell is given by:

= I0 exp{−αeffCl} (2)

here the “effective” absorption coefficient is defined by

eff = N0

∞∫

0

σ(ν)f (ν)dν (3)

ere,σ(ν) is the absorption cross-section per moleculhe gas,N0 = 2.5× 1019 molecules/cm3 is the atmospheroncentration of molecules at 25◦C, and C is the gasoncentration fraction (C = N/N0, 0 < C < 1, C =ppm/106

r Cl = (ppm m)/106, with N as the concentration of golecules per unit volume). Note that Eq.(3) represents th

1 For methane, concentration in ppm is given by multiplying the percef LEL by 500.

832 C. Massie et al. / Sensors and Actuators B 113 (2006) 830–836

Fig. 2. Measured absorption bands and line spectra in the near-IR for several hydrocarbon gases.


We can quickly estimate the effective absorption coeffi-cient with the above data by assuming an idealised spec-tral distribution, as shown inFig. 1(b), for an LED with∼60 nm spectral width and centred on 1667 nm. In this case,f (v) = 1/(v2 − v1), and the integral in Eq.(3) takes the sim-ple form:

αeff = N0

(ν2 − ν1)

ν2∫

ν1

σ(ν)dν = N0S

(ν2 − ν1)(4)

Substituting values gives an effective absorption coeffi-cient of 0.006 cm−1 or, from Eq.(2), approximately 0.15%change in light intensity on passage through a 10 cm celllength with 50%LEL methane (approximately 40 timesweaker than using a narrow-linewidth laser source centredon the Q6 line).

However, the effective absorption coefficient can be im-proved by appropriate filtering of the LED, blocking spectralregions that are not affected by gas while targeting stronglyabsorbing regions, such as the central Q-band lines. Ef-fective absorption coefficients for filtered sources can beestimated as above, using the HITRAN database whichprovides individual line strengths (hence, the integratedline strength for all lines within the bandwidth of a filterthrough summation). For example, for 20 and 7 nm filters,b nm,tSaso ab-s e tot

andt filterl nec-e l in-s idthw

F forfi

is predicted to give∼0.6% change in light intensity on pas-sage through a 12 cm cell length with 50%LEL methane andsimilar values were obtained in practice.

It should be noted that the above theoretical descriptionis based on the condition thatαCl � 1 for each absorptionline within the spectral bandwidth considered. For highconcentrations and/or long path lengths, the strongestabsorption lines may deplete the available light intensity attheir respective wavelengths, as shown inFig. 3(c), and theLED intensity distribution is no longer constant along thecell; so, as a result, the absorption signal does not changelinearly with concentration. However, the work here is con-cerned with obtaining sufficient sensitivity for LEL levelsof methane, so the assumption ofαCl � 1 is justified, as forthe strongest Q6 line withα = 0.25 cm−1 and 100%LEL (5%methane) in a 12 cm length cell, we haveαCl ∼ 0.1.

3. Portable sensor design options

Various LED and detector configurations were consideredin the development of the near-IR portable methane detector,all based around the same gas detection cell, as shown inFig. 4. The cell consists of a 12 cm long polished stainlesssteel tube with an internal diameter of 1 cm, providinga sealed volume through which gas can be drawn. Them cellp thed thed ngtho PSD)m malldp nalf as)s ulseso nalsa DCl s isp tputa ,t gasc

sin-g eL sorp-t e op-t ticali inceb talt s ins . Thisa twoL theyo sig-n re

oth with centre wavelength on the Q6 line at 1665.5he integrated line strengths areS20nm= 2.65× 10−20 and7nm= 2.14× 10−20 cm2 molecule−1 cm−1, giving effectivebsorption coefficients of 0.0092 cm−1 and 0.0212 cm−1, re-pectively. The theoretical plot ofFig. 3 for filters, centredn 1665.5 nm, shows the marked effect on the effectiveorption coefficient, as the filter linewidth is reduced duhe dense spacing of lines within the Q-band.

In practice, the filter tolerances, angular alignmentemperature effects, all become more critical, as theinewidth is reduced, and hence, a compromise is oftenssary between sensitivity, cost and stability of the finatrument. With these considerations in mind, a 7 nm was used in this work, as discussed later in Section3. This

ig. 3. Effective absorption coefficient as a function of filter line-widthlters centred on 1665.5 nm.

ultiple reflections from the polished walls of the tuberovide a light pipe that guides light from the source toetector, greatly increasing the amount of light reachingetector and increasing the effective absorption path lef the cell. For all cases, a phase-sensitive detection (ethod was used for signal extraction to attain a setection noise-bandwidth of∼1 Hz. By 1800 out-of-phaseulsing of the LEDs (or inversion of one output sig

or the single LED/two-detector system), an active (gignal and a reference signal is created consisting of pf similar magnitude but opposite phase. The two sigre appropriately scaled and summed, giving a fixed

evel (with no target gas present in the cell). When garesent, the signals will be unbalanced, giving an out the pulse modulation frequency (∼1 kHz), and hence

he PSD produces an output voltage related to theoncentration.

The first design considered was based on a dual LEDle detector system, as shown inFig. 4(a). Here, one of thEDs is chosen to emit at wavelengths outside the ab

ion band of the target gas (methane), and provides thical reference signal to compensate for changes in opntensity due to dirt or condensation, etc., in the cell, soth LEDs will be affected in a similar way. Experimen

ests on this configuration, however, revealed problemensor operation over the required temperature rangerises from the different temperature coefficients of theEDs (which are, of necessity, different models, sinceperate at different wavelengths), which can lead to falseals from temperature change.Fig. 5shows the temperatu


Fig. 4. Various configurations for source/detector layout in the sensor cell.(a) Dual source, single detector; (b) dual source with stabilisation detector;(c) folded path; (d) single source, dual detector.

characteristics of the two LEDs used, namely, HamamatsuL8245 (1650 nm) and L7866 (1300 nm reference), givingtemperature coefficients of approximately−0.0372 dB/◦Cfor L8245 and−0.0242 dB/◦C for L7866. One possible so-lution to this problem is to introduce a second detector, as

Fig. 5. Relative radiant output against ambient temperature for HamamatsuLEDs L8245 (1650 nm active LED) and L7866 (1300 nm reference LED).

Fig. 6. (a) Photograph of a standard detector and the lensed detector; (b)comparison of absorption from 100% methane using lensed and unlenseddetectors for single and double path length configurations.

shown inFig. 4(b). This second detector, positioned at theLED end of the cell, monitors the output of both LEDs (in anarea shielded from gas). If there is any intensity differencebetween the LEDs, the stabilisation detector circuitry feedsa signal back into the reference LED-drive circuitry to adjustthe drive current and compensate for the intensity difference.By using this process, the output of both LEDs can be main-tained at the same intensity. However, for this system to op-erate correctly, the two detectors will have to track each otherwith temperature, and because the detectors are at oppositeends of the cell, this is not easily achievable. Also, the numberof components and cost is increased with this approach.

Because temperature drift may be a problem and absorp-tion by the gas is very weak in the near-IR, it is clearly ben-eficial if the absorption signal can be improved through anincreased path-length without having to change dimensionsof the stainless steel cell (and the final instrument). Hence,the configuration shown inFig. 4(c) was investigated, wherethe detector and the source are positioned at the same end ofthe cell, with a mirror at the opposite end of the cell, appar-ently doubling the available path length. Experiments werecarried out on this layout, using a standard G8421-03 InGaAsphotodiode as the detector, and then repeated with a G8421-03 photodiode that used a lens instead of a window on thephotodiode can, as shown inFig. 6(a). The mirror used was a15× 60 mm concave mirror from Edmund Scientific. Results


from tests are shown inFig. 6(b), where it can be seen thatvery little improvement was observed for the standard detec-tor, but the lensed detector showed an increase by a factor of∼1.7. The reason for these results lies in the acceptance an-gles of the detectors. The lensed detector mainly accepts lightthat has travelled directly down the cell and back again. How-ever, the unlensed detector will pick up a substantial amountof back-scattered light from inside the cell which does nothave a large path-length through the gas.

The final configuration and the one considered to be thebest option is illustrated inFig. 4(d), consisting of a singleLED and two detectors, each with a narrowband spectral fil-ter to create reference and active channels. The active filter ischosen to have a spectral distribution over a strongly absorb-ing region of methane absorption lines, whereas the referencefilter has a spectral distribution outside the absorption lines.As discussed in Section2, the narrowband filter used forthe active channel will also increase the effective absorptioncoefficient, thereby increasing the measured absorption.Moreover, this configuration uses two detectors of the samemodel number and at the same position within the cell.Common changes in detector performance with temperaturewill tend to cancel, when the reference and active signals arecompared, and hence, temperature sensitivity should be lessof a problem than a dual LED system. We shall now discussthe experimental design and test of this system in some detail.

4

ingleH PINp ctorsn ngtha ctivea f thes ld bew tredo wn erf le,c entrew turei ces.T if thefi ndt ntlyl chp tionl ctivefib ar , andt mentis the

filter is manufactured at the edge of the central wavelengthtolerance. Similar concerns apply for the reference filter, butan additional point to be considered is the behaviour of theLED spectral distribution with temperature. If the position ofthe reference filter is badly chosen, a situation similar to thedual LED set-up could arise, where the active and referencechannel behave very differently with temperature. To be safe,the reference filter was chosen to have its centre wavelengthclose to that of the active filter (1670± 1.5 nm), but far awayenough and with a sufficiently narrow bandwidth (3 nm) toavoid the Q-band absorption lines.

In order to make the sensor methane-specific, and to avoidcross-sensitivity to other gases, it is also important that thewavelengths chosen for both the active and reference LEDslie outside the absorption lines or bands of other hydrocarbongases (ethane, propane and butane) that may be present whenmonitoring gas leaks in domestic or commercial environ-ments. Careful examination of the experimentally-measuredspectral data ofFig. 2shows that the choice of wavelengthsfor the LEDs (1665.5 and 1670± 1.5 nm) does avoid ab-sorption by the other gases and this was confirmed throughindependent tests performed during the design stages of thesensor.

As noted, temperature stability is important to minimisedrift and false alarms. To improve the temperature character-istics of the sensor, the two detectors were housed in a singlea re thes eatedu ovea tureb cellw theL gasi

sen-s that0 ane.

F thanes

. Design and test of the dual detector system

The dual detector system was constructed, using a samamatsu L8245 LED and two Hamamatsu InGaAshotodiodes, G8371-01. In the design, a number of faeed to be considered carefully, including: (i) the wavelend bandwidth chosen for the filters to create the and reference channels, and (ii) temperature stability oensor. For the active filter, the target wavelength shouithin the dense Q-band absorption line structure, cenn 1665.5 nm (seeFig. 2), with a narrow bandwidth of a feanometres or less (seeFig. 3). However, in practice, oth

actors limit the choice in filter specifications, for exampost of manufacture, temperature effects on the cavelength, tilt sensitivity, etc. The cost of manufac

ncreases with narrowing bandwidth and tighter toleranhe manufacturing tolerances are important becauselter bandwidth is sufficiently narrow, for example 3 nm, ahe tolerance on the filter centre wavelength is sufficiearge, for example±1.5 nm, then some filters within a batroduction may include very little of the Q-band absorp

ines. For our sensor, the specification chosen for the alter had a centre wavelength of 1665.5± 1.5 nm with aandwidth of 7± 0.15 nm. This was found to provideeasonable compromise in terms of tolerances, costemperature sensitivity, and gives considerable improven absorption over a non-filtered LED (seeFig. 3), but istill wide enough to have a satisfactory performance if

luminium block to increase the thermal mass and ensuame temperature at both detectors. The block was hsing a 5.6�, 0.5 W resistor to raise the temperature abmbient to∼35◦C and was maintained at this temperay a control circuit using a thermistor sensor. Also theas sealed using sapphire windows which also shieldED and detectors from the cooling effect created when

s drawn through the cell.Fig. 7shows the fully assembled prototype portable

or. For the instrument, the electronic gain was set so–1 V range corresponded to 0% to 100%LEL meth

ig. 7. Photograph showing the fully-assembled prototype portable meensor.


Fig. 8. Time response of the instrument to 1.6%LEL and 50% LEL methane.

To further improve the stability of the reading, output sig-nals were averaged over 32 samples through software fil-tering. A time response trace of the instrument output for1.6%LEL (800 ppm) and 50%LEL methane is shown inFig. 8. The magnitude of change for 1.6%LEL is∼40 mV,and the change in signal for 50%LEL is∼580 mV. The ul-timate sensitivity of the detection system is limited by thesignal to noise ratio and was determined to be∼0.2%LEL(100 ppm).

5. Conclusion

In this paper, we have described in detail the designand construction of a low cost, rugged, portable, methane-specific sensor. The optical sensor has potentially a low costof ownership compared with the standard pellistor sensor andthe component cost is relatively low through use of standardLEDs and detectors operating in the near-IR region around1660 nm. However, due to the low absorption from overtoneabsorption lines in the near-IR, the main challenge is toachieve the required sensitivity over a wide temperature rangeof −20◦C to +50◦C and the best design option for achievingthis has been identified. A prototype instrument has beenconstructed with a sensitivity down to 0.2%LEL (100 ppm).The knowledge gained from this investigation provides ag otherh and fot ighers

A

up-p ship( GasM Pro-f andt r W

Johnstone of Strathclyde University are gratefully acknowl-edged.

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[ Ed-J.-Y.tson,no,

andant.

[ oth,rbe,ase,

B

C ni-v nicsf t an-a withG rans-f , in2

G de,G rs in1 , bothf ellowa nicsG sen-s ptersi n 100t ptics.

ood basis for the development of portable sensors forydrocarbon gases such as ethane, propane and butane

he development of portable laser-based systems for hensitivity.

cknowledgments

The authors gratefully acknowledge the financial sort provided through a Knowledge Transfer PartnerKTP3876) between The University of Strathclyde andeasurement Instruments Ltd. Assistance provided by

essor G Duxbury in obtaining spectral data for gasesechnical discussions with Professor B Culshaw and D

r

Optical fibre instrumentation for environmental monitoring appltions, J. Opt. A: Pure Appl. Opt. 5 (2003) S140–S145.

10] L.S. Rothman, C.P. Rinsland, A. Goldman, S.T. Massie, D.P.wards, J.-M. Flaud, A. Perrin, C. Camy-Peyret, V. Dana,Mandin, J. Schroeder, A. McCann, R.R. Gamache, R.B. WatK. Yoshino, K.V. Chance, K.W. Jucks, L.R. Brown, V. NemtchiP. Varanasi, The HITRAN molecular spectroscopic databaseHAWKS (HITRAN atmospheric workstation, 1996 edition, J. QuSpectrosc. Radiat. Transfer 60 (5) (1998) 665–710.

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iographies

rawford Massie graduated with a BSc(Hon) in Physics from the Uersity of Paisley in 1998, and an MSc in Lasers & Optoelectrorom the University of St. Andrews in 1999. He worked as a patenlyst with Derwent Information before taking up an Associate Postas Measurement Instruments (GMI) Ltd. under the Knowledge T

er Partnership (KTP) programme with the University of Strathclyde002.

eorge Stewart is currently a Reader at the University of Strathclylasgow. He was awarded a BSc Degree with First Class Honou974, and a PhD degree for research on integrated optics in 1979

rom the University of Glasgow. He was appointed as a Research Ft the University of Glasgow until 1985, and then joined the Photoroup at Strathclyde University to work on fibre optic components,

ors and fibre lasers. Dr. Stewart is the primary author of five chan various specialist textbooks and is author/co-author of more thaechnical and scientific papers in the field of fibre and integrated o

design of a portable optical sensor for methane gas detection

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