920 IEEE SENSORS JOURNAL, VOL. 14, NO. 3, MARCH 2014

Improvement of Measurement Accuracy of InfraredMoisture Meter by Considering the Impact

of Moisture Inside Optical ComponentsCun Guang Zhu, Jun Chang, Peng Peng Wang, Bo Ning Sun, Qiang Wang,

Wei Wei, Xiang Zhi Liu, and Sa Sa Zhang

Abstract— The impact of moisture inside internal end-face gapsof optical components on the measurements quality of infraredmoisture meter for sulfur hexafluoride (SF6) gas-insulated equip-ment (GIE) is explored. The distortion of observed absorption lineshapes occurs when the background absorption spectrum of mois-ture existing inside internal end-face gaps of optical componentsat atmospheric pressure is superposed on the desired absorptionspectrum of moisture in SF6 GIE where the pressure exceeds 1atm. Errors that arise with the measurements of moisture con-centration and pressure because of line shape distortion effectsin infrared absorption spectroscopy are quantitatively analyzed.A correction algorithm is proposed for the dual-beam balancedratiometric detector strategy to improve the measuring precision.This algorithm eliminates the line shape distortion effects causedby moisture inside optical components and conducts a successfulmoisture measurement and control program for SF6 GIE. In ourexperiments, the impacts of moisture inside optical componentshave been suppressed, and the mean absolute errors have beendecreased by 86.0% and 76.1%, respectively.

Index Terms— Infrared moisture meter, SF6 GIE, absorptionspectrum, pressure measurement.

I. INTRODUCTION

MOISTURE concentration measurement and control havebeen applied in a wide range of fields [1-3], it is par-

ticularly essential in sulphur hexafluoride (SF6) gas-insulatedequipment (GIE) [4]. Moisture will affect dielectric withstandstrength of GIE. Electrical breakdown in the high voltagesystem dissociates SF6 into sulphur fluorides, sulphur and

Manuscript received August 21, 2013; revised October 12, 2013; acceptedNovember 11, 2013. Date of publication November 19, 2013; date ofcurrent version January 17, 2014. This work was supported in part bythe Natural Science Foundation of China under Grant 60977058, in partby the Independent Innovation Foundation of Shandong University underGrants IIFSDU2010JC002 and 2012JC015, in part by the Key TechnologyProjects of Shandong Province under Grant 2010GGX10137, and in partby the Promotive Research Fund for Excellent Young And Middle-AgedScientists of Shandong Province under Grant BS2010DX028. The associateeditor coordinating the review of this paper and approving it for publicationwas Prof. Julian C. C. Chan.

C. G. Zhu, J. Chang, P. P. Wang, B. N. Sun, Q. Wang, W. Wei, andS. S. Zhang are with the School of Information Science and Engineering andShandong Provincial Key Laboratory of Laser Technology and Application,Shandong University, Jinan 250100, China (e-mail: 2003410572@163.com;changjun@sdu.edu.cn; wozhisi12345@163.com; wadesun0913@qq.com;iamwq1989@gmail.com; 931103056@qq.com; sasazhang@sdu.edu.cn).

X. Z. Liu is with the Institute of Automation, Shandong Academy ofSciences, Jinan 250014, China (e-mail: liuxzh@mail.sdu.edu.cn).

Color versions of one or more of the figures in this paper are availableonline at http://ieeexplore.ieee.org.

Digital Object Identifier 10.1109/JSEN.2013.2291033

fluorine. Some of these products react with excessive levelsof moisture to produce by-products including gaseous sul-phur oxyfluorides, hydrogen fluoride and solid by-products.A reduction in SF6 concentration occurs, and the insulatingefficiency of the system is degraded, which will result inequipment damage, or even serious personal injury. Moisturemeasurement and control constitute a major part of an effectivemaintenance program for GIE.

Conventional methods of moisture detection include gravi-metric method, electrolytic method and dew-point method.As distributed feedback light diode (DFB-LD) technologymatures, infrared absorption spectroscopy has been extensivelyapplied to detection of methane [5, 6], moisture [7] and othercomponents. Because of its low detection limit, fast response,cheap cost, and long durability, this technology has potentialas a monitoring tool for moisture concentration and pressure inSF6 GIE. The emphasis in previous moisture paper has beenmostly on the applications at pressures near one atmosphere.However, SF6 GIE commonly operates under pressure higherthan 2 atm, and the absorption spectra at elevated pressures aremore complicated due to pressure-induced broadening, shiftingand line shape distortion effects, which will be discussed inmore detail in this article. Thus it is important and practicalto improve the accuracy of the infrared gas sensor system atelevated pressures.

In current study, it is found that the moisture inside opticalcomponents influences the quality of the measuring resultsof infrared moisture meter for SF6 GIE seriously. The back-ground absorption spectrum of moisture inside optical compo-nents at atmospheric pressure is superposed on the absorptionspectrum of moisture in SF6 GIE where the pressure exceeds1 atm, and the observed absorption line shape would dis-tort. Thus it is no longer accurately described by a Lorentzprofile. We evaluate moisture concentration and pressure atpressures above 1atm by fitting experimental absorption datato a Lorentz profile, and unsatisfied fitting prediction result isobtained. This is often a problem in the practical applicationbecause most of the optical components exists obvious gas gapwhere moisture could be potentially packaged. For example,end-face gaps of photoelectric detector (PD) exist between thelens and the photosensitive elements can be up to about 1 mmin length. And in the collimator which is composed of fiber, aninserter (quartz glass tube), gradient-index (GRIN) lens, anda metal sleeve pipe, the end-face air gaps exist between the

1530-437X © 2013 IEEE. Personal use is permitted, but republication/redistribution requires IEEE permission.See http://www.ieee.org/publications_standards/publications/rights/index.html for more information.

ZHU et al.: IMPROVEMENT OF MEASUREMENT ACCURACY OF INFRARED MOISTURE METER 921

Fig. 1. Stack absorption spectrum and the Lorentz fit.

inserter and GRIN lens at one end of the collimator is nearly0.2 mm in length (the moisture concentration in the air canbe attained to 9000 ppm-20000 ppm).

A stack spectrum formed by the superposition of the back-ground absorption spectrum at atmospheric pressure and theabsorption spectrum of 460 ppm moisture in a 10-cm chamberis shown as the solid line in Fig. 1. The internal air pressureof the chamber is set at 3.438 atm to mimic internal operatingenvironments of SF6 GIE. It can be directly observed thatthe absorption profile is not a standard Lorentz curve due tothe superposition of two Lorentz lines of different widths, andfitting it to a Lorentz function may give misleading results.Combining inverse relations obtained by laboratory tests,moisture concentration (513 ppm) and the internal pressure ofthe chamber (4.167 atm) are derived from parameters used inthe Lorentz fitting function. The accuracy of concentration andpressure was 53 ppm and 0.729 atm respectively, but it cannotmeet the industrial moisture measurement needs. The influenceof moisture inside optical components must be addressedto further enhance the measuring accuracy. Unfortunately, inprevious studies, this problem has drawn little attention. In thisstudy, we focus on the elimination of line shape distortioneffects in infrared absorption spectroscopy caused by moistureinside optical components.

II. BEER–LAMBERT LAW AND THE CORRECTION THEORY

When the light emitted from the laser source covers awavelength range of one or more gas absorption lines, thetransmission of a probe beam I (P , ν) of light through auniform absorbing medium follows the law of Beer–Lambert:

I (P, ν) = I0(P, ν) exp(−α(P, ν)C L) (1)

Where α(P , ν) represents the absorption coefficient at thewave number ν(cm−1), P(atm) is the total pressure, C is thetarget gas concentration, L(cm) is the length of the absorptionpath. I0(P , ν) is incident intensity of the probe beam.

α(P, ν) = S(T )g(P, ν)P/kT (2)

Where k (J· K−1) is Boltzmann’s constant. S(T )(cm· mol−1) is the line strength at an arbitrary temperature

and g (P , ν) (cm) the line shape function. The line shapefunction g(P , ν) is determined by the physical mechanismsthat perturb the energy levels of the transition or the way inwhich the absorbing molecules interact with the laser beam.When P is greater than 1atm, the broadening effect can bedescribed by a Lorentz profile.

g(P, ν) = 1

π

�νc(P)/2

(ν − ν0 − �νs(P))2 + (�νc(P)/2)2 (3)

Where νc (P) (cm−1) is the collisional full width at halfmaximum (FWHM) and νs(P) (cm−1) is the pressure-inducedfrequency shift. Within the binary collision assumption, bothνc (P) and νs(P) should be proportional to the systempressure [8].

�νc(P) = 2γ (T )

(296

T

)n

P (4)

�νs(P) = δ(T )

(296

T

)m

P (5)

Where γ (T ) (cm−1 atm−1) is the broadening coefficient, δ(T ) (cm−1atm−1) represents the broadening coefficient. γ (T )and δ (T ) can be scaled from the values at the reference tem-perature with the temperature exponents n and m, respectively.

A new correction algorithm based on the dual-beam bal-anced ratiometric detector (BRD) strategy [9-11] is proposedto eliminate the impact of line shape distortion effects ininfrared absorption spectroscopy caused by moisture insideoptical components. We simplify the analysis procedure byassuming that moisture exists in the cavities of PD, collimatorand DFB-LD only. Moisture has strong absorption lines atthe wavelength of 1368.597 nm. Near this wavelength, theeffects of OH absorption (the line strength at 296 K is8.189*10−51 cm· mol−1) in fiber are several orders ofmagnitude weaker than H2O absorption (the line strengthat 296 K is 1.795*10−20 cm· mol−1) [12]. OH has rela-tively strong absorption lines in the wavelength region of1379.621 nm-1379.659 nm (the line strength at 296 K isfrom 1.138*10−23 cm*mol−1 to 1.139*10−23 cm*mol−1).From Corning SMF-28e+ optical fiber product informationsheet, we can know that maximum attenuation of CorningSMF-28e+ optical fiber at wavelength of 1383± 3 nm is0.31-0.35 dB/km. In dual-beam optical system, differencein fiber lengths of the two beams can be controlled within5 cm, and the corresponding attenuation is 0.0000155 dB to0.0000175 dB. So effects of OH absorption in fiber are notsignificant and can be ignored. The scattering effects in the gaschamber and in the single-mode fiber are also small enough tobe ignored [13]. The output voltage V (ν) of the BRD circuitcould be given by:

V (ν) = G ln

(Ire f

Ipro− 1

)(6)

Ipro and Ire f represent intensities of probe and referencebeams after absorption by moisture in all propagation paths.G is the synthetic gain of the system.

We used I ′re f and I ′

pro to represent intensities of referenceand probe beams without consideration for the absorption loss.Substituting law of Beer-Lambert, we derive the relationship

922 IEEE SENSORS JOURNAL, VOL. 14, NO. 3, MARCH 2014

Fig. 2. Schematic of experiment apparatus used in our work.

Fig. 3. BRD output voltage versus moisture concentration.

between the output voltage and the light absorbance (supposeboth PDs have the same effective cavity length L pd ), asshown at the bottom of the page, where Pc(atm) representsthe internal pressure (1-7 atm) of the gas chamber, P(atm)is the pressure surrounding the other optical components (1atm). Cc is moisture concentration to be measured in the gaschamber. Cd f b represents the moisture concentration in thelaser cavity. Crpd is moisture concentration in the cavity of thePD which is used to receive the reference beam. Cppd is themoisture concentration in the cavity of the PD which is usedto receive the probe beam. Ccol represents the concentrationof moisture in the end-face gaps of the collimator. Lcol(cm) isthe lengths of end-face gaps in the collimator. Lc(cm) andLd f b (cm) are the cavity lengths of the gas chamber andthe DFB-LD respectively. β is defined as the I ′

re f /I ′pro ratio.

Fig. 4. Profile of correction term V1.

Fig. 5. The correction effect of absorption spectrum of 460 ppm moistureinside the gas chamber at 3.438 atm pressure.

The same absorption terms can be offset.

V (ν)=G ln(βeα(Pc,ν)Cc Lc+α(P,ν)[(C ppd−Crpd )L pd+Ccol Lcol ]−1)

(8)

Assuming a small absorbance so that

exp(α(Pc, ν)Cc Lc + α(P, ν)(Cppd − Crpd )L pd

+α(P, ν)Ccol Lcol)

≈ (α(Pc, ν)Cc Lc + α(P, ν)(Cppd − Crpd )L pd

+α(P, ν)Ccol Lcol + 1) (9)

dV (ν) = (G/u)d(βα(Pc, ν)Cc Lc)

+(G/u)d(βα(P, ν)Ccol Lcol)

+(G/u)d(βα(P, ν)(Cppd − Crpd )L pd + β − 1)

(10)

dV (ν) = (G/u)βd(α(Pc, ν)Cc Lc)

+(G/u)βd(α(P, ν)Ccol Lcol)

+(G/u)βd(α(P, ν)(Cppd − Crpd )L pd) (11)

V (ν) = G ln

⎛⎝ I ′

re f e−α(P,ν)Cdf b Ld f b−α(P,ν)Crpd L pd

I ′proe−α(P,ν)Cdf b Ld f b−α(Pc,ν)Cc Lc−α(P,ν)C ppd L pd−α(P,ν)Ccol Lcol

− 1

⎞⎠ (7)

ZHU et al.: IMPROVEMENT OF MEASUREMENT ACCURACY OF INFRARED MOISTURE METER 923

TABLE I

RESULTS INFERRED FROM THE DIRECTLY MEASURED ABSORPTION SPECTRA

TABLE II

RESULTS INFERRED FROM THE CORRECTED ABSORPTION SPECTRA

Where u is defined as

u = Ire f /Ipro − 1 (12)

When Cc ≈ 0, the output voltage of the BRD circuit V1(ν)may serve as the correction term which can be expressed as

dV1(ν) = (G/u)βd(α(P, ν)(Cppd − Crpd )L pd

+α(P, ν)Ccol Lcol) (13)

Considering the different humidity inside the two PDs andthe end-face gaps in the collimator, V1(ν) �= 0. The correctedoutput voltage V0(ν) can be expressed as

V0(ν) = V (ν) − V1(ν) (14)

V (ν) = G ln(βeα(P,ν)[(C ppd−Crpd )L pd+Ccol Lcol ] − 1) (15)

In the infrared moisture meter with fiber structure, fibercollimator is utilized in the gas chamber to collimate thedivergent emitting light from fiber. Increase of pressure canenable the metal sleeve pipe to be deformed to a certainextent and cause mismatching of two fiber collimators, whichwould result in an additional coupling loss of the probe beam.The correction factors κ and η are introduced to compensatefor variations of V1(ν) and V0(ν) respectively (The values ofboth κ and η are 1 and there is no signal loss due to pressureforces ). So V0(ν) can be described as

V0(ν) = η(V (ν) − κV1(ν)) (16)

III. VERIFICATION EXPERIMENT

Figure 2 shows a schematic of the experimental arrange-ment, which is used to validate the effectiveness of thecorrection algorithm. The system uses a distributed feed-back InGaAsP diode laser, emitting at ∼1368 nm. The laseroperated in single mode and was modulated with a saw-tooth function (66 mA amplitude) superimposed on a meancurrent 13.5 mA. This means that the current ranges from13.5 mA–79.5 mA. The fiber-coupled output of the laser wassplit into reference beam and probe beam by the coupler.The reference-beam was directly fiber coupled on PD1. Twocollimators at 5 cm in length are connected by a 20 cm singlemode fiber; the total effective optic route length is 10 cm.The probe-beam was collimated by the collimator in the gaschamber. After transmitting through the sample gas (above1 atm), the probe beam was detected by PD2. The intensityratio β was 2.74725. Using an oscilloscope, the detectedsignals were sampled and averaged over 128 scans to removestochastic noise from the laser and detectors. These detectedsignals were the output of a BRD circuit. Pure nitrogen (with apurity of 99.999%) was aerated into the gas chamber to changethe moisture concentration in the gas chamber. We utilizedthe dew-point meter (S8000INTEGRALE S8K-I, MICHELL,UK), connected to the gas chamber to compare control pro-gram. And digital pressure sensor (BMP085 LCC8 BOSCH)was also used to monitor the air pressure in the gas chamber.

The absorption spectra of H2O-air in the pressure range1–7 atm were measured to obtain the relationship between the

924 IEEE SENSORS JOURNAL, VOL. 14, NO. 3, MARCH 2014

extinction ratio (%) of the probe beam yloss and the innerpressure of the chamber Pc.

yloss = 1.31Pc + 98.76 (17)

At an atmospheric pressure, the proportional relationshipbetween the moisture concentration and the BRD outputvoltages is described by a linear equation, which is shownin Fig. 3.

We can draw a conclusion that the difference of moisturebetween the two optical paths (the probe beam and the refer-ence beam) is corresponding to 130.5 ppm in a 10-cm absorp-tion path length. Amplitude of the correction term V1 canbe derived from the absorption spectrum of any gas samplescontaining a known concentration of moisture at standardatmospheric pressure with its amplitude scaled down that wasmultiplied by a factor of κ(Cppd − Crpd )/Cc. The absorptionspectrum of moisture concentration of 32 ppm in the gaschamber is shown by the dashed line at the top of Fig. 4.The solid line indicates the profile of correction term V1.

Line shape distortion effects in infrared absorption spec-troscopy caused by moisture inside optical components canbe eliminated by deducting V1 from the distorted absorptionspectrum. Corrective effect of an absorption spectrum of460 ppm moisture inside a 10 cm gas chamber at 3.438 atmpressures is shown in Fig. 5. We can see obvious line shapedistortion effects in the original captured waveform as shownat the top of Fig. 5. At the bottom, the dashed line indicatesdistortion corrections using the method outlined in this paper.A Lorentz can be fitted properly shown by the solid line.The pressure P in the chamber can be calculated by takingw and dividing it by 0.00960 (w = 0.00960 at about 1atm).Moisture concentration Cc is determined by 2ηA/πw.

An experiment was designed to compare the quality of mea-suring results before and after implementation of the correctionalgorithm. Pressure and water concentration, as well as waterline parameters, which are listed in Table I and Table II,were inferred from the directly measured absorption spectraand the corrected absorption spectra respectively. The meanabsolute error in estimating concentrations has decreased from62.8 ppm to 8.8 ppm, and that in pressure measurementsalso decreased from 0.661 atm to 0.158 atm. The impacts onthe measurement of moisture concentration and pressure havebeen suppressed in our experiments, and the mean absoluteerrors are decreased by 86.0% and 76.1% respectively. It lookspromising for adopting our proposed correction algorithm.

IV. CONCLUSION

The impact of moisture inside optical components on themeasuring results quality of infrared moisture meter for SF6GIE is explored. A correction algorithm is proposed for theBRD strategy to improve the measuring precision, whichmakes it possible to eliminate line shape distortion effectscaused by moisture inside optical components. The impactof line shape distortion effects in infrared absorption spec-troscopy caused by moisture inside optical components canbe effectively reduced. A comparison of measuring resultsderived from distortion corrected and uncorrected spectrawas performed. The results support the theoretical analysis.

The mean absolute error in estimating concentrations hasdecreased from 62.8 ppm to 8.8 ppm, and that in pressuremeasurements also decreased from 0.661 atm to 0.158 atm.The impacts on the measurement of moisture concentra-tion and pressure have been suppressed in our experiments,and the mean absolute errors are decreased by 86.0% and76.1%respectively.

The same algorithm can be applied to the detection ofN2, O2 and CO2, which would inevitably exist inside opticalcomponents and affect the accuracy of the pressurized absorp-tion gas detection system.

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Cun Guang Zhu was born in Shandong, China,in 1985. He received the M.S. degree from YantaiUniversity in 2010. He is currently pursuing thePh.D. degree at the School of Information Scienceand Engineering, Shandong University, Jinan, China.His current research interests include optical gassensing device, optical fiber sensor fabrication, andengineering application.

ZHU et al.: IMPROVEMENT OF MEASUREMENT ACCURACY OF INFRARED MOISTURE METER 925

Jun Chang received the B.S., M.S., and Ph.D.degrees from Shandong University in 1988, 1991,and 2006, respectively. He was a Visiting Fel-low with the School of Electrical Engineeringand Telecommunications, University of New SouthWales, from 2004 to 2005. He is currently aProfessor with the School of Information Scienceand Engineering, Shandong University. His researchinterest include optical fiber sensor and fiber laser.

Peng Peng Wang was born in Shandong, China,in 1985. She received the M.S. degree from YantaiUniversity in 2010. She is currently pursuing thePh.D. degree at the School of Information Scienceand Engineering, Shandong University, Jinan, China.Her current research interests include distributed-feedback fiber laser, optical fiber sensor fabrication,and engineering application.

Bo Ning Sun was born in Fujian, China, in 1989. Hereceived the B.S. degree from Shandong University,Jinan, China, in 2011. He is currently pursuing themaster’s degree at the School of Information Scienceand Engineering, Shandong University. His currentresearch interests include Raman optical fiber dis-tributed sensors and engineering application.

Qiang Wang was born in Shandong, China, in1989. He received the B.S. degree from ShandongUniversity, Jinan, China, in 2011. He is currentlypursuing the Ph.D. degree at the School of Informa-tion Science and Engineering, Shandong University.His current research interests include optical gassensing device, optical fiber sensor fabrication, andengineering application.

Wei Wei was born in Shandong, China, in 1989. Hereceived the B.S. degree from Shandong University,Jinan, China, in 2012. He is currently pursuing themaster’s degree at the School of Information Scienceand Engineering, Shandong University. His currentresearch interests include optical gas sensing deviceand engineering application.

Xiang Zhi Liu was born in Jiangxi, China, in1973. He received the M.S. and Ph.D. degrees fromShandong University in 1999 and 2008, respectively.His current research interests include computer-aided modeling, numerical analysis, optical commu-nication systems, and networks.

Sa Sa Zhang is a Professor with Shandong Uni-versity, Jinan, China. He received the B.S. degreefrom the Optics Department, Shandong University,in 1986, and the Ph.D. degree from the Schoolof Information Science and Engineering, ShandongUniversity, in 2008.

His main research interests focus on laser physicsand technology, optical fiber sensing, infrared mate-rials and devices, crystal physics, photonic crystalsimulation, and fabrication.