IEEE TRANSACTIONS ON INDUSTRIAL ELECTRONICS, VOL. 61, NO. 10, OCTOBER 2014 5599
A Digital Hygrometer for Trace MoistureMeasurement
Tarikul Islam, Anwar Ulla Khan, Jamil Akhtar, Member, IEEE, and Mohammad Zia Ur Rahman
Abstract—A microcontroller-based digital hygrometer systemusing a low-cost moisture sensor in the range of 3.7–100 ppmhas been developed. The sensor is of capacitive type and consistsof nanoporous thin film of alumina (γ − Al2O3) dip coated inbetween two parallel gold electrodes onto an alumina substrate.The behavior of the sensor has been modeled with an object to de-velop a signal processing circuit to convert capacitance change intofrequency. The detection electronics circuit is based on a relaxationoscillator whose output is suitable for interfacing with a digitaldevice. The sensor has been characterized with the circuit and thenoutput frequency is calibrated in terms of ppm. The accuracy ofthe digital hygrometer when compared with the commercial dewpoint meter is found to be ±1 PPM.
HUMIDITY sensors play an important role in processindustries and environment control. In domestic
applications, it is used for humidity control in microwave ovens,in buildings, and in laundry, etc. Humidity sensors are also usedin automobile industries in rear-window defoggers, automobilefuel cells, and motor assembly lines [1], [2]. In medical field,respiratory equipment, sterilizers, incubators, pharmaceuticalprocessing, and biological products employ humidity sensors.Humidity sensors are also used for green-house air-conditioning, plantation protection (dew prevention), soil mois-ture monitoring, and cereal storage in agriculture. In industry,humidity sensors are used for chemical gas purification,health monitoring of transformer oil, wafer processing insemiconductors, and paper and textile production [1]. Hence,there are endless applications where humidity measurementis needed. The type of humidity sensor required dependson the measurement range and application. A low-cost RHsensor covering the measuring range from 10% to 98% canbe fabricated easily in few dollars, but trace moisture sensors
Manuscript received April 11, 2013; revised July 28, 2013; acceptedOctober 28, 2013. Date of publication January 2, 2014; date of current versionMay 2, 2014. This work was supported by the Department of Atomic Energy(DAE) (2011/34/1/BRNS), Board of Research in Nuclear Science, India.
T. Islam is with the Department of Electrical Engineering, Faculty ofEngineering and Technology, Jamia Millia Islamia (Central University), NewDelhi 110 025, India (e-mail: [email protected]).
A. U. Khan and M. Z. U. Rahman are with the Department of ElectricalEngineering, Jamia Millia Islamia (Central University), New Delhi 110 025,India.
J. Akhtar is with Sensors and Nano-Technology Group, Council of Sci-entific and Industrial Research/Central Electronics Engineering Research(CSIR-CEERI), Pilani 333 031, India.
Color versions of one or more of the figures in this paper are available onlineat http://ieeexplore.ieee.org.
Digital Object Identifier 10.1109/TIE.2013.2297295
with measuring range from 0 to 1000 ppm requires thousandsof dollars [3]–[5]. The quality of a humidity sensor is assessednormally by high sensitivity, low response and recovery time,good reproducibility, and negligible drift due to aging andtemperature. Also, a low-cost and linear response is alwaysdesirable [1]. Practically, it is very difficult to fabricate a sensorhaving all desirable characteristics with measurement rangefrom few ppm to % RH. Different materials can be used tofabricate humidity sensors such as organic polymer, porousceramic, porous silicon etc. [6], [7]. Polymer material workswell in high humidity range, but suffers from drift, durabilityand temperature stability whereas porous ceramic materialscan works in trace moisture range, below 1% RH because of itsexcellent mechanical, chemical, and thermal stability [8], [9].γ − Al2O3 film a porous ceramic material prepared by sol-gelmethod is hydrophilic to water molecule but insoluble in waterand may withstand several harsh environmental conditionsin certain applications such as in fuel cell. The humiditysensors may be of capacitive type, resistive type, optical,and gravimetric but a large number of commercial humiditysensors are of capacitive type because of its high sensitivity,good temperature stability and ease of fabrication, and lowcost [1]. In capacitive type, when a thin porous dielectric layerbetween the parallel electrodes absorbs moisture, its apparentdielectric constant increases and thus increasing capacitance.Parallel plate structure may be suitable for developing low-costtrace moisture meter. Very recently, effort has been madeby the authors to fabricate thin film parallel plate capacitivesensor for measuring trace moisture from 2.5 to 25 ppm. Thedevice shows most of the desirable characteristics for industrialapplication [8].
In the present work, effort has been made to develop amicrocontroller-based digital hygrometer to measure moisturefrom 3.7 to 100 ppm using the similar sensor reported in [8].The sensor is fabricated using nanoporous ceramic material(γ − Al2O3 prepared by sol-gel method) having large surface tovolume ratio [10]–[12]. The capacitance of the sensor changesnonlinearly with ppm but almost independent with changein temperature and one can neglect its change for accuratemeasurement [1], [8]. The device response has been modeledto develop suitable detection electronics circuit to convert thecapacitance change in digital form [13]. The digital output canbe easily interfaced with microcontroller. A simple relaxationoscillator can perform this function easily. But, in conventionalform of relaxation oscillator circuit the sensitivity is verylow. The capacitance change of the sensor due to moisturein ppm is small in comparison to its offset capacitance whenhumidity was absent. This offset capacitance can be reflected
5600 IEEE TRANSACTIONS ON INDUSTRIAL ELECTRONICS, VOL. 61, NO. 10, OCTOBER 2014
Fig. 1. Photograph of the thin film parallel-electrode capacitive sensor.
as dry capacitance. To cancel this dry capacitance, a relaxationoscillator circuit reported in [9] is used, which only encodesthe capacitance change due to humidity into digital form. Thecircuit is partly modified to get pulse wave modulated signalby comparing the triangular wave with constant dc voltage.The pulse modulated wave is then converted into dc signal foranalog display of the output. Finally, the digital data are used bymicrocontroller for further signal conditioning. The calibratedoutput in terms of ppm is displayed with the help of LCD.
II. FABRICATION OF SENSOR AND ITS MODELING FOR
SIGNAL CONDITIONING
Fig. 1 shows the actual picture of parallel electrode ca-pacitive moisture sensor fabricated on an alumina substrateof dimension 2 cm × 2 cm. Then, by using manual screenprinting equipment, a gold electrode of dimensions 1.6 cm ×1.4 cm was screen printed onto an alumina substrate and fired at900 ◦C for 1 h. γ − Al2O3 film was deposited on the electrodewith the help of dip-coater by dipping the substrate 6 timesin the sol-gel solution. To ensure uniform thickness of film,the film was deposited by using PC controlled automatic dipcoater. Deposited thin film of ∼6 μm thickness on electrodeis then dried slowly at a room temperature for several hours,and another gold electrode of dimension 1.3 cm × 1.2 cm wasscreen printed on the film. An optimal thickness of the film isdesired because, if the thickness is small, the two electrodesmay easily short and if the thickness is large the sensitivity ofthe device will reduce. Also, the gold electrode is made porouswith average pore dimension (∼1.7 μm) larger than the poredimension of the film (10 Å). Larger pores in electrode willallow water molecules to penetrate then be absorbed by theporous γ − Al2O3 film with very large specific surface area [8],[14]. Finally, the film was sintered in a programmable furnaceby firing it initially at 450 ◦C for 1 h and then subsequentlyat 900 ◦C for another 1 h for curing the top gold electrode.Electrical connection to the sensor is made by two silver wiresoldered on both electrodes. The actual sensing area of thedevice is 1.3 cm × 1.2 cm. The capacitive response Ch(x)ofthe sensor normalized by its dry capacitance Cd when ppmis minimum as a function of ppm is plotted in Fig. 2. The
Fig. 2. Normalized capacitance Cn of the humidity sensor as a function ofmoisture (ppm).
capacitance change from 0 to 100 ppm moisture is measuredby precision impedance analyzer (Agilent 4294A) at 1 kHzsignal frequency with 500 mV sinusoidal ac voltage. As film isuniform, absorbed water can be regarded reasonably as beinguniformly distributed throughout the film. Hence, the sensorcapacitance Ch(x) can be given as
Ch(x)
Cd= Cn =
La
εa
La∫0
dl
εs=
εsεa
(1)
where εa is the dielectric constant of γ − Al2O3 without watermolecules (εa ≈ 9− 11), εs is apparent dielectric constantin presence of water, and La is the thickness of the film(∼6 μm) [8], [9].
The trace moisture (ppm) in moist N2 gas is measured withthe help of a commercial SHAW dew point meter (model noSADPTR-R, U.K.) [5]. The plot shows that the capacitancechange of the sensor increases with increase in apparent dielec-tric constant εs of hygroscopic film due to physical absorptionof water molecules.
Looyenga’s empirical equation gives the apparent dielectricconstant εs of the film as [15], [16]
ε1/3s =[γ(ε1/3w − ε1/3a
)+ ε1/3a
]
then the fractional volume γ of water in the film can begiven by
γ =ε1/3s − ε
1/3a
ε1/3w − ε
1/3a
(2)
where εw is the dielectric constant of water at any temperatureθ and is given by [17]
εw=78.54{1−4.6×4.6−4(θ−298)+8.8×10−6(θ−298)2
}.
(3)
At room temperature of 25 ◦C or 298 K, εw = 78.54 and attemperature of 50 ◦C or 323 K, εw = 78.10.
The % change in εw due to 25 ◦C change in ambient temper-ature of sensor working is 0.6% only, which can be neglected.Some sensing material depends on the ambient temperatureparticularly polymer or porous silicon, causing major changein capacitance [9], [15], [18]. In case of polyimide film RH
ISLAM et al.: DIGITAL HYGROMETER FOR TRACE MOISTURE MEASUREMENT 5601
Fig. 3. Fractional volume γ as a function of moisture (ppm).
sensor, the volume fraction γ is a function of both temperatureand humidity [9]. But, in our case γ − Al2O3 ceramic materialis used as a sensing element for which effect of temperature isnegligible [1], [8], [18].
Substituting εs from (1) in (2), we get
γ =(Cnεa)
1/3 − ε1/3a
ε1/3w − ε
1/3a
.
For εw = 78.54 and εa = 9
γ = 0.9446C1/3n − 0.9446. (4)
By using (4), the fractional volume γ is plotted in Fig. 3 as afunction of moisture (ppm). With the help of this plot, we canderive an empirical approximate relation as [9]
γ = γmynψ(θ)
ψ(θ) = 1− βo(θ − θo) (5)
where γm is the maximum fractional volume at θo, βo is tem-perature coefficient of capacitance, yγ is humidity level mea-sured in ppm, and ψ(θ) is the temperature-dependent volumefraction. In the present case, ψ(θ) = 1 because of temperaturestability of the sensor [8] and (5) can be written as
γ = γmyn. (6)
From (4) and (6), we get,
γmyn =0.9946C1/3n − 0.9446
y =
[0.9446
γmC1/3
n − 0.9446
γm
] 1n
. (7)
Above, expression gives direct moisture in ppm as a functionof the capacitance response of the sensor. But, implementationof this expression by an interfacing circuit is very difficult.
With the help of observations from Fig. 2, which indicates thenormalized capacitance versus moisture (ppm) at temperature(298 K), a simpler expression can be obtained as [9]
Ch(x)
Cd= Cn = 1 + εdsy
m (8)
Fig. 4. Error (%) between normalized values of Cn estimated from the modelequation to the measured experimentally.
where εds is the dielectric sensitivity at θo and m is a nonlinearindex.
By using curve fitting technique (method of least square),values of εds = 3.229× 10−4 and m = 1.091375 are obtained.
The normalized values of Cn estimated from the relationgiven in (8) were compared to those measured experimentally.The result is depicted in Fig. 4. It is observed that the nor-malized capacitance obtained from the model equation differsonly by ±0.79%, from the experimental value which is quiteacceptable.
This comparison shows that (8) gives an acceptable modelof γ − Al2O3 film-based humidity sensors for developing anapproximate detection electronics circuit for digital display ofmoisture. An interfacing circuit based on this model will beobtained in the next section.
III. INTERFACE ELECTRONICS
Humidity level in ppm which is to be detected as a functionof measured capacitance value can be given by (8)
Moisture (ppm) =
{1
εds
Ch(x)− Cd
Cd
}1/m
. (9)
The above expression indicates that moisture is related withrelative capacitive difference value. With the help of conven-tional relaxation oscillator consisting of an integrator followedby a comparator, one can easily detect capacitance in a digitalform as oscillation period of oscillator is proportional to theintegration time constant τc = CR. But (9) shows that the inte-gration time constant should be proportional to the capacitancedifference.
This requirement can be fulfilled by expressing an integratorin a transfer function D(s) as follows [9]:
D(s) =Vc(s)
Vi(s)= − 1
sR (Ch(x)− Cd). (10)
In state variable form
Vc(s) = − 1
sRCh(x)(Vi(s)− sRCdVc(s)) . (11)
Above expression can be realized by an integrator and adifferentiator. Hence, an op-amp OA2 as a differentiator is
5602 IEEE TRANSACTIONS ON INDUSTRIAL ELECTRONICS, VOL. 61, NO. 10, OCTOBER 2014
Fig. 5. Circuit diagram of the interface electronics [9].
Fig. 6. Voltage waveforms at different points of the interfacing circuit.
added in a conventional relaxation oscillator consisting of op-amp OA1 as an integrator and op-amp OA3 as a comparator asshown in Fig. 5.
The humidity sensor is connected as a capacitance Ch (x)for integrator. Assuming ideal op-amps with integrator andcomparator signals Vd and VC shown in Fig. 6, the oscillationperiod T can be derived as
T = Tp+Tn = G(2+k+1/k)Ra
{Ch(x)−
Rb
RcCa
}(12)
where
G =Rd
Re, k =
Vs−Vs+
and Vs+ and Vs− are the output voltage level of compara-tor when VI ≥ 0 and VI < 0, respectively. For symmetricalcomparator output with negligible offset voltages for op-ampsOA1 and OA2 (assume Vs+ = Vs−), the time period T can berepresented by
T = 4GRa
{Ch(x)−
Rb
RcCa
}. (13)
For experimental Vs+ = 4.08 V and Vs− = 4.68 V, the valueof term (2 + k + 1/k) is equal to 4.01, which can be approxi-mate as 4 as used in (13).
Fig. 7. Photograph of digital storage oscilloscope (DSO) showing outputwaveforms.
The time delay of comparator IC OA3 may cause error inthe overall time period of the signal. Utilizing fast op-ampfor comparator (LF-351, AD-844) and ultralow offset op-amps(Op-07), the (13) can be used with acceptable accuracy of timeperiod measurement. If Rb is adjusted in such a way that
Rb
RcCa = Cd
then period of relaxation oscillator
T = 4GRa (Ch(x)− Cd) (14)
becomes exactly proportional to the capacitance change. If theoutput of the integrator is compared with a constant dc voltage,then output of the comparator OA4 is a pulse wave modulation(PWM). For analog display of the moisture, the PWM can beconverted into dc voltage after low-pass filtering [19].
IV. HARDWARE IMPLEMENTATION OF THE
ELECTRONICS CIRCUIT
The circuit was first implemented on bread board and thenon veroboard by using op-amps with high slew rate, high inputimpedance, ultralow offset voltages, and low response timeto minimize the effect of offset voltage and time delay. AnOP-07 was used for integrator OA1 and for remaining op-ampLF-351 was used. Frequency of the output was adjusted to2.833 kHz when the sensor capacitance Ch(x) = 538.95 pFunder dry condition by using the following component val-ues: Ca = 500 pF, Ra = 5.1 MΩ, Rb = Rc = 9.83 KΩ,Rd = 0.976 KΩ, Re = 2.185 KΩ, Vs+ = +4.08 V, Vs− =−4.68 V.With the help of this relaxation oscillator circuit, thechange in capacitance was converted into frequency of oscilla-tion. A digital oscilloscope (Agilent Technologies DSO1002A)was used to measure the frequency change of the relaxationoscillator output. Fig. 7 shows the snap shot photo of DSOshowing output waveforms.
ISLAM et al.: DIGITAL HYGROMETER FOR TRACE MOISTURE MEASUREMENT 5603
Fig. 8. Photograph of the experimental setup.
Fig. 9. Moisture (ppm) as a function of frequency of relaxation oscillatoroutput.
V. EXPERIMENTAL SETUP AND RESPONSE
CHARACTERISTICS OF THE SENSOR BY
ELECTRONICS CIRCUIT
A robust measurement system is employed to obtain anaccurate and reliable response of the sensor to the differentconcentrations of moisture [18]. A rectangular shape steel testchamber of 5 cm in diameter and 10 cm in height is used forplacement of sensor and is fitted in series with a commercialdew point meter. Desired moisture concentration in the rangeof 0–100 ppm has been generated by mixing dry N2 gaswith humidity content gas at room temperature (25 ◦C in ourcase). Humidity content gas is obtained by passing dry N2 gasthrough water bubbler which generates vapor. Moisture contentof dry N2 gas used for creating moist gas is ∼2–3 ppm. Forsensor testing, concentration level of moist gas was varied from3.7 ppm to 100 ppm and monitored by dew point meter. Acomplete experimental setup can be seen in Fig. 8. Experimentswere conducted at 25 ◦C.
Fig. 9 shows the variation of the frequency of pulses withmoisture in ppm. It shows that the frequency is inversely pro-portional to the moisture. As we know, frequency is inverselyproportional to period and from (14) it shows that periodof relaxation oscillator is exactly proportional to capacitancechange. Frequency change of the circuit is significant and it isnearly 695 Hz for the change in moisture of 95 ppm.
The frequency sensitivity over the full scale defined asSf = (Δf/Δppm)× 100 = 692%. Variation of frequency in
Fig. 10. Transient response of the circuit to 96.3 ppm moisture change.
TABLE ICOMPARISONS OF THE HYGROMETER
Fig. 11. Repeatability of the circuit output for 96.3 ppm moisture change.
full scale is nonlinear (30%). If the response curve is seg-mented in two parts, then above 20 ppm frequency change isalmost linear, but below 20 ppm, nonlinearity is nearly (4%).This nonlinearity is due to nonlinear capacitive response ofthe moisture sensor as shown in Fig. 2. This small nonlinearchange of frequency has been taken care of by microcontroller-based signal conditioning algorithm. For practical application,response time, recovery time, and repeatability are some im-portant parameters of the sensor. These are also determinedby the signal conditioning circuit. Transient response curve for96.3 ppm change in moisture concentration is shown in Fig. 10.The response and recovery times of the sensor are approxi-mately 28 s and 102 s, respectively. The comparison of the someof the data to its commercial dew point meter is shown in Table I[3], [4], [20].
Fig. 11 shows the repeatability of the sensor output at thesame moisture level for several moisture cycles. The sensoroutput is highly repeatable. The frequency output increases withsudden increase in moisture and then reaches to the saturationvalue monotonically. During refreshing, the frequency outputreaches to its initial value.
5604 IEEE TRANSACTIONS ON INDUSTRIAL ELECTRONICS, VOL. 61, NO. 10, OCTOBER 2014
Fig. 12. Block diagram of hygrometer system.
Fig. 13. (a) Picture of hygrometer system. (b) ppm output measured from thesignal conditioning circuit displayed on LCD.
VI. SIGNAL CONDITIONING FOR MOISTURE DISPLAY
Block diagram of the complete system is shown in Fig. 12.For detection of oscillation frequency and subsequent signalprocessing, an 8-bit PIC16F877A microcontroller was used.Since the output of the sensor is in digital form, it can beinterfaced directly to the microcontroller. For this, we use timerTMR0 of PIC16F877A in 8 bit mode to count the input edgesappearing from the relaxation oscillator circuit. The pulses aremeasured precisely for 1 s and then the number of overflowsmultiplied by 256 plus the counts in TMR0 will give us thetotal number of pulses in 1 s. Since we are measuring it exactlyfor 1 s, there is no need for further conversion, and the count isthe frequency in Hz. By keeping the time base at 1 s, there isanother advantage that the minimum number of counts detectedcan be 1, so we can measure from as low as 1 Hz to as highas 50 MHz. For calibrating frequency of oscillation in terms
TABLE IIPARAMETERS OF THE SYSTEM
of ppm, we use the look-up table which is a widely usedconcept in microcontroller programming. Here, microcontrolleris programmed with the help of Flow codes, which is one ofthe world’s most advanced graphical programming languagesfor microcontrollers [21]. Moisture (ppm) corresponding to aparticular frequency was finally displayed on the LCD as shownin Fig. 13. To evaluate the performance of the measurementsystem, readings on the LCD were compared to those measuredby commercial dew point meter, and it was found that themaximum error was ±1% over the specified range of moisture(ppm). The characteristics of the hygrometer examined for3.7 to 100 ppm moisture content in dry gas are shown in Table II.
VII. CONCLUSION
A digital hygrometer for measuring moisture in the range of3.7 to 100 ppm has been proposed. The sensor is parallel platecapacitive type and is made of thin film porous alumina (γ −Al2O3). A simple low-cost mass producible sol-gel method hasbeen followed to fabricate the sensor. The response behaviorof the sensor has been modeled with an aim to develop asuitable signal conditioning circuit. Then, an interfacing circuitwith both digital (frequency change) as well as analog dcoutput has been developed. The output of the circuit has beeninterfaced with microcontroller to display the moisture in ppm.The accuracy of the digital hygrometer has been compared withthe commercial meter over a 3.7 to 100 ppm range and it isfound to be ±1 PPM. The performance characteristics of thesystem are comparable to the commercial dew point meter. Itis possible to develop a low-cost system as it neither requirescomplex steps nor the costly equipment for the fabrication ofsensor. Moreover, the sensing layer is prepared by low-cost sol-gel method where batch fabrication is possible. The interfaceelectronics circuit is simple requiring few components and hasdigital output which is easy to interface with microcontroller.
ACKNOWLEDGMENT
Authors would like to thank A. Hossain and K. Senguptafor their constant feedback and mental support for performingthis work.
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Tarikul Islam received the M.Sc. Eng. degree ininstrumentation and control from Aligarh MuslimUniversity, Aligarh, India, in 1997 and the Ph.D. de-gree in engineering from the Department of Electron-ics and Telecommunication Engineering, JadavpurUniversity, Kolkata, India, in 2007.
Currently, he is working as a Professor in theDepartment of Electronics and Communication En-gineering, Faculty of Engineering and Technology,Jamia Millia Islamia, New Delhi, India. He haspublished 33 papers in international peer-reviewed
journals, and 40 papers in international and national conferences. His researchinterests include thin film sensors, sensor arrays, electronic instrumentation,and soft computing techniques for signal conditioning.
Anwar Ulla Khan was born in Lucknow, India, in1983. He received the M.Tech degree in instrumen-tation and control from Aligarh Muslim University,Aligarh, India, in 2011.
He is currently working as a Ph.D. ResearchScholar in the Department of Electrical Engineering,Faculty of Engineering and Technology, Jamia MilliaIslamia, New Delhi, India.
His research interests include thin-film humiditysensors and microcontroller-based signal condition-ing circuits with frequency output.
Jamil Akhtar (M’14) was born in Ghaziabad, India,in 1959. He received the M.Sc. degree in physicswith a specialization in physics and electronics fromAligarh Muslim University, Aligarh, India, in 1980,and the Ph.D. degree in semiconductor devices andtechnology from Jawaharlal Nehru University, NewDelhi, India, in 2001.
Since March 1983, he has been with the Council ofScientific and Industrial Research/Central Electron-ics Engineering Research Institute, Pilani, India, asa Scientist for R&D in semiconductor devices and
technology. At present, he holds the grade of Scientist-G (Chief Scientist) andheads the Sensors and Nano-Technology Group. His research interests includetechnology for silicon-based IMPATTs and BARITTs for x-band and w-bandapplications, numerical techniques for semiconductor device simulation, designand fabrication of microstrip detectors, piezoresistive microsensors based onMEMS technology, and MeV ion-assisted techniques for nanostructure forma-tion in single crystalline silicon.
Mohammad Zia Ur Rahman received the M.Techdegree in instrumentation and control from AligarhMuslim University, Aligarh, India, in 2011.
He is currently working as a Ph.D. ResearchScholar in the Department of Electrical Engineer-ing, Faculty of Engineering and Technology, JamiaMillia Islamia, New Delhi, India. His research in-terests include modeling of sensors for humiditymeasurement.
