An optical fiber temperature sensor using a thermochromic solution

213

Sensors and Actuators A, 24 (1990) 213-216

An Optical Fiber Temperature Sensor Using a Thermochromic Solution TIANYOU

HA0

and CHUNG-CHIUN

LIU*

Electronics Design Center, Case Western Reserve Vm’versily, Cleveland, OH 44106 (U.S.A.) (Received April 5 1990; in revised form July 11, 1990; accepted July 30, 1990)

Abstract

An optical temperature sensor operated in the temperature range 25 to 55°C has been constructed and evaluated. This sensor combines an optical fibre with a thermochromic solution, cobalt chloride, and is operated based on the changes in the optical absorption spectrum of the thermochromic solution caused by the temperature variation. Good response and reproducibility are obtained and the measurement principles, the construction of the sensor, and the experimental results of this investigation are reported and discussed. -, Introduction

In recent years, optical fibers have been used for temperature sensing. The preliminary use of these sensors was as thermosensitive cladding in rare earth doping of the frontal tip of an optical fiber. This resulted in a light back-transmission which could be modulated by temperature. Other optical temperature sensors have also been proposed using different materials, such as birefringent crystals [l], liquid crystals [2], fluorescent materials [3,4] and semiconductors including GaAs [5-71. Our research focuses on the development of an optical fibre thermometer which can be operated in the temperature range 25-55°C. This device would have practical applications in biomedical and other areas. The relatively small size, physical flexibility, and lack of electromagnetic interference are the inherent attractive features of an optical fiber sensor. Brenci et al. [8] have suggested the utilization of a thermochromic solution as one approach in developing a low-temperature optical fiber thermometer. They used cobalt chloride dissolved in isopropyl alcohol and water as the thermochromic solution. Absorption spectra of this solution in the range 400-860 nm over a temperature range 5-75°C were reported.

In our research, an optical fiber was used in combination with a thermochromic solution, CoCl,*6H,O. As in the work reported by Brenci et al., the changes in the optical absorption spectra of this cobalt ion solution are caused by variations in temperature. This phenomenon can be used for the detection of temperature. Furthermore, the absorption spectra, due to temperature changes, which occur at two different wavelength ranges can be measured and used to establish a sensing and a reference signal. The difference in these two absorption spectra is then used to indicate the sensed temperature. This paper describes the measurement principles, the construction of the sensor, the experimental assembly and the test results of this optical fiber thermometer using a CoC1,*6H,O thermochromic solution. Principles of the Sensing Approach Brenci et al. [8] showed that the absorption

spectrum of a Co(B) thermochromic solution in the visible range 400-800 nm can be related to the temperature of a test medium. Hence, the application of a thermochromic solution such as CoCl,-6H,O was adopted for the development of an optical fiber sensor. The measurement principle used in our research is based on the radiation of a stabilized light beam which passes through the cobalt chloride solution and can be modulated by the temperature. According to Planck’s law of radiation 4 =f(T,rl)C,A-'(exp$-

where 4 = spectral radiant intensity, f(T, 2) = transmission coefficient of solution, which is a function of temperature, T, and spectral wavelength, A = spectral wavelength, t = radiant temperature, which is a constant, and C,, C,= constants. Also, one realizes that the voltage output of a photodiode, V, can be expressed as V=kln$

*Author to whom correspondence

0924-4247/90/$3.50

should be addressed.

l)-’

(2)

where k is the change-over coefficient of the 0 Elsevier Sequoia/Printed in The Netherlands

214

photodiode. Equation (2) can be used for two photodiodes over wavelength ranges of 6001100 nm and 720- 1100 nm:

/4

OPTICAL FIBERS

/

OPTICAL FIBER

1100

V=kln

4,(K

2) dA

(3)

&(T, A) dA

(4)

I 600 1100

V, = k In s 720

where subscripts 1 and 2 represent the outputs of the two photodiodes, respectively. According to the mean value theorem of integrals, VI = k In 41 (F, ni) Ali

(5)

V2 = k In &(T, nii) A&

(6)

where k is the wavelength between 600 and 1100 nm, iii is the wavelength between 720 and 1100 nm, A& is the difference of the wavelengths between 600 and 1100 nm and A& is the difference of the wavelengths between 720 and 1100 nm. V, = V, - V2 = k In &(T, ;li) Al, -k

In &(7’, iii) A&

=klnZ,-klnZ2 =

k

ln

(7)

fdT AK5(exp C2lAt- I)-’ AA fz(T, lii)lli,‘(exp

Cz/&l - 1))’ AA2

2, represents 42( T, iii) A&, which is related to the reference point and is insensitive to the temperature sensed, when the pi is a wavelength more than 720 nm. Thus V3 = k In Z, + A = F((T) with A = -k

(8)

In Z,.

Thus, the differential voltage V, is a direct function of the sensing temperature of the test medium. This is the basic principle of the temperature measurement of this fiber optic temperature sensor.

Experimental Construction of the Sensor Two types of optical fiber were used in this study. One was low-loss quartz-core silicon-cladded fiber (HCP-Mo300T-06, Ensign-Bickford Optics Co., Avon, CT), and the other was plastic fiber (10-500-A General Fiber Optics, Inc., Cedar Grove, NJ). Two configurations of the fiber optic temperature sensor were constructed in this study and are shown in Fig. 1. One configuration uses a small glass tube, 1.8 mm in diameter and 9 mm in length, which is filled with the thermochromic solution, cobalt chloride. The bottom of the glass tube is relatively thin and a single quartz optical fiber is inserted in the glass tube then sealed with

’ COPPER MIRROR (4

HOUSING FINISHED

WITH BOTTOM (b)

‘SEALED OPTICAL

WITH GRADE

EPOXY

Fig. 1. Configurations of the optical fiber-based temperature sensors.

epoxy (Dercon). The glass tube is then placed inside copper tubing with a diameter of 3.5 mm and a length of 7.5 mm. As mentioned, the temperature-sensing principle is based on the changes in the optical absorption spectrum of the thermochromic solution due to a temperature change. This temperature change can result from the thermal radiation of the test medium. Therefore, the construction of the housing for the thermochromic solution must be taken into consideration. Thus, the bottom of the copper tube is vacuum-deposited with a thin palladium film, providing a mirrorfinished surface. A second quartz optical fiber is then placed inside the copper tubing as shown in Fig. l(a). The second configuration, shown in Fig. l(b), uses two different types of optical fibers. Similarly to the first configuration, a quartz fiber is inserted inside the glass tubing and sealed with epoxy. A second optical fiber, a plastic one, is attached with epoxy at the thin bottom of the glass tube. This plastic fiber is flexible enough to bend as shown in Fig. l(b). The thermochromic solution was prepared in the following manner. Cobalt chloride, CoC12~6H20, dissolved in an alcohol-based solvent, was used. Spectroscopic grade 2-propanol and HPLC grade isopropyl alcohol in 0.1 M concentration were used, and the 2-propanol showed better results in our studies. The alcohol-water mixture contains 15% water. Cobalt chloride, CoClz.6HzO, dissolved in an alcohol-based solution is a known thermochromic solution. Figure 2 shows the absorption spectra of the solution at different temperatures. It is apparent that when the absorption intensities are measured at approximately 600 to 720 nm (as a baseline comparison), the difference of these absorption intensities can be used to indicate the temperature of the test solution. The results shown in Fig. 2 agree well with those reported by Brenci et al. [8].

215

diodes were then processed. The difference between these two voltage outputs was then used as an indication for the temperature sensed. In the evaluation of the sensor, a constant-temperature water bath was used to maintain the test medium at a constant temperature and a strip chart recorder was used for the recording. Results and Dlscussioo

X,nm

Fig. 2. Absorption spectra of the CoCl,.6H,O function of temperature.

Experimbtal

solution as a

Arrangement

Figure 3 shows the experimental schematic of this study, and Fig. 4 gives details of the signalprocessing arrangement. A 50 W quartz tungsten filament lamp and a stabilized power supply (model # 68735), both from Oriel Corporation, Stratford, CT, were used. Two colored glasses were used as the light filters; one transmitted light at wavelengths more than 600 nm, whereas the other transmitted at wavelengths more than 720 mn. Photodiodes (model 7180 silicon photovol) were used and the output voltages of these two photo-

Calibration of the developing fiber optic temperature sensor was carried out in a constant-temperature water bath over the temperature range 2%55°C. Figure 5 shows the sensor output as a function of the temperature sensed. It is apparent that the sensor is less sensitive at the lower temperature range, namely 25-4O”C, and more sensitive at the higher temperature range, 40-55°C. The developing sensor is operable beyond 55”C, but our range of interest in this study is 25-55°C. As shown in Fig. 5, the reproducibility among experimental runs is excellent. There is no significant difference or variation in the experimental results using either configuration of the sensor constructed (Fig. 1). The accuracy of controlling the operating temperature of the constant-temperature bath is +O.lo”C in this study, and the voltage output can be measured experimentally to within f 1 mV. This voltage output measurement suggests that the developing sensor has a temperature sensitivity of +O.l5”C. In a series of experimental runs, the temperature of the test medium was raised from 25 to 55°C by controlling the rate of the constant-temperature bath. Both the sensor response in signal output and repeatability are very good, as shown in Fig. 6. At a fixed temperature the sensor output reach 90% steady-state readings are approximately one minute. In another series of experimental runs, the temperature of the test medium was decreased

Fig. 3. Experimental schematic of the testing system. 1.2 II RUNI

I

II

1.0

4

A RUN 2 o RUN 3

+I

4 4

-9,

4 0.2 -

4 32

36

40

44

46

52

TEMPERATURE,=‘C

Fig. 4. Schematic of the signal-prccessing

arrangement.

Fig. 5. Sensor output as a function of temperature.

56

216 RUN 4

55°C

Acknowledgements

I

This work was supported by Fiber Optic Sensor Technology, Inc., the Edison Sensor Technology Center and the Resource for Biomedical Sensor Technology of the National Institutes of Health (Grant No. RR02024). References Fig. 6. Sensor response as a function of time.

from 55°C to 25°C within two minutes. Similar to the experiments in raising the temperature from 25°C to 55°C the sensor response and repeatability are also very good. No hysteresis effect was observed in the sensor performance, regardless of the direction of the temperature changes of the testing solution. In this investigation, one of the more tedious but important steps in the construction of the sensor is the sealing of the CoCl,*6H,O inside the glass tube. This has to be completed without trapping any air or gas bubbles inside the tube, otherwise errors in the measurement will result. One of our approaches is to first seal a short section of a glass tube by epoxy to the glass tube holder of the CoCl,*6H,O solution. The solution is then injected through the short tube by a syringe until the solution partially fills the short tube. The optical fiber is then inserted inside the short glass tube. Due to the capillary effect, the tip of the optical fiber can now be easily immersed in the solution without introducing any gas bubbles. The space between the upper part of the short tube and the optical fiber is then sealed with epoxy. When an optical measurement involves absolute optical intensity, as in our case, the length and the bending of the optical fiber will sometimes contribute to distortion of the signal output. Thus one must be cautious about the arrangement and positioning of the optical fibers when measurements are made. In summary, we have demonstrated a fiber optical temperature sensor using a thermochromic solution, CoCl,-6Hz0. The reproducibility and calibration of the sensor were assessed and show promise.

1 T. C. Cetas, A birefringent crystal optical thermometer for measurements: electromagnetically induced heating, in C. C. Johnson and J. L. Short (cds.), Proc. 1975 USNC/ URSI Symp., Bureau of Radiological Health, Rockville, MD, 1976, pp. 275-278. 2 T. H. Windhom and C. A. Cam, Optically active binary liquid crystal thermometry, IEEE Trans. Biomed. Eng., BME-26 (1979) 148. 3 T. Samulski and P. N. Shrivaatara, Photoluminescent thermometer probes: temperature measurements in microwave fields, Science, 208 (1980) 193-194. 4 K. T. V. Grattan and A. W. Palmer, Infrared fluroexence decay-time temperature sensor, Reu. Sci. Insrrum., 569) (1985) 1784- 1787. 5 K. Kyuma, S. Tai, T. Sawada and M. Nunoshita, Fiberoptic instruments for temperature. measurement, IEEE J. Quantum Electron., QE-I8 (1982) 676-679. 6 K. Hilgers and I. Kaufman, A fiber optic differential temperature probe, Fiber Optic and Laser Sensors V, SPIE, 838 (1987) 223. 7 D. A. Christiansen and V. A. Vajuine, A fiber optic temper-

ature Sensor using wavelengthdependent

detection, Fiber

Optic and Laser Sensors V, SPIE, 838 (1987) 253. 8 M. Brenci, G. Conforti, R. Falciai, A. G. Mignani and A.

M. Scheggi, Thermochromic transducer optical fiber temperature sensor, 2hd Conf. Optical F&r Sensors, Lieakrhalle Stuttgart, F.R.G, Sept. 5-7, 1984, p. 155.

Biographies Tianyou Hao is an associate professor of automation instruments, Beijing University of Iron and Steel Technology, Beijing, China. He is currently a visiting scholar at the Electronics Design Center, working on optical fiber-based chemical sensors. Chung-Chiun Liu is the Wallace R. Persons professor of sensor technology and control as well as the director of the Electronics Design Center at Case Western Reserve University. His research interests include electrochemical sensors, microelectronic fabrication processes and electrochemical sciences.