A Miniature Contact Thermometer for Student Use Thomas J. B ~ n and o Jemy G. Shepherd Thermophysics Division, Center for Chemical Engineering, National Bureau of Standards, Boulder. CO 80303
Isothermal temperature control for components of apparatus used in most physical chemistry courses is usually obtained using a well insulated, vigorously stirred water Electrical power is usually applied to immersion heaters using a solid state relay controlled by a suitable sensor. The most common sensor used in undergraduate laboratories is the reliable mercury contact switch or contact "thermometer". Unfortunately, this device is quite fragileand is easily broken when subjected to careless handling. Adjusting the temperature set point of commercial units can he a time-consuming procedure, especially inconvenient if measurements are needed a t several temperatures. In addition, the relatively large size of the commercial units restricts their use to large baths. A recently designed miniature contact switch can be of meat value as a low-cost substitute for the commercial units. ~~. while at the same time addressing some of the difficulties associated with their use. Originally constructed for chrw n~atogrnphirtemperature-cmtrol applications, the device can he easily mudified tbr student use. A schematic diagram of this mt~dificationi i prcienred in Figure I. The witch is made of boroiilicatr class tubing of 1.0 mm 0.d. and 0.3 mm i d . and incorporates tungsten cokacts. The bulbs a t each contact were made from uranium glass to grade the seal to the contact. These bulbs also orovide for - ~ -.~ -the .--. expansion of the mercury, with the upper bulb senring as an overflow. This overflow volume orevents the unit from ~ -shat- -~- --~ ~ tering in the unlikely event of a large temperature overshoot or circuit failure. Each switch so constructed will complete the contact circuit a t a specific temperature. This control temperaturecan he set (to within2 OC) bv heating theunit in a water hath to the desired set point. ~ h excesswmercury k in the upper bulb is then removed by gentle tapping, and the unit is evacuated and sealed in a hydrogen flame. The switch will then control a t the set point to within 0.05 O C . Since a separate contact switch is used for each temperature set point, precious student lab time is conserved. In practice, the student modification is potted in a commercial, water-curing silicone elastomer. This makes the switch more durable since the glass tubing itself is quite fragile. The solid state relay circuit that the contact switch ~~
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1.3 cm.
Fig. 1. Mercury contact switch for student use.
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Shoemaker. D. P.; Garland, C. W.; Stelnfeld. J. I. "Experiments in Physical Chemistry".3rd ed.; McGraw-Hill: New York. 1974. White, J. M. "Physical Chemistry Laboratory Experiments"; Prentice-Hall: Englewood Cliffs. NJ. 1975. Frederick. N. V.; McFarlane. J.; private communication.
452
Journal of Chemical Education
Fig. 2. Circuit diagram of a simple solid state relay temperature controller.
gates is shown in Figure Z3The circuit is easily constructed (for under $90) using readily available components. In the present configuration, a total heating power of 1000 w may he applied to the hath. Heavier loads can be handled upon making simple modifications to the circuit. Acknowledgments The financial support of the Gas Research Institute and the U.S. Department of Energy (Office of Basic Energy Sciences) is gratefully acknowledged.
