Patrick Picker Carmel Jolicoeur and Jacques E. Clesnoyersl
Unversitb de Sherbrooke Sherbrooke, Qubbec, Canada
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Pretise d0~d-LOOP Thermostat
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Modern systems are often kept isothermal by circulating a thermostated liquid through a jacket. Constant temperature baths equipped with a circulation system are now commercially available, but most of these thermostats are not precise to better than O.Ol°C; furthermore, their temperature cannot be changed rapidly because of the large quantity of liquid involved. Recent work2 on microcalorimetry in this laboratory required a thermostat stable to *0.00l0C, the temperature of which could he changed rapidly for heat capacity measurements. Therefore, the following closed-loop thermostat, which eliminates the large water reservoir, was designed. The overall thermostating system is shown schematically in the figure. The circulating liquid is cooled by a thermoelectricimmersion cooler (Whirlpool Corporation, Benton Harbor, Mich.) and then heated by a fase response controlled heating element. The size of this heater depends on the overall design, the power of the immersion cooler (40 w), and the heat liberated by the
circulating pump. In the present case, a 25 w showcase bulb was found to be quite adequate. One of the most critical aspects of the system is the position of the thermistor relative to that of the heating element. If it is placed too far, there is a time lag between the detection and the control and the thermostating system oscillates. If it is too close, the thermal inertia of the heating element introduces an error in the absolute value of the temperature. In the present system, a distance of in. was found appropriate to cancel all periodic temperature fluctuations in the loop. A fine adjustment of this distance is possible if the stainless steel casing of the thermistor is fitted in a Swagelok type "0" ring joint. Auy sensitive proportional temperature controller may be used (for exampIe, Model PTC-1000, Tronac Inc., Provo, Utah). All tubing and jackets must be well insulated. The advantages of this closed-loop thermostat are: 1) elimination of rapid fluctuations due to temperature gradients in the liquid, 2) overall compactness, 3) ease of changing the temperature of the system rapidly because of the small quantity of liquid involved, 4) possibility of programming the temperature change if the rate of temperature change is low compared with the time conitant of the system. Two such thermostating systems have now been in use for over six months on a continuous basis on two different microcalorimeters; a regulation of *O.OOl°C near 25'C has been achieved with a peak to peak ripple under 0.005'C. One of us (C. J.) would like to thank Hydro Qut5bec for the award of a scholarship. 'This research was supported by the Defense Research Board Canada, Grant number 5401-05. z P ~P., JOLICOEUR, ~ ~ ~ C.,~ AND, DESNOYERS, J. E., Rev. Sci Inst., 39, (1968), in press. of
clolsd-loop thermostot.
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Journal of Chemical Education
