A CHEAP and ACCURATE STUDENT-TYPE THERMOSTAT VINCENT E. PARKER, PHILIP E. HATFIELD,
AND
A. STRICKLER
Evansville College, Evansville, Indiana
T
HE purpose of this paper is to describe a portable, on the sides with cross-bars of iron rods to make a inexpensive, yet accurate student type thermostat suitable means of support for associated apparatus. for use in the small college laboratory. For heaters, sockets are provided for two 32-candle The most imperative requirement is that the appara- power 175-watt 115-volt carbon filament bulbs, altus be inexpensive, but at the same time accuracy though in practice it was found that one bulb was cannot be sacrificed. The bath is made from an su5cient to maintain the bath a t constant temperaeight-gallon pottery crock which is placed in a movable ture and during the summer weather a %watt 115wooden container and insulated on all sides by two volt showcase bulb was found to be satisfactory. In inches of rock wool. The container is covered with fact, when the current was cut off there was still a plywood and mounted on ~bber-tiredcasters so as slight glow due to the passage of the current through the to be readily movable from room to room. The over- condenser Cz which facilitated reading of the apparatus all dimensions are: height to table top, 36$/&inches; in the bath and gave less temperature lag when it was height overall, 56'/, inches; table top, 25'/2 inches turned on again. A cooling coil the diameter of the square. For convenience the apparatus is made desk- inside of the bath and consisting of three turns of I/,high. Standard clamp-type ringstands are mounted inch copper tubing is provided for use in hot weather.
No attempt has been made to control the water flow thermostatically, but good results have been obtained by simply adjusting the flow to meet changing conditions. The stirrer is made from a small 115-volt stirrer, or cream whipper, with the paddle replaced by a long, '/&ch brass shaft and 2-inch propeller cut
bulb in series with the filament permits use of the 115volt line for filament current. This eliminates the necessity of a filament transformer. For plate voltage, the 115-volt line is self-rectified. The accompanying diagram, giving the working circuit of the complete set-up, is a modified version of the circuit described by D. J. Beaver and J. J. Beaver.' The circuit has been redesigned to permit the use of alternating current, and simplified, so as to eliminate resistors R1,Rz,Rs, R4, and condenser Cl. A heavier current-drain tube is employed in order that a cheaper, less sensitive relay may be used. The complete cost of the thermostat, built as far as possible from parts picked up around the laboratory, was about fifteen dollars. The performance to date has been very satisfactory. In a typical performance test, after allowing approximately an hour for adjust-
CI-Two-microfarad. 200-volt paper condenser. Ct--Two-microfarad, 200-volt paper condenser. I,-Forty-watt, 115-volt lamp. +Sixty-watt. 115-volt lamp. (Other wattage lamps may be necessary for some stirrer motors.) &One hundred fifty-watt, 115-volt lamp. LI, Ls-One hundred seventy-five-watt. 32-candlepower, 115-volt heater lamps. R-Five thousand-ohm, ten-watt resistor. (The y various value of this resistor may ~ l with relays.) Rd-Telephane-type relay. T-Mercury thermoregulator. V T-Vacuum-tube, type LiX 210.
from 16-gage sheet copper. Two lamp sockets in series with the motor and arranged in parallel enable the motor speed to be controlled by means of light hulb resistances. The mercury regulator used to control the heater circuit is of the conventional type with acetone-filled bulb. The capillary stem is of small diameter to assure fine regulation, the rise and fall of the mercury column being about Za/p inches per degree Centigrade. In order to prevent arcing a t the regular contacts, and to facilitate more accurate control, a vacuum tube relay is incorporated. A type U X 210 tube is employed, since i t will handle a relatively heavy current, thus permitting the use of an ordinary telephone-type relay to control the heater current. A 150-watt 115-volt
ments, the bath maintained a constant temperature for an eight-hour continuous run. Measurements of the temperature were made every fifteen minutes with a calorimeter thermometer which was read through a magnifying lens. The temperature a t each reading was accurately 25"C., and there was no visible fluctuation during the heating-non-heating cycle. The photograph gives a general view of the entire set-up.
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BEAWR,D. J. AND J. J. BEAVER, Ind. Ena. Chem., 15,3.5M1 (1923).
