Continuous Control Thermoregulator

Abstract

Conditions determining constancy of furnace temperature are discussed, and it is pointed out that the effectiveness of a thermostat should be assessed by comparing the temperature fluctuations of the uncontrolled furnace with those of the controlled furnace. A continuous control thermoregulator of the photocell-thyratron type in which the controlling influence is applied through a saturated choke is described. Records obtained with different furnaces show that variations in supply voltage are mainly responsible for furnace temperature fluctuations, and that the thermoregulator reduces these fluctuations by a factor of 1/100. The use of a photocell in conjunction with a phase-controlled thyratron for producing in the latter an anode current which would follow progressive changes in the illumination of the photocell was described by Hull(1) in 1929. The device has been employed by Zabel and Hancox(2) for maintaining a constant temperature in a furnace. The temperature is measured by thermocouple or resistance thermometer, the out-of-balance potential being applied to a galvanometer the mirror of which reflects light on to a V-slit in front of the photocell. The amount of light falling on the cell determines the thyratron current, a portion of which passes through the furnace windings. This method of applying the thyratron current would appear to require, in order to correct for mains voltage fluctuations of the magnitude generally experienced, a greater anode current than may be safely passed by the ordinary type of gas-filled triode. Provided, however, that the system possesses the necessary range of control, it is possible to maintain the temperature constant within very fine limits on account of the continuous action of the correcting influence. (Zabel and Hancox claim a deviation of only 0.06° at 880° C.) Other types of thermoregulator (Sturtevant(3), Coates(4)), eliminate the galvanometer-photo-cell arrangement. In these, the out-of-balance potential from a resistance thermometer d.c. or a.c. bridge is amplified and the output operates a relay. An account of the use, at temperatures up to 1100° C., of this type of thermostat has been given recently by Yates(5). The system devised by Coates can be used to provide the continuous correcting influence (as opposed to on-off control by relays) which is necessary for precise temperature control, but no reports of its performance, used in this manner, have yet been published.