A Simple Automatic Furnace Temperature Control

A description is given of a simple furnace temperature control with a platinum resistance thermometer in one arm of an a.c. bridge. The out-of-balance potential is amplified, limited and applied to the grid of an output pentode, the auxiliary grid of which is fed from an a c. supply. The a.c. anode current of the output pentode is used to operate a thermal relay, which controls the furnace current Novel features in the circuit render the instrument equally sensitive at all temperatures between 0° C. and 1000° C., stable, portable and inexpensive. The instrument controls the temperature of an annealing furnace to less than ± 0 5° C. over the whole temperature range, the sensitivity depending on the design of the furnace and the resistance thermometer An additional note describes a simple method of reducing or increasing the furnace temperature slowly with time.