A proportional thermostat with 10 microdegree stability

The general problem of designing active proportional thermostats is discussed, and a design philosophy is deduced. The application of this philosophy to the construction and operation of a precision proportional thermostat for geophysical accelerometers in boreholes is described in detail. The thermostat discussed is of small size and weight, is simple to operate, and requires less than 5 W of power. It maintains the temperature of an instrument and its vacuum enclosure of 1688 cm3 total volume and 7 kg mass constant within 10 μ°C over periods of days in the presence of fluctuations in the ambient temperature of several degrees. Various models of the thermostat, with several different configurations of thermal insulation, have been tested extensively in both laboratory and borehole environments. Stable proportional operation has been achieved with setpoint temperatures ranging from 15 to 35 °C and with the difference between the setpoint and mean ambient temperatures varying from 4 to 10 °C. The electroni...