Temperature Measurement of an Isolated, Isochoric Dewar

The time limit of operation per liquid helium charge of the FM-1 dewar is determined by its thermal characteristics and the maximum allowable internal pressure. Efficient machine operation demands that as much operating time be obtained per helium charge as practical. It is, therefore, desirable to measure accurately the dewar temperature in order to cease operating shortly before the dewar´s burstdiaphragm operates to flood the vacuum system. This paper describes a telemeter system designed to monitor the temperature of an isolated dewar. The output frequency of the transmitter is made proportional to temperature via a current-controlled oscillator. Input is from the same temperature-dependent composition resistor sensors that are employed to determine dewar temperature in the nonisolated mode. Power is supplied to the dewar electronics via the inductively coupled telemeter link.