Automated 3He melting curve thermometer

A 3He melting curve thermometer is described which uses a self-balancing capacitance bridge for pressure measurement. The capacitive pressure transducer and the fixed reference capacitor are integrated into one compact unit. Wide dynamic range and high resolution of the bridge are obtained by combining a four-digit ratio transformer and a four-digit integrating voltmeter to determine bridge balance. The simple and fast balance strategy uses a single-step programmable attenuator to control bridge excitation. Application examples of the instrument as a secondary thermometer with mu K resolution and less than 0.1% non-linearity over the 20-200 mK range, using a calibration of only two constants, and as a temperature controller are presented.