Tuned-circuit dual-mode Johnson noise thermometers

摘要

Dual-mode Johnson noise and dc resistance thermometers can be used in control systems where prompt indications of temperature changes and long-term accuracy are needed. Such a thermometer is being developed for the SP-100 space nuclear electric power system that requires temperature measurement at 1400 K in space for 10 years, of which 7 are expected to be at full reactor power. Several direct-coupled and transformer-coupled, tuned resistance-inductance-capacitance (RLC) circuits that produce a single, continuous voltage signal were evaluated for noise temperature measurement. The simple direct-coupled RLC circuit selected provides a mean-squared noise voltage that depends only on the capacitance used and the temperature of the sensor, and it is independent of the value of or changes in the sensor resistance. These circuits provide a noise signal with long-term accuracy but require integrating noise signals for a finite length of time. The four-wire resistor for the noise temperature sensor allows simultaneous dc resistance measurements to be made that provide a prompt, continuous temperature indication signal. The dc current mode is employed continuously, and a noise voltage measurement is made periodically to correct the temperature indication. The differential noise voltage preamplifier used substantially reduces electromagnetic interference (EMI) in the system. A sensor has been tested that should provide good performance ((plus minus)1% accuracy) and long-term (10-y) reliability in space environments. Accurate noise temperature measurements were made at temperatures above 1300 K, where significant insulator shunting occurs, even though shunting does affect the dc resistance measurements and makes the system more susceptible to EMI. 14 refs.