The Model 431B Power Meter, with temperature compensated thermistor mounts, measures rf power from 10 microwatts (-20 dBm) to 10 milliwatts (+10 dBm) in the 10 MHz to 40 GHz frequency range. Direct reading accuracy of the instrument is ±3% of full scale.

The design of the Model 431 and its thermistor mount, results in almost complete freedom from measurement error caused by ambient temperature changes. The instrument incorporates two self balancing bridges with one arm of each bridge being a thermistor. The two matched thermistors, both located within the mount, are thermally coupled, but electrically isolated. One thermistor is used to absorb rf power; the other is used to provide temperature compensation. Thus, the thermal drift problems normally associated with the thermistor-power meter arrangement have been greatly reduced. A single setting of the ZERO control on the most sensitive power range is maintained within 1% for all higher power ranges.

The temperature compensated thermistor mounts used with the instrument are specifically designed for Model 431A/B Power Meters. Coaxial (HP478A and HP8478B) and waveguide thermistor mounts (486A series) cover the 10 MHz to 40 GHz frequency range.