Measuring RF power is not an easy job. Testing power sensors is much more difficult.

Bolometers are very good transfer sensors and are especially stable over a long time.

Errors in Instrumentation accuracy can be small, depending on the capabilities in DC metrology.

The limiting factor always is the knowledge of the calibration factor of the reference power sensor.

Some NMIs specify around 0.2 % as their best CMCs. A calibration at this level of accuracy is time consuming and expensive.

A second influence is stability of the environment temperature. ?One never should forget, RF power measurement ??is a thermal measurement.

Instruments to control thermistor mounts had been made by some companies, like HP/ Keysight and Tegam. The old HP 432A (Type II bridge) is a good low cost choice,

If you do not mind DC Substitution. The N432A and Tegam use a NBS Type IV bridge. I attach three links that reflect on power measurement.

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regards,

Hubert

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