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HP 432A / HP 8478B accuracy problem

Question asked by Werner on Jun 29, 2012
Latest reply on Jun 28, 2013 by matthew_rose
I have a problem with the measurement accuracy of a HP 432A power meter in conjunction with a 8478B thermistor mount. In all ranges the measured power level is about 1 to 2 dB to high. The zeroing procedure is working as specified. The resistance of all four thermistors can be measured "cold" with an ohm meter in the range of 5 to 6 kohms for two in series and they seem to be equal. After warming up I measured the input reflection (swr) with an NWA up 3GHz. The values are significantly better than the specification. 

Everything seems to be OK, but there is a measurement error up to 2dB. The malfunction is not in the power meter but in the sensor.
I read something about two compensation screws in the thermistor mount. But I do not understand precise what their function is.
What is the main problem and what can I do to fix it?
Best regards, Werner