Primary power standards in the microwave domain are realized using a
calorimetric technique, usually identified with the used measurement system,
i.e., the microcalorimeter. It is adjusted for measurement of power ratios
with a relative accuracy that, after an appropriate system calibration, is
of order of 10−3, at least in the microwave domain (1 GHz–18 GHz).
Hereby we describe the calibration process implemented at the Istituto
Nazionale di Ricerca Metrologica (Italy) for realizing a coaxial power
standard based on indirect heating thermocouples. Particular regard is
devoted to describe the nearly ideal thermal load used for determining the
microcalorimeter losses and their influence on the measurand accuracy.