  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.
