An isothermal method was developed in order to reduce the efficiency measur
ement time of;microwave power standards, using microcalorimeters, The first
results given in the two waveguide standards IEC R100, (8.2-12.4) GHz and
R220 (18-26.5) GHz show good agreement between classical and isothermal met
hods. The measurement time has been reduced by a factor of more than three,
while keeping the same uncertainties.