AbstractThe measurement of the total microwave power in ECRH applications is commonly accomplished by directional couplers or mirrors with gratings which deflect a small portion of the power in conjunction with diode detectors. The nearly quadratic characteristic of a solid state diode allows for the direct measurement of the radiated power. It has an instantaneous response, but the quasi-quadratic behavior is limited to a certain range of the characteristic and the power is measured only at the (quite localized) position of the coupler. The first constraint limits the dynamic range of the measurement and the second introduces errors due to the transverse shift of the beam during long pulses. These drawbacks can be avoided by measuring the window loss power calorimetrically. A calorimeter is described which evaluates the flow rate and temperature difference of the cooling circuit of a vacuum window or mirror.