Static and dry calorimeters

A calorimeter measures microwave power by turning all of it into heat in a matched, thermally isolated load and measuring the resulting temperature change. It is an absolute method, mainly for medium and high power.

Basic principle

Heat energy absorbed = power × time = m · c · ΔT, so

P = m · c · ΔT / t

where m = mass of the absorbing material, c = specific heat, ΔT = temperature rise in time t.

Static calorimeter

In a static calorimeter the absorbing medium does not circulate. A fixed quantity of liquid (water or oil) forms or surrounds the matched load inside a thermally insulated vessel.

   RF in
 =====+=====================
      |  insulated vessel   |
      |   +-------------+   |
      |   | water/oil   |   |-- thermometer /
      |   | (mass m)    |   |   thermocouple
      |   +-------------+   |
       =====================

Working:

1. The waveguide or coax is terminated in a tapered water/oil load matched to the line, so no power reflects.
2. RF power is applied for a known time t.
3. The liquid temperature rises by ΔT. Since the vessel is insulated, almost all heat stays in the liquid.
4. P = m c ΔT / t.

Example: m = 0.2 kg water, c = 4186 J/kg·°C, ΔT = 5 °C in t = 120 s P = 0.2 × 4186 × 5 / 120 ≈ 34.9 W.

Calibration: often a known DC heater in the same load is used to find the effective heat capacity, which removes error due to vessel and loss.

Dry calorimeter

A dry calorimeter uses a solid absorber instead of a liquid, e.g. a resistive film, lossy ceramic or a carbon/iron-loaded load with high thermal capacity.

Working:

1. The solid load is matched to the line and placed in an insulated enclosure.
2. Absorbed RF heats the load. Its temperature is sensed by a thermopile or thermocouple attached to it.
3. Either the rate of temperature rise gives P (= m c dT/dt), or the steady temperature is compared with that produced by a known DC power in the same load (DC substitution): RF power equals the DC power that gives the same thermopile output.
4. Twin-load dry calorimeters use two identical loads – one with RF, one as reference – and a differential thermopile to cancel ambient drift.

Comparison