Power measurement with static calorimeter

A calorimeter measures high microwave power by converting it completely into heat in a load and measuring the temperature rise. In a static calorimeter, a fixed (non-flowing) mass of absorbing material is heated.

Principle

P = m·c·ΔT / t
m  = mass of absorbing load (kg)
c  = specific heat (J/kg·°C)
ΔT = temperature rise (°C) in time t (s)

Construction and working

 RF in      insulated (thermally isolated) load
 ======[ waveguide ]==[ absorbing load ]
                         |  thermometer /
                         |  thermocouple
                      [ ΔT readout ]

1. A matched, thermally insulated load (water, lossy dielectric or absorbing block) terminates the waveguide.
2. The microwave power is applied for a known time t; the load absorbs all of it.
3. The temperature rise ΔT is measured with a thermometer or thermocouple.
4. Power is calculated from P = m·c·ΔT/t. To remove the need to know m and c exactly, the same load is calibrated by applying a known DC or 50 Hz power and noting its temperature rise (substitution method):

P_RF = P_DC × (ΔT_RF / ΔT_DC)   (same time)

Example

A 0.1 kg water load (c = 4186 J/kg·°C) rises by 2.4 °C in 60 s:

P = 0.1 × 4186 × 2.4 / 60 = 16.74 W

Features

• Suitable for medium to high power (watts to kilowatts); accurate (about ±1 %) because it is an absolute method.
• Slow; heat losses through insulation and the mount cause error, so good thermal isolation is needed.
• For continuous high power, the circulating (flow) calorimeter is used instead.