Static and dynamic circulating calorimetry; measurables

A calorimeter measures microwave power by converting it fully into heat in a matched load and measuring the temperature rise. It is used for medium and high power (about 10 mW up to kW).

Static calorimetry

In a static calorimeter the load (a fixed mass of water, oil or a solid absorber) is thermally insulated and does not flow.

 RF in ===> [ matched load in insulated ] -- thermometer
            [ fixed mass m, sp. heat c  ]

1. Microwave power is absorbed in the load for time t.
2. The temperature rises by ΔT, read with a thermometer or thermocouple.
3. Power absorbed:

P = m · c · ΔT / t     (watts)

Example: 0.2 kg of water (c = 4186 J/kg·°C) warms by 5 °C in 120 s → P = 0.2 × 4186 × 5 / 120 ≈ 34.9 W.

The method is simple but slow, and heat loss to the surroundings must be corrected.

Dynamic (circulating / flow) calorimetry

The fluid (usually water or oil) flows continuously through the load. In steady state all absorbed power is carried away by the fluid.

cold fluid in --> [ matched RF load ] --> hot fluid out
   T_in              ^ RF power           T_out
   flow rate v       |

1. The fluid enters at T_in and leaves at T_out.
2. With flow rate v (volume/s), density ρ and specific heat c:

P = ρ · v · c · (T_out − T_in)
For water: P (W) ≈ 4.187 × v (cm³/s) × ΔT (°C)

3. The flow rate is fixed with a flowmeter; ΔT is measured by thermocouples or a temperature bridge.

Being continuous, it suits high power (kW) transmitters and has a faster reading than the static method.

Microwave measurables and tools