Calorimeter wattmeter and single bridge bolometer limits

A calorimeter wattmeter measures high microwave power (watts to kilowatts) by absorbing it in a fluid load and comparing the heating with a known low-frequency or DC power.

Calorimeter wattmeter

Most practical calorimeter wattmeters are flow (circulating) type with a substitution balance:

            +------------+       +------------+
 oil/water->| RF load    |------>| Input head |--+
 stream     | (absorbs   |       | (temp.     |  |
            |  RF power) |       |  sensor)   |  |  bridge
            +------------+       +------------+  +--> null
            +------------+       +------------+  |  meter
 same    -> | DC/LF load |------>| Ref. head  |--+
 stream     | (heater)   |       | (temp.     |
            +------------+       |  sensor)   |
                                 +------------+

Working principle:

1. A liquid (water or oil) is pumped at a constant rate through the RF load, which is matched to the line and absorbs all the microwave power.
2. The liquid heats up; its temperature rise is sensed by a temperature-sensitive element (thermistor or thermocouple) in the input head.
3. A second, identical path has a reference head heated by a known DC or 60 Hz power.
4. Both heads form arms of a bridge. The DC power is adjusted until the bridge balances, meaning both streams have the same temperature rise.
5. Then RF power = known DC power, read on an ordinary wattmeter.

Without the reference, power can be found from the flow directly:

P = ρ · v · c · ΔT
Water: P ≈ 4.187 · v(cm³/s) · ΔT  W

Features: wide range (10 mW to several kW), accuracy about ±1–5 %, broadband (load is well matched), but slow response and bulky (pump, plumbing).

Limitations of the single bridge bolometer

1. Ambient temperature drift: the bolometer also responds to room temperature, so zero drifts; there is no compensating element.
2. Low power only: limited to about 10 mW; higher power burns out the barretter or thermistor.
3. Manual balancing: each reading needs the DC to be adjusted by hand, so it is slow and not direct reading.
4. Mismatch error: bolometer resistance changes with power, so the mount match and reflected power change.
5. DC–RF substitution error: RF and DC currents do not heat the element in exactly the same way (skin effect, current distribution).
6. Average power only: it cannot show pulse peak power directly.