---
title: "Static and dry calorimeters"
subject: "RF and Microwave Engineering"
chapter: 8
key: "semester_7/rf_and_microwave_engineering:static-dry-calorimeters"
source: https://www.bctengineer.com/q/semester_7/rf_and_microwave_engineering/static-dry-calorimeters
site: BCT Engineering
---

# Static and dry calorimeters

RF and Microwave Engineering · Chapter 8: RF and Microwave Measurements

- Asked in: 2076 Bhadra, 10 marks
- Video: [Power Measurement of Microwave Signal | Microwave Engineering | R K Classes | Lec 63 | Join Telegram](https://www.youtube.com/watch?v=i6x2HIbO0hA) — R K Classes
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_7/rf_and_microwave_engineering/static-dry-calorimeters
- Whole chapter: https://www.bctengineer.com/md/semester_7/rf_and_microwave_engineering/chapter-8

## Question

Explain power measurement using static and dry calorimeters.

## Short answer

Static calorimeter: a fixed mass of liquid (water/oil) in an insulated load absorbs RF for time t; P = m c ΔT/t. Dry calorimeter: a solid matched load with thermocouple/thermopile; power is obtained from temperature rise rate or by DC substitution (equal thermopile output).

## Answer

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

```text
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.

```text
   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

| Point | Static (liquid) | Dry |
|---|---|---|
| Absorber | Fixed water/oil | Solid resistive load |
| Sensor | Thermometer, thermocouple | Thermopile |
| Power range | Medium–high (W to kW for short time) | mW to tens of W |
| Accuracy | Good, needs heat-loss correction | Very good (used as standard) |
| Response | Slow | Slow |

## Check yourself

The answer's key points as multiple-choice questions, with the correct option and why.

1. Which relation gives power in a calorimeter?
   A. P = m c dT / t
   B. P = m c t / dT
   C. P = c dT t / m
   D. P = m dT / (c t)
   Answer: A. Energy absorbed (P x t) equals m c dT, so P = m c dT / t. (Key point: Basic calorimeter principle)

2. Why is a known DC heater used in the static calorimeter load?
   A. To increase RF absorption
   B. To find the effective heat capacity, removing vessel and loss errors
   C. To change the frequency
   D. To measure VSWR
   Answer: B. Comparing RF heating with known DC heating calibrates out heat capacity and loss errors. (Key point: Role of calibration)

3. What distinguishes a dry calorimeter from a static one?
   A. It uses a flowing liquid
   B. It uses a bolometer bridge
   C. It uses a solid absorber with a thermopile sensor
   D. It needs no matched load
   Answer: C. A dry calorimeter has a solid resistive/lossy load sensed by a thermopile or thermocouple. (Key point: Dry calorimeter absorber)

4. What is the purpose of the reference load and differential thermopile in a twin-load dry calorimeter?
   A. To double the RF power
   B. To measure frequency
   C. To measure reflection
   D. To cancel ambient temperature drift
   Answer: D. Both loads see the same ambient; only one gets RF, so the differential output cancels drift. (Key point: Twin-load dry calorimeter)

## Related in RF and Microwave Engineering

- [Power measurement with static calorimeter](https://www.bctengineer.com/question.md?key=semester_7%2Frf_and_microwave_engineering%3Astatic-calorimeter-power) — RF and Microwave Engineering, chapter 8, asked 2×
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- [VSWR meter and low power (bolometer) measurement](https://www.bctengineer.com/question.md?key=semester_7%2Frf_and_microwave_engineering%3Avswr-meter-power-measurement) — RF and Microwave Engineering, chapter 8, asked 2×
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- [Measurement parameters and low power device](https://www.bctengineer.com/question.md?key=semester_7%2Frf_and_microwave_engineering%3Alow-power-measurement-device) — RF and Microwave Engineering, chapter 8

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