Calorimeters
Introduction to Calorimeters
The calorimetric principle is considered the most accurate method for the measurement of RF power. The measurement of electrical power using this principle depends upon the conversion of the electrical energy, as delivered by an RF source, into thermal energy in a resistive load. This temperature rise can be measured (with a thermopile located between the load coolant inlet and outlet, for example), and when applied properly, can provide the means for accurate RF measurement.
A calorimeter is a device which uses the calorimetric principle to accurately measure RF power over a wide range of frequency and power levels and has the ability to measure true average power, even with complex waveforms.
A calorimeter is comprised of:
A calorimeter can provide highly accurate RF power measurements traceable to NIST. Calibration procedures are based on AC substitution calorimetry. A source of AC power is applied to the load to simulate the heating effect of the unknown RF power, and is measured using readily available AC power sources and measuring equipment.
Calorimeter accuracy is expressed as a percentage of full scale, less load error. Only the power that is actually absorbed by the load is measured. All other power (reflected, etc.) is considered part of load error and is not measured. Load error can be predicted by calculation.
Figure 1 shows a typical calorimeter design. Circulating coolant flows through the RF load. A closely coupled thermopile senses the temperature rise across the coolant and produces a DC voltage directly proportional to the RF (or calibration) power applied, assuming flow remains constant. A heat exchanger removes the heat picked up by the fluid and exhausts it to ambient air or another liquid. The cooled fluid is then recirculated to the load in a closed loop. The DC voltage from the thermopile is fed into a signal conditioning board and then to a display, calibrated in RF watts (or a computer interface).
Figure 1
Calorimeter Load Accuracy
Figure 2 shows a typical calorimeter load design. The incident power, is shown as PI (the power that is to be measured). PA represents the power which is actually absorbed by the termination for conversion to heat. Power reflected by the termination and connector is shown as PR. This portion of the load error can be predicted by calculation from VSWR data. PM represents power absorbed by the load, but not transferred to the coolant stream. This factor is minimized by the design of the calorimeter.
Figure 2
PI = Incident Power
PA = Power absorbed by load resistor
PR = Power reflected at input connector
PM = Power absorbed by load parts other than resistor
Calorimeter Calibaration
Calibration is an essential element in maintaining the accuracy of a calorimeter. The specific procedure is referred to as AC substitution calorimetry. A precisely measured amount of AC power is applied to the RF load input (or calibration resistor) which simulates the heating effect of the RF power. The known power is then used to adjust the calorimeter thermopile readout device to match the AC standard. The AC standard is measured with voltmeters and ammeters that have calibration traceable to NIST. This allows the calorimeter to act as a transfer standard to provide the rated accuracy, less load error, offered by the calorimeter.
Calibration equipment usually consists of:
a) An adjustable, regulated source of low distortion AC power.
b) High accuracy, true RMS reading, voltmeter and ammeter (or wattmeter).
Calibration power is applied directly to the RF load 50 ohm coaxial connector, if the load is of the internal resistor type. Many loads, such as waveguide, do not use a resistor; water is both the coolant and dielectric material. In this case a calibration resistor is supplied to simulate the heating effect of the RF load. Refer to Figure 1 for a typical calibration set up.
Calorimeter Selection Chart
These charts allow you to quickly select the proper calorimeter to meet your requirements and match it to a load. Special calorimeters and loads also available.
All calorimeters are offered as systems consisting of the calorimeter and one or more loads.
Model |
Accuracy |
Type Readout |
Power Range (Watts) |
Approximate Weigth (lbs. Calorimeter Only) |
|---|---|---|---|---|
HA-100 HA-1000 HA-5000 HA-10,000 |
½% ½% ½% ½% |
Digital Digital Digital Digital |
15-100 15-1000 50-5000 300-10,000 |
70 70 100 280 |
ADR-100 ADR-1000 ADR-5000 ADR-10,000 |
1% 1% 1% 1% |
Digital Digital Digital Digital |
15-100 15-1000 50-5000 300-10,000 |
70 70 100 280 |
CPM-15,000 CPM-25,000 CPM-30,000 CPM-50,000 CPM-80,000 |
2% 2% 2% 2% 2% |
Digital Digital Digital Digital Digital |
150-15,000 300-25,000 500-30,000+ 500-50,000 500-80,000 |
175 215 250 500 600 |
Load Selection Chart
These charts allow you to quickly select the proper calorimeter to meet your requirements and match it to a load. Special calorimeters and loads also available.
All calorimeters are offered as systems consisting of the calorimeter and one or more loads.
Load Model |
Frequency Range (GHz) |
VSWR |
Connector |
Power (Watts) |
|---|---|---|---|---|
CLWT-13 |
DC-11-33-3.5 |
1.1:11.3:11.35:1 |
N,7/8 EIA |
1000 |
CLWT-53 |
DC-500500-900900-1200 |
1.15:11.1:11.25:1 |
1-5/8 EIA7/8 EIA |
5000 |
CLWT-153 |
1kHz-1 |
1.1:1 |
3-1/8 EIA |
15,000 |
CLWT-253 |
1kHz-900 |
1.1:1 |
3-1/8EIA |
25,000 |
CLWT-54 |
1kHz-900 |
1.1:1 |
3-1/8EIA |
50,000 |
CLWT-84 |
1kHz-800 |
1.15:1 |
6-1/8EIA |
80,000 |
19118 |
1-2.5 |
1.15:1 |
1- 5/8EIA |
3000 |
WLT-650 |
1.12-1.70 |
1.2:1 |
UG-417/U |
50,000 |
WLT-430 |
1.70-2.60 |
1.2:1 |
UG-435/U |
30,000 |
WLT-340 |
2.20-3.30 |
1.2:1 |
UG-553/U |
30,000 |
WLT-284 |
2.60-3.95 |
1.15:1 |
UG-53/U |
25,000 |
WLT-187 |
3.95-5.85 |
1.15:1 |
UG149/U |
10,000 |
WLT-159 |
4.90-7.05 |
1.15:1 |
CMR-159 |
10,000 |
WLT-137 |
5.85-8.20 |
1.15:1 |
UG-344/U |
5000 |
WLT-112 |
7.05-10.00 |
1.15:1 |
UG-51/U |
5000 |
WLT-90 |
8.20-12.40 |
1.15:1 |
UG-39/U |
3000 |
WLT-62 |
12.40-18.0 |
1.15:1 |
UG-419/U |
3000 |
WLT-42 |
18.0-26.5 |
1.15:1 |
UG-596/U |
500 |
WLT-28 |
26.5-40.0 |
1.15:1 |
UG-600/U |
300 |
WLDT-24 |
2.0-4.0 |
1.25:1 |
UG-1572/U |
2000 |
WLDT-29 |
4.0-8.0 |
1.2:1 |
UG-1575/U |
3000 |
WLDT-19 |
5.0-11.0 |
1.2:1 |
Specify |
3000 |