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Calorimeters

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Calorimeters

  • Introduction to Calorimeters
  • Calorimeter Load Accuracy
  • Calorimeter Calibaration
  • Calorimeter Selection Chart
  • Load Selection Chart
  • 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 liquid cooled RF load with closely coupled thermopile and calibration resistor.
  • Circulating pump/heat exchanger.
  • Signal conditioning board with display.
  • 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
    %
    %
    %
    %
    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

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