HF ABSOLUTE CAVITY RADIOMETER

Model HF, AHF

The self-calibrating Absolute Cavity Pyrheliometer, Model HF, has been a reference standard level device for many years. The sensor consists of a balanced cavity receiver pair attached to a circular wire-wound and plated thermopile. The blackened cavity receivers are fitted with heater windings which allow for absolute operation using the electrical substitution method, which relates radiant power to electrical power in SI units. The forward cavity views the direct beam through a precision aperture. The precision aperture area is nominally 50 mm² and is measured for each unit. The rear receiver views an ambient temperature blackbody. The HF radiometer element with baffle tube and blackbody are fitted into an outer tube which acts as the enclosure of the instrument. The Model AHF has an automatic shutter attached to the outer tube.

The operation of the cavity radiometer, and the measurement of the required parameters is performed using an appropriate control box. The control functions include setting of the calibration heater power level, activation of the calibration heater, selection of the signals to be measured and control of the meter measurement functions and ranges. The measured parameters include the thermopile signal, the heater voltage and the heater current which is measured as the voltage drop across a 10 Ohms precision resistor. The instrument temperature may also be measured using an internally mounted thermistor. The meter resolution of 100 nV allows for a thermopile signal equivalent in radiation of approximately 0.1 Wm^-2.

Control boxes for manual or manual/automatic are available. The control box can operate either one radiometer in the measurement mode or two radiometers in the comparison mode. Automatic operation allows for computer control of shuttering, calibration heating and measurement functions. Calculation operations and data storage are also possible under computer control. Programs for independent, automatic measurement and cavity radiometer comparison are supplied with automatic units.

Although these are absolute devices, the radiometers are compared with the EPLAB reference cavity radiometers which have participated in the International Pyrheliometric Comparison (IPC) and other intercomparisons and are directly traceable to the World Radiation Reference (WRR).