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B 3. Cavity Radiometers and Pyrheliometers
B 3.1 Eppley Laboratory HF/AHF Cavity Radiometer
The self-calibrating Absolute Cavity Pyrheliometer has been a reference standard device for many years.
The sensor consists of a balanced cavity receiver pair attached to a circular wirewound 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 solar beam through a precision aperture. The precision aperture area is nominally
50 mm and is measured for each unit. The rear receiver views an ambient temperature blackbody. The
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HF radiometer element with baffle tube and blackbody are fitted into an outer tube which acts as the enclosure
of the instrument. The instrument is weather proof when the window is mounted. The model AHF has
an autom atic shutter attached to the outer tube. A separate, m ounted window is supplied with each unit
for continuous operation of the radiometer, but at reduced accuracy. An adaptor is supplied for mounting
to Eppley solar trackers. The HF cavity radiometer has been used for measurement of the extraterrestrial
solar radiation from the Nimbus 7 ( 14 years) and the LDEF (6 years) satellites and is space proven.
The operation of the cavity radiometer, and the measurement of the required parameters, is performed
using the 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 LI 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 W m .
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Control boxes for manual only, manual and automatic and automatic only operation are available. The
control box can operate either one radiometer in the measurement mode or two radiometers in the comparison
mode by changing from single to dual operations cable. Two cables are supplied with each unit. The automatic
operation allows for computer control of all 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 EPC's and other inter-comparisons and are directly traceable
to the WRR. The References are HF’s SN 14915 and SN 27798. The HF which is part of the WSG is
SN 18748.
Specifications
Radiometer:
Sensor: 60 junction circular wirewound and plated thermopile with
balanced cavity receivers (. 350 S)
Cavity: Inverted cone within a cylinder coated with specular black paint.
em issivity $ 0.999
Aperture area: nominal 50 mm : each measured using precision pins
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Field of view: 5° central; 1.6° unencum bered (0.8° slope); 8.5° m ax.
Heater resistance: 150 S (Approx.)
Irradiance sensitivity: I :V W m (approx)
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Size: 5.5 in. diameter at base; 13.32 in long without connector and
adaptor, 7 in x 5 in at shutter housing; 3.5 in. dia. outer tube
Weight 9.25 lb; 11.5 lb with tracker adaptor
Window material: Corning 7940
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