Kipp&Zonen BSRN Scientific Solar Monitoring System - Manual (Page 111)

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B 3. Cavity Radiometers and Pyrheliometers
B 3.1 Eppley Laboratory HF/AHF Cavity Radiom eter
The self-calibrating Absolute Cavity Pyrheliom eter has been a reference standard device for m any years.
The sensor consists of a balanced cavity receiver pair attached to a circular wirewound and plated therm opile.
The blackened cavity receivers are fitted with heater windings which allow for absolute operation using
the electrical substitution m ethod, 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 nom inally
50 m m and is m easured for each unit. The rear receiver views an am bient tem perature blackbody. The
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HF radiom eter elem ent with baffle tube and blackbody are fitted into an outer tube which acts as the enclosure
of the instrum ent. The instrum ent is weather proof when the window is m ounted. The m odel 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 radiom eter, but at reduced accuracy. An adaptor is supplied for m ounting
to Eppley solar trackers. The HF cavity radiom eter has been used for m easurem ent of the extraterrestrial
solar radiation from the Nim bus 7 ( 14 years) and the LDEF (6 years) satellites and is space proven.
The operation of the cavity radiom eter, and the m easurem ent of the required param eters, is perform ed
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 m easured and control of the m eter
m easurem ent functions and ranges. The m easured param eters include the therm opile signal, the heater
voltage and the heater current which is m easured as the voltage drop across a 10 LI precision resistor.
The instrum ent tem perature m ay also be m easured using an internally m ounted therm istor. The m eter
resolution of 100 nV allows for a therm opile signal equivalent in radiation of approxim ately 0.1 W m .
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Control boxes for m anual only, m anual and autom atic and autom atic only operation are available. The
control box can operate either one radiom eter in the m easurem ent m ode or two radiom eters in the com parison
m ode by changing from single to dual operations cable. Two cables are supplied with each unit. The autom atic
operation allows for com puter control of all shuttering, calibration heating and m easurem ent functions.
Calculation operations and data storage are also possible under com puter control. Program s for independent,
autom atic m easurem ent and cavity radiom eter com parison are supplied with autom atic units.
Although these are absolute devices, the radiom eters are com pared with the EPLAB reference cavity
radiom eters which have participated in the EPC's and other inter-com parisons and are directly traceable
to the W RR. The References are HF’s SN 14915 and SN 27798. The H F which is part of the W SG is
SN 18748.
Specifications
Radiom eter :
Sensor: 60 junction circular wirewound and plated therm opile with
balanced cavity receivers (. 350 S)
Cavity: Inverted cone within a cylinder coated with specular black paint.
em issivity $ 0.999
Aperture area: nom inal 50 m m : each m easured using precision pins 2
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) -1 2
Size: 5.5 in. diam eter at base; 13.32 in long without connector and
adaptor, 7 in x 5 in at shutter housing; 3.5 in. dia. outer tube
W eight 9.25 lb; 11.5 lb with tracker adaptor
W indow m aterial: Corning 7940