Kipp&Zonen BSRN Scientific Solar Monitoring System - Manual (Page 112)

100
CONTROL BOX:
Size: 7 in. high x 17 in. wide x 16 in. deep
W eight: 23 lb (approx)
Power requirem ent: 115 VAC 60 @ or 230 VAC 50 Hz selectable
B 3.2 PMOD/PMO6
PMO-6 Absolute radiom eter (excerpted from Applied Optics, Vol. 25, Page 4173, Novem ber 15, 1986)
The PMO6 radiom eter is based on the m easurem ent of a heat flux using an electrically calibrated heat
flux transducer. The radiation is absorbed in a cavity which ensures a high absorptivity over the spectral -
range of interest for solar radiom etry. The heat flux transducer consists of a therm al im pedance with resistance
therm om eters to sense the tem perature difference across it. Heat developed in the cavity is conducted
to the heat sink of the instrum ent and the resulting tem perature difference across the therm al im pedance
is sensed. The sensitivity of the heat flux transducer is calibrated by shading the cavity and m easuring
the tem perature difference while dissipating a known am ount of electrical power in a heater elem ent which
is m ounted inside the cavity It is advantageous to determ ine the electrical power which is needed to produce
the sam e tem perature difference as was observed with the cavity irradiated, because in this case the
heat losses are the sam e during radiative and electrical heating—even if nonlinear effects are involved.
During practical operation of the instrum ent, an electronic circuit m aintains the tem perature signal constant
by controlling the power fed to the cavity heater—independent of the m ode, that is, whether the cavity
is shaded or irradiated. The substituted radiative power is then equal to the difference in electrical power
as m easured during the shaded and irradiated periods, respectively.
Changes of the tem perature of the heat sink m ay also produce a tem perature signal. Therefore, two heat
flux transducers with m atched tim e constants are com bined to form a differential heat flux transducer.
The temperature difference m easured between the two tops of the therm al im pedances is then—depending
on the quality of the m atching—largely insensitive to changes of the tem perature of the heat sink.
The instrum ent m easures irradiance, hence its receiver area has to be accurately known. A precision
aperture of nom inally 5-m m diam eter is placed in front of the prim ary cavity. A second aperture of 8.35-m m
diam eter acting as a view-lim iting aperture and defining a field of view of 5/ is placed 95.4 m m in front
of the precision aperture. This geom etry puts only a m oderate ~ 0.75/ requirem ent on the solar pointing.
All the apertures of the so-called m uffler are in the shadow of the view-lim iting aperture. The purpose
of the m uffler is to reduce the sensitivity to wind effects and to increase the therm al m ass of the heat sink
of the instrum ent.
The cavities are m ade of electro-deposited silver and are gold-plated on their outside. They are soldered
onto the therm al im pedances m ade from stainless steel. The therm al im pedances are in turn soldered
to the copper heat sink of the instrum ent. The heater elem ent in the cavities is a flexible printed circuit.
It is etched in a 5 :m constantan foil supported by a 20 :m Kapton foil. It is glued to the cone-shaped
part of the cavity at the sam e spot as the radiation entering the cavity first im pinges on the cavity walls.
Its resistance is ~90 S and a four-wire term inal configuration is provided to allow for accurate m easurem ents
of the electrical power dissipated in the heater. All the inner surfaces of the cavity are coated with a thin
layer of specularly reflecting black paint. The resistance therm om eters are m ade from copper wire of 0.03-m m
diam eter by winding it around the joint of the therm al im pedance with the cavity and the heat sink, respectively.
The four therm om eters of the two heat flux transducers, each with a resistance of ~100 S, are wired in
a bridge circuit to sense the difference of tem perature between the two cavities. The bridge is trim m ed
with a piece of the sam e copper wire to yield zero response with the two cavities held at the sam e tem perature.
The precision aperture is fabricated from tem pered stainless steel. Its roundness is better than 0.2 :m
and the cylindrical part of the aperture edges has a length of only 20 :m .
Characteristics
W orking Principle Electrically calibrated cavity radiom eter. Autom atic operation with alternating
observation and reference phases.