Kippzonen BSRN Scientific Solar Monitoring System Manual de usuario descargar pdf (Pagina 112)

100

CONTROL BOX:

Size: 7 in. high x 17 in. wide x 16 in. deep

W eight: 23 lb (approx)

Power requirement: 115 VAC 60 @ or 230 VAC 50 Hz selectable

B 3.2 PMOD/PMO6

PMO-6 Absolute radiometer (excerpted from Applied Optics, Vol. 25, Page 4173, November 15, 1986)

The PMO6 radiometer is based on the measurement 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 radiometry. The heat flux transducer consists of a thermal impedance with resistance

thermometers to sense the temperature difference across it. Heat developed in the cavity is conducted

to the heat sink of the instrument and the resulting temperature difference across the thermal impedance

is sensed. The sensitivity of the heat flux transducer is calibrated by shading the cavity and measuring

the temperature difference while dissipating a known amount of electrical power in a heater element which

is mounted inside the cavity It is advantageous to determine the electrical power which is needed to produce

the same temperature difference as was observed with the cavity irradiated, because in this case the

heat losses are the same during radiative and electrical heating—even if nonlinear effects are involved.

During practical operation of the instrument, an electronic circuit maintains the temperature signal constant

by controlling the power fed to the cavity heater—independent of the mode, that is, whether the cavity

is shaded or irradiated. The substituted radiative power is then equal to the difference in electrical power

as measured during the shaded and irradiated periods, respectively.

Changes of the temperature of the heat sink may also produce a temperature signal. Therefore, two heat

flux transducers with matched time constants are combined to form a differential heat flux transducer.

The temperature difference measured between the two tops of the thermal impedances is then—depending

on the quality of the matching—largely insensitive to changes of the temperature of the heat sink.

The instrument measures irradiance, hence its receiver area has to be accurately known. A precision

aperture of nominally 5-mm diameter is placed in front of the primary cavity. A second aperture of 8.35-mm

diameter acting as a view-limiting aperture and defining a field of view of 5/ is placed 95.4 mm in front

of the precision aperture. This geometry puts only a moderate ~ 0.75/ requirement on the solar pointing.

All the apertures of the so-called muffler are in the shadow of the view-limiting aperture. The purpose

of the muffler is to reduce the sensitivity to wind effects and to increase the thermal mass of the heat sink

of the instrument.

The cavities are made of electro-deposited silver and are gold-plated on their outside. They are soldered

onto the thermal impedances made from stainless steel. The thermal impedances are in turn soldered

to the copper heat sink of the instrument. The heater element 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 same spot as the radiation entering the cavity first impinges on the cavity walls.

Its resistance is ~90 S and a four-wire terminal configuration is provided to allow for accurate measurements

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 thermometers are made from copper wire of 0.03-mm

diam eter by winding it around the joint of the therm al im pedance with the cavity and the heat sink, respectively.

The four thermometers of the two heat flux transducers, each with a resistance of ~100 S, are wired in

a bridge circuit to sense the difference of temperature between the two cavities. The bridge is trimmed

with a piece of the same copper wire to yield zero response with the two cavities held at the same temperature.

The precision aperture is fabricated from tempered 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

Working Principle Electrically calibrated cavity radiometer. Automatic operation with alternating

observation and reference phases.

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