A pyrheliometer is an instrument for direct measurement of solar irradiance.
The Eppley pyrheliometer measures the length of time that the surface receives sunlight
and the sunshine’s intensity as well.
The Eppley pyrheliometer consists of a thermopile mounted under receivers inside a clear
glass spherical bulb about 3 inches in diameter. The thermocouples are platinum-rhodium (90 - 10 percent)
and gold palladium (60 - 40 percent) and the thermopile comprises several thermocouples. The receivers are
concentric flat metal rings exposed in a common plane; the rings are thermally insulated from each other and
from the mounting. One ring is coated with magnesium oxide; to the underside of this the electrically insulated
cold junctions are attached in close thermal contact. The other ring is coated with lampblack; the hot
junctions are similarly attached to its undersurface. The magnesium oxide has a high reflectivity for
radiation in the solar wavelengths and is a good absorber and emitter in the longer wavelenghts, making for
a low equilibrium temperature on exposure to solar radiation. Lampblack has good absorption in both wavelength
intervals, and due to its greater absorption of the solar wavelengths its equilibrium temperature on exposure
to solar radiation is higher than that of the magnesium oxide. The similarity of the receivers with regard to
absorption of long wave radiation tends to minimize the effect of any long wave radiation (as from the glass
cover) which may fall on the receivers. The bulb contains dry air. The overall response of the instrument to
solar radiation is an e.m.f. rather closely proportional to solar radiation flux density through the plane
of the receivers.
Info as printed on the plate:
Serial No. 6108
This pyrheliometer has been compared with the Meteorological branch reference standard.
The derived value of the constant for the instrument is 6.65 millivolts/cal. cm.-2 min.-1 (int.)
Canada Department of Transport
Eppley is still making radiation measuring equipment, although they no longer look like this one:
THE EPPLEY LABORATORY, INC.
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