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Eppley Pyrheliometer

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.


Eppley Pyrheliometer

Info as printed on the plate:
Eppley Pyrheliometer
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
Meteorological Branch

Eppley Pyrheliometer

Eppley Pyrheliometer

Eppley is still making radiation measuring equipment, although they no longer look like this one:


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