Dutton, E.G., J.J. Michalsky, T. Stoffel, B.W . Forgan, J. Hickey, D. W . Nelson, T.L. Alberta and I. Reda,
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2001: Measurement of broadband diffuse solar irradiance using current commercial instrumentation with a
correction for thermal offset errors. Jour. Atmos. Ocean. Tech., 18, 297 - 314.
W ardle, D.I. et al., 1996: Improved measurements of solar irradiance by means of detailed pyranometer
24
chracacterisation. Solar Heating and Cooling Programme Task 9, International Energy Agency Report
IEA-SHCP-9C2, Atmospheric Environment Service, Downsview, Ontario.
Philipona, R. C. Fröhlich, Ch. Betz, 1995: Characterization of pyrgeometers and the accuracy of
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atmospheric long-wave radiation instruments. Applied Optics, 34(9) 1598-1605.
71
changes by greater than 0.5% over the operating temperature range of the instrument, a responsivity
correction factor should be applied.
The zero-offset due to thermal emittance should be corrected, but presently no agreement on the correction
methodology has been reached. Two common methods are presented below:
(1) An empirical correction factor dependent on instrument type can be used of the form :
23
01 2
os = b + b NetIR + b DC
where: os = offset in W m
-2
NetIR = the net IR irradiance measured by the thermopile of
a shaded and ventilated pyrgeometer
dome case
DC = F [T - T ]
44
T = measured pyrgeometer temperatures
F = Stefan-Boltzmann constant
x
b = regression coefficients dependent on instrument
(2) A similar correction using the IR irradiance has been used with several instruments when
they are mounted in the Canadian Meteorological Service venilated housing: This methodology
correlates night time offset with the incoming infrared radiation. The relationship for Eppley
PSP’s and Kipp and Zonen CM11's is approximately :
24
Z = -1.5 + 0.025 P
where: Z = the zero offset
P = the infrared signal as measured by the pyrgeometer
As both of these methods are determined empirically on a limited number of instruments, the coefficients
should not be applied without first testing the results on the individual instruments on which they are
to be used.
A less accurate method of determining the offset, is through the linear interpolation of the 60-minute-mean
value of the pyranometer signal from before and after astronomical twilight ($108°) for the same day.
This assumes that the infrared response of the instrument remains reasonably constant throughout
the solar day, which is not the case in conditions of scattered cloud.
9.2.3 Pyrgeometers
The pyrgeometer signal should be based upon the calibration constants directly traceable to the WRC
blackbody. The flux should be calculated as :
25
where = thermopile responsivity (:V W m )
-1 2
= instrument dependent calibration constants
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