Forgan, B. W ., 1996: A new method for calibrating reference and field pyranometers, Journal of
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Atmospheric and Oceanic Technology, 13 638 - 645.
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(7) An averaging period where the difference between the reference instrument and the field
instruments is greater than 1%, and is greater than 3 standard deviations away from the mean
difference should be discarded.
(8) A minimum of 25 acceptable series must be completed for each comparison.
(9) All changes in the ratio between the reference and the field instrument(s) must be recorded.
Changes of less than 0.1%/year (normalized) need not be reported to the archive.
Changes greater than 0.5%/year (normalized) indicate significant drift in one or both of the
instruments and remedial action should be taken immediately. If a third cavity radiometer
is available, the comparison should be repeated in an attempt to isolate the changes. Once
the problem instrument is isolated, it should be returned to the manufacturer to identify the
cause of the change in responsivity.
(i) If the field cavity radiometer is faulty, the reference instrument should be used as a
replacement until the field instrument is returned. At that time, the reference instrument
should be compared against the WRR.
(ii) If one of the field pyrheliometers is found to be in need of service, another pyrheliometer
of the same manufacture and model should be substituted while it is sent for service.
(iii) If the reference instrument has apparently changed its responsivity, the instrument
should be sent to the manufacturer to determine the reason(s) for the change and then
compared against the WRR before a new com parison with the field instrum ents is
performed.
The archive should be informed of the problem and its solution once obtained. Large changes
of this nature may require re-evaluation of previously obtained data. If expertise is not available
to analyse the effects such a change in responsivity may have on previous determinations of the
responsivity of other instruments or on the data, contact the BSRN archive for further advice.
8.3 Pyranometer Calibration
The standard procedure for the calibration of pyranometers adopted by the BSRN is that of Forgan
(1996) . This method recognizes that the best calibration accounts for the climatic regime in which
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the pyranometer is located. The following steps provide a brief outline of the procedure.
(1) Two (or four for redundancy) pyranometers are required with approxim ately equal sensitivity.
The original sensitivities can be determined by the sun/shade technique against a standard
radiometer.
(2) One instrument is used as the global instrument, while the second is installed on site as the
shaded radiometer. Care must be taken to ensure that the area blocked by the diffuse disk
matches the field of view of the cavity radiometer (or working pyrheliometer) being used for
the measurement of direct beam radiation.
(3) At about the time of the summer solstice, the two pyranometers are switched during a period
of sunny weather.
(4) Using a series of simultaneous equations the sensitivity of the two pyranometers can then
be calculated from the data obtained during the several days before and after the instrument
swap.
This procedure has several significant advantages over the sun/shade method of pyranometer calibration.
Firstly, the procedure does not require the instrument to be removed from service during the calibration
procedure with the exception of changing its location, which can be accomplished during darkness.
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