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To maintain the traceability of pyrgeometer measurements the following procedure has been established:
(1) Each BSRN station requires a minimum of two pyrgeometers, initially calibrated at the WRC.
One of these instruments is to be declared a site reference instrument and used only during
times of comparison. The other instrument(s) will be classified as the field instrument(s). An
initial comparison of the two instruments should be made immediately upon deployment at
the station to determine the relationships between the thermistors and thermopiles of the
instruments.
(2) Comparisons between instruments should occur a minimum of once every 4 months at sites
where no significant seasonal variations occur and once each season (nom inally every 3
months) at locations with significant change. IT SHOULD NOT BE NECESSARY FOR THE
FIELD INSTRUMENTS TO BE TAKEN OUT OF SERVICE DURING THE PERIOD OF THE
COMPARISON.
(3) Wherever possible, the instruments being compared should use the same tracking shade
device and data acquisition system to reduce systematic biases. Instrument ventilation must
be with the same style ventilator. In cases where separate tracking shade devices are used,
care must be taken to ensure that both the field and reference instruments are shaded correctly.
When the same data acquisition system cannot be used, a comparison between data acquisition
systems must be performed. Comparison data should not be collected until the reference
instrument has come to thermal equilibrium with its surroundings.
(4) Normal station sampling protocols should be used during the comparison.
(5) The comparison should last a minimum of 2 days and a maximum of 5 days. It is desirable
that data be obtained which corresponds to a typical range of irradiances for the period. This
may be accomplished by acquiring measurements under a variety of non-precipitating weather
conditions during both daylight and nighttime hours.
(6) From the continuous data set collected during the comparison period, only steady-state
conditions should be used for the comparison. This can be somewhat arbitrarily defined as
those periods where the standard deviations of the thermopile and thermistor signals are
less than 0.25% of the magnitude of the respective mean signal for the averaging period.
(7) Analyses should be performed to determine if there are any changes in the following:
(i) the ratio of a given reference instrument thermistor to the respective field instrument
thermistor (dome and body).
(ii) the ratio of the reference instrument thermopile output to the field instrument thermopile
output.
(iii) the ratio of the calculated irradiance of the reference instrument to the calculated
irradiance from the the field instrument.
Note: For typical irradiances, the temperature of the case accounts for between 75 and 100%
of the signal for clear to isothermal conditions. The Eppley PIR uses Yellow Springs Instruments
(YSI) thermistor YSI 44031 for these measurements. The thermistor is specified to have an
interchangeability of ±0.1 °C between 0 °C and 70 °C and is nominally 10 KS at 25 °C.
Conversion from resistance to temperature is accomplished through the Steinhart and Hart
equation,
T = a + b(lnR) + c(lnR)
-1 3
where T = tem perature in Kelvin
R = resistance in ohms
The coefficients for the YSI 44031 thermistor are: a = 0.0010295, b = 0.0002391 and
c = 1.568e-07.
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