Absolute calibration of pyrgeometers is difficult because of the com plex interaction between the instrum ent and the incom ing signal. This is prim arily due to the difficulty in producing an hem ispheric interference filter to transm it the broadband infrared signal (approxim ately 4 - 50 :m ) em itted by the atm osphere and/or earth’s surface to the therm opile detector. Two com plications to be surm ounted through characterization and calibration are: (1) The absorptance of solar radiation by the dom e causing heating and thus therm al em issions from the dom e to the sensor surface. (2) The variation of transm issivity of the dom e over the wavelength range. The first is overcom e by m onitoring the dom e tem perature and correcting for the increase in signal reaching the therm opile, while the second requires calibrating the instrum ent in a therm al radiation regim e sim ilar to that in which the instrum ent is to be deployed.
At present no standard m ethod exists for the calibration of pyrgeom eters, but m ost characterizations are accom plished by applying the Stefan-Boltzm ann Law to a blackbody calibration source. Therefore, to reduce the overall uncertainty between m easurem ents m ade in various countries using different calibration techniques, the BSRN Scientific Panel recom m ends that the prim ary calibration of pyrgeom eters be perform ed at the WRC, or other authorized centres, following the procedures developed by Philipona et al. (1995). W hile not yet recognized as an absolute calibration, this procedure reduces m easurem ent uncertainty through the inclusion of varying both the cavity and dom e tem peratures of the pyrgeom eter, as well as varying the radiative tem perature of the blackbody. All three tem peratures are varied respecting the m ean annual tem perature of the location of the final deploym ent of the pyrgeom eter. In this m anner, each instrum ent is characterized for a specific radiation regim e.
To maintain the traceability of pyrgeom eter m easurem ents the following procedure has been established:
(1) Each BSRN station requires a m inim um of two pyrgeom eters, initially calibrated at the W RC.
One of these instrum ents is to be declared a site reference instrum ent and used only during tim es of com parison. The other instrum ent(s) will be classified as the field instrum ent(s). An initial com parison of the two instrum ents should be m ade im m ediately upon deploym ent at the station to determ ine the relationships between the therm istors and therm opiles of the instrum ents.
(2) Com parisons between instrum ents should occur a m inim um of once every 4 m onths at sites where no significant seasonal variations occur and once each season (nom inally every 3 m onths) 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) W herever possible, the instrum ents being com pared should use the sam e tracking shade device and data acquisition system to reduce system atic biases. Instrum ent ventilation m ust be with the sam e style ventilator. In cases where separate tracking shade devices are used, care m ust be taken to ensure that both the field and reference instrum ents are shaded correctly.
W hen the sam e data acquisition system cannot be used, a com parison between data acquisition system s m ust be perform ed. Com parison data should not be collected until the reference instrum ent has com e to therm al equilibrium with its surroundings.
(4) Norm al station sam pling protocols should be used during the com parison.
(5) The com parison should last a m inim um of 2 days and a m axim um of 5 days. It is desirable that data be obtained which corresponds to a typical range of irradiances for the period. This m ay be accom plished by acquiring m easurem ents under a variety of non-precipitating weather conditions during both daylight and nighttim e hours.
(6) From the continuous data set collected during the com parison period, only steady-state conditions should be used for the com parison. This can be som ewhat arbitrarily defined as those periods where the standard deviations of the therm opile and therm istor signals are less than 0.25% of the m agnitude of the respective m ean signal for the averaging period.
(7) Analyses should be perform ed to determ ine if there are any changes in the following:
(i) the ratio of a given reference instrum ent therm istor to the respective field instrum ent therm istor (dom e and body).
(ii) the ratio of the reference instrum ent therm opile output to the field instrum ent therm opile output.
(iii) the ratio of the calculated irradiance of the reference instrum ent to the calculated irradiance from the the field instrum ent.
Note: For typical irradiances, the tem perature of the case accounts for between 75 and 100%
of the signal for clear to isotherm al conditions. The Eppley PIR uses Yellow Springs Instrum ents (YSI) therm istor YSI 44031 for these m easurem ents. The therm istor is specified to have an interchangeability of ±0.1 °C between 0 °C and 70 °C and is nom inally 10 KS at 25 °C.
Conversion from resistance to tem perature is accom plished through the Steinhart and Hart equation,
T = a + b(lnR) + c(lnR)-1 3
where T = tem perature in Kelvin R = resistance in ohm s
The coefficients for the YSI 44031 therm istor are: a = 0.0010295, b = 0.0002391 and c = 1.568e-07.
Figure 8.1. Percentage change in infrared flux due to case therm istor errors.
Using these values, the difference between the m easured tem perature at a given resistance and the calculation at that resistance is no m ore than 0.02 °C through the tem perature range -60 °C to 50 °C. Figure 8.1 illustrates the effect of a positive deviation in the determ ination of the case tem perature on the calculated ‘case flux’ from the correct value. This difference m ay be due to sam pling errors, an incorrect therm istor reading (e.g. the case therm istor is not representing the actual cold junction tem perature) or an incorrect therm istor inversion equation. As can be seen, unless the error is greater than approxim ately 5%, the overall accuracy of the m easurem ent is not greatly affected.
For exam ple: International Pyrheliom eter C om parisons IPC VII, 24 Septem ber to 12 O ctober 1990,
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R esults and Sym posium. W orking R eport N o. 162, Swiss M eteorological Institute, D avos and Zurich, March 1991, 91 pages.
9.0 Radiation Data Reduction and Quality Assurance Procedures