Kippzonen BSRN Scientific Solar Monitoring System Bedienungsanleitung PDF herunterladen (Seite 79)

Secondly, it alleviates the potential of thermal shock to the instrument which occurs first when the

instrument is exposed to direct beam radiation and then again when the instrument is shaded. The

actual extent of such shock has not been measured for all instruments, but may be significant. Thirdly,

the pair of instruments being used to measure diffuse and global (the redundant measurement) solar

radiation are calibrated simultaneously.

A similar transfer method of calibration can also be undertaken during days where there are periods

where the solar line of sight is clear and periods where the sun is covered by cloud. By assuming that

the disk subtending the angular extent of the sun removes an insignificant amount of diffuse radiation

during overcast conditions, the responsivity of the direct beam radiometer can be transferred using

a similar set of simultaneous equations of two variables and two unknowns.

By grouping all the data obtained from either of these procedures, the uncertainty due to the instrument

directional responsivity (cosine and azimuth error) becomes inherent in the coefficients over the multitudes

of samples that make up the calibration procedure. Conversely, by grouping samples with respect

to zenith angle and intensity, the cosine response and the linearity of the instrument can also be

determined.

In the calibration procedure, care must be taken to eliminate the zero offset components associated

with the net thermal radiation of the sensor and its surroundings. ISO 9060 considers a ventilated first

class instrument to be one for which this negative flux is less than ± 15 Wm for a net thermal flux

-2

of 200 Wm . This offset becomes important when two instruments have significantly different offsets

-2

and when the responsivity is transferred from the pyrheliom eter to the shaded radiom eter. If care is

not taken to eliminate the offset, it will be incorporated into the responsivity as an uncertainty in the

calibration slope. At large radiation levels the error is minor, however, in the diffuse flux, it can be lead

to as much as a 20% underestimation.

Although the zero offset is observed at night it remains part of the pyranometer signal throughout the

daylight hours, especially during clear days. Several methods have been used to estimate the magnitude

of the offset, with the general consensus being that the most accurate measurements of solar irradiance

are those that correct the the zero offset of individual pyranometers. A methodology, however, has

yet to be agreed upon within the BSRN community. Section 9.2.2 outlines two experimental techniques.

Individual thermal offset calibration tests should be performed on site and with the instrumentation

used for the global and diffuse measurements.

Not all locations, nor all instruments experience nighttime thermal offsets. Black and white thermopile

instruments are self-compensating with respect to infrared emissions. Pyranometers that use different

transducers for the measurement of solar radiation are also not affected in the same manner.

8.4 Pyrgeometer Calibration

Absolute calibration of pyrgeometers is difficult because of the complex interaction between the instrument

and the incoming signal. This is primarily due to the difficulty in producing an hemispheric interference

filter to transmit the broadband infrared signal (approximately 4 - 50 :m) emitted by the atmosphere

and/or earth’s surface to the thermopile detector. Two complications to be surmounted through

characterization and calibration are: (1) The absorptance of solar radiation by the dome causing heating

and thus thermal emissions from the dome to the sensor surface. (2) The variation of transmissivity

of the dome over the wavelength range. The first is overcome by monitoring the dome temperature

and correcting for the increase in signal reaching the thermopile, while the second requires calibrating

the instrument in a thermal radiation regime similar to that in which the instrument is to be deployed.

At present no standard method exists for the calibration of pyrgeometers, but most characterizations

are accomplished by applying the Stefan-Boltzmann Law to a blackbody calibration source. Therefore,

to reduce the overall uncertainty between measurements made in various countries using different

calibration techniques, the BSRN Scientific Panel recommends that the primary calibration of pyrgeometers

be performed at the WRC, or other authorized centres, following the procedures developed by Philipona

et al. (1995). While not yet recognized as an absolute calibration, this procedure reduces measurement

uncertainty through the inclusion of varying both the cavity and dome temperatures of the pyrgeometer,

as well as varying the radiative temperature of the blackbody. All three temperatures are varied respecting

the mean annual temperature of the location of the final deployment of the pyrgeometer. In this manner,

each instrument is characterized for a specific radiation regime.

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