The direct beam radiation measurement is done with the help of pyrheliometer. Its calibration is done mainly by following two processes.I) Intercomparision II) Calibration with standard instrument.
I) Intercomparision:
In thiis method, one pyrheliometer is taken as reference among the numbers of field instruments. The pyrheliometer which is taken as reference is calibrated against standard one and the calibration factor is determined for all field instruments.
II)Calibration with standard instrument:
An instrument whose calculated performance has been determined with a high degree of precision is called an absolute instrument. Abbot water-flow is taken as absoulute pyrheliometer. Number of pyrheliometers are calibrated against the absolute one are standard pyrheliometer. Generally field instruments are used for daily measurement of radiation. Field instruments are callibrated against standard pyrheliometer. Calibration is done by operating the standard and field instrument simultaneously in a clearday then calibration factor is ratio of measurement from field to that of measurement from the standard pyrheliometer.
I) Intercomparision:
In thiis method, one pyrheliometer is taken as reference among the numbers of field instruments. The pyrheliometer which is taken as reference is calibrated against standard one and the calibration factor is determined for all field instruments.
II)Calibration with standard instrument:
An instrument whose calculated performance has been determined with a high degree of precision is called an absolute instrument. Abbot water-flow is taken as absoulute pyrheliometer. Number of pyrheliometers are calibrated against the absolute one are standard pyrheliometer. Generally field instruments are used for daily measurement of radiation. Field instruments are callibrated against standard pyrheliometer. Calibration is done by operating the standard and field instrument simultaneously in a clearday then calibration factor is ratio of measurement from field to that of measurement from the standard pyrheliometer.
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