New Calibration Method May Purge Old Procedure

Published: 31 August 2007

This photo shows a pyrgeometer entering the blackbody calibration chamber.  A blackbody is an object that absorbs all electromagnetic radiation that falls onto it. This lack of both transmission and reflection properties make blackbodies ideal sources for calibrating instruments that measure radiation, like the pyrgeometer.

A pyrgeometer is a type of radiometer that measures energy in the spectral range from approximately 3.5 to 50 micrometers. ARM uses more than 100 pyrgeometers for longwave irradiance data supporting climate studies, so accuracy and consistency in their operations is critical for obtaining reliable data. A new method for calibrating these instruments was tested with seven pyrgeometers during an outdoor study at the ARM Southern Great Plains site, and showed excellent agreement with two instruments traceable to the World Infrared Standards Group (WISG).

Presently, pyrgeometers deployed by ARM are calibrated with traceability to the manufacturer’s blackbody. Though this traceability method is adequate, it is not recognized by the international community. To achieve acceptance within the international community, ARM researchers conducted a test on a collection of seven pyrgeometers using a blackbody system to characterize their thermal properties. These instruments were then deployed outdoors along with two reference pyrgeometers that are traceable to the WISG. The blackbody coefficients of the test instruments were then adjusted so that the measured irradiance by the test pyrgeometers would equal the irradiance measured by the reference instruments.

The resulting analysis, presented at the 2007 ARM Science Team Meeting in March, showed that the seven pyrgeometers agreed to better than ± 1.5 W/m2 uncertainty with 95% confidence level with respect to the two reference pyrgeometers under all sky conditions and during daytime and nighttime. Their method, analysis, and recommendations are documented on their Science Team Meeting poster.