Infrared calibration of net radiometers and infrared thermometers
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A standard, convenient and accurate method for calibrating net radiometers would assist in unravelling reasons for the perplexing lack of surface energy balance closure as well as improving on the accuracy of the energy balance residual method for estimating evaporation. A relatively inexpensive, accurate and quick laboratory method for non-steady radiative conditions above a large water-heated or water-cooled radiator containing circulated water, with surface-embedded thermocouples, was used to obtain reproducible net radiometer calibration factors for the infrared waveband for a wide range in net irradiance. Infrared calibration factors for two-, four-component, miniature polyethylene, polyethylene-domed (with and without ventilation) and domeless net radiometers were obtained. A method was also used for the shortwave calibration of net radiometers by placement of a net radiometer adjacent to a standard shortwave radiometer with both instruments placed above the radiator. Measurements from heated-needle anemometers demonstrated that thermally induced wind speed was not a significant factor in the infrared calibrations. Furthermore, the temperature gradient across the radiator was fairly uniform at any time. For the infrared calibrations, the two- and four-component net radiometers yielded average root mean square errors of 0.88 and 0.97 W m-2 respectively compared to 0.92 W m-2 for the polyethylene-domed net radiometers, 2.59 W m-2 for four domeless units and 2.27 W m-2 for a polyethylene-domed miniature net radiometer. Theory presented and collected measurements allowed the net radiometer infrared calibration factor to be determined for cases when the infrared irradiance from the environment was not constant. For the broadband domeless net radiometers used, the shortwave and infrared calibration factors were within 6.5% of each other and yet 24.3% different for some of the polyethylene-domed instruments. The use of ventilators, for polyethylene-domed net radiometers, resulted in more variable data and larger-than-expected infrared calibration factors. The radiator method also provides a convenient method for calibrating a number of infrared thermometers (IRTs) simultaneously for a wide temperature range. Different regression procedures for the non-linear calibration relationships, for IRTs with and without a body temperature sensor, were applied to obtain estimates of the radiator surface temperature. The residuals between the average radiator surface temperature and the corresponding IRT target temperature measurements were within 0.15 C for all IRT types except for a handheld IRT unit which was within 0.2 C. The radiator method used allows net radiometers to be calibrated for both infrared and shortwave under near-identical laboratory conditions, as well as IRTs to be calibrated, and is relatively simple to set up and operate. 2009 Elsevier B.V. All rights reserved.
