Calibration of Shielded Net Radiometers

Net radiation is an important term because of its use in water balance and other studies. Faulty data results from net radiometers that are not calibrated properly. Using the total shading method to calibrate shielded net radiometers (NRs) results in faulty calibration coefficients because large negative net radiation is included in the results. The proposed partial shading method corrects this problem and yields valid calibration coefficients. Outdoor calibrations are time consuming and require clear skies that are rare in many areas. We propose laboratory methods, requiring less time, for precise and repeatable short- and long-wave calibrations. A detailed model of a shielded NR was used to compute the magnitude of the various heat transfers from each side of a transducer. Under high net radiant conditions, the sensible heat transfer upward from the transducer averaged 22% larger than downward. This gave rise to the need for separate calibration coefficients for daytime conditions (heated surface facing upward) and for nighttime conditions (heated surface facing downward). The effect of wind speed on the signals of several net radiometer designs was investigated. The results indicate that NRs should be ventilated when they are calibrated and for precise measurements.