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Published by the American Society of Agricultural and Biological Engineers, St. Joseph, Michigan

Citation:  Transactions of the ASAE. 44(5): 1299–1306. (doi: 10.13031/2013.6435) @2001
Authors:   X. Lin, K. G. Hubbard, E. A. Walter–Shea
Keywords:   Radiation shield, Air temperature error, Temperature sensor, Heat transfer, Simulation.

Nonaspirated radiation shields create their own microclimate, which is a source of temperature measurement errors. A radiation loading model was developed for the shield and sensor system. The model is based on the energy balance of the sensor, which is influenced by the interior solar radiation loading due to solar radiation penetration into the radiation shield, net infrared radiation exchange between the shield surface and sensor surface, air speed inside or outside the radiation shield, and the surface radiative properties of the sensor and the radiation shield. Air temperature measurement errors for a nonaspirated radiation shield were simulated. The results demonstrate that air temperature measurement errors are inversely proportional to air speed. Daytime air temperature errors were much larger than nighttime errors. The air temperature errors ranged from +2.0 C to +4.0 C during midday under clear skies and with no wind. The air temperature sensor with larger emissivity and small solar absorptivity was the best choice for daytime measurements, while an air temperature sensor with small emissivity (regardless of solar absorptivity) was preferable for nighttime. Therefore, larger sensor emissivity with a lower solar absorptivity is desirable overall because the daytime errors are larger than the nighttime errors. The common nonaspirated radiation shield cannot provide an environment in which the sensor can reliably measure air temperature with the accuracy claimed by the sensor manufacturer.

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