RADIATION LOADING MODEL FOR EVALUATING AIR TEMPERATURE ERRORS WITH A NONASPIRATED RADIATION SHIELD

Abstract

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