Natural Resources, School of


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Published in Transactions of the ASAE Vol. 46(2): 443–452 Copyright 2003 American Society of Agricultural Engineers ISSN 0001–2351


For non–ventilated air temperature measurements at weather stations, both ambient wind speed and solar radiation are known to affect the magnitude of air temperature measurement errors. The objective of this study is to explore the effect of the sensor’s housing and to quantify any stagnation or conduction errors. The HMP45C temperature and relative humidity sensor with a Gill radiation shield is widely used in remote monitoring sites. The use of a filter in the HMP45C leads to loss of ventilation, while the protrusion of the sensor housing below the Gill shield exposes it to radiation loading and potentially increased conduction of heat to the sensor. The HMP45C sensors were deployed with and without an air filter in both standard Gill shields and in a Gill shield modified with extra plates to completely cover the base of the sensor housing. The data collected were examined using spectral analysis and statistical methods. Results show that both average air temperature errors and variations of air temperature errors were smaller in the HMP45C sensors when the manufacturer–supplied air filter was removed. The ranges of the three–sigma errors can be decreased by 0.4°C to 0.7°C and the accuracy of monthly average air temperature can be improved at least 0.1°C by employing an HMP45C without the air filter. Results suggest that the maximum air temperature taken with the filter may reach more than 1.0°C higher than that taken without the filter. The major frequency component contributing to air temperature errors using the HMP45C sensor in the Gill shield is the frequency of one day per cycle, which is expected. Partial radiation blocking combined with aspiration significantly reduces the contribution of the one–day cycle. In field tests, the R. M. Young aspirated temperature system proved very accurate compared to an aspirated precision industrial platinum resistance thermometer (PRT).