AIR TEMPERATURE ERRORS CAUSED BY AIR FILTER AND CONDUCTION EFFECTS ON HMP45C TEMPERATURE SENSORS IN WEATHER STATIONS

Abstract

For nonventilated air temperature measurements at weather stations, both ambient wind speed and solarradiation are known to affect the magnitude of air temperature measurement errors. The objective of this study is to explorethe effect of the sensors housing and to quantify any stagnation or conduction errors. The HMP45C temperature and relativehumidity sensor with a Gill radiation shield is widely used in remote monitoring sites. The use of a filter in the HMP45C leadsto loss of ventilation, while the protrusion of the sensor housing below the Gill shield exposes it to radiation loading andpotentially increased conduction of heat to the sensor. The HMP45C sensors were deployed with and without an air filter inboth 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 airtemperature errors and variations of air temperature errors were smaller in the HMP45C sensors when themanufacturersupplied air filter was removed. The ranges of the threesigma errors can be decreased by 0.4.C to 0.7.C andthe accuracy of monthly average air temperature can be improved at least 0.1.C by employing an HMP45C without the airfilter. Results suggest that the maximum air temperature taken with the filter may reach more than 1.0.C higher than that takenwithout the filter. The major frequency component contributing to air temperature errors using the HMP45C sensor in theGill shield is the frequency of one day per cycle, which is expected. Partial radiation blocking combined with aspirationsignificantly reduces the contribution of the oneday cycle. In field tests, the R. M. Young aspirated temperature system provedvery accurate compared to an aspirated precision industrial platinum resistance thermometer (PRT).