Author
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Tarara, Julie |
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HOHEISEL, GWEN-ALYN - WASHINGTON STATE UNIV |
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Submitted to: HortScience
Publication Type: Peer Reviewed Journal Publication Acceptance Date: 3/18/2007 Publication Date: 10/1/2007 Citation: Tarara, J.M., Hoheisel, G. 2007. Low-cost shielding to minimize radiation errors of temperature sensors in the field. HortScience. 42(6):1372-1379. Interpretive Summary: When scientists measure air temperature in field experiments, they must shield the temperature sensor from sunlight that will heat the sensor and create an erroneous reading of air temperature. The most effective solar radiation shields are ventilated by fans, but their drawback is the need for power to the fan that must be supplied in a remote field setting. Commercially-built solar radiation shields without fans or the need for battery or electric power are common for weather stations. However, because of their expense (up to $180 per shield), they are not affordable for most agricultural and ecological scientists who must place large numbers of temperature sensors across field experiments to acquire representative measurements. This paper provides information to agricultural scientists on design considerations for solar radiation shields that are based on the physical principles of heat or energy movement in the environment. Solar radiation and wind essentially determine the extent to which a shielded temperature sensor returns erroneous measurements. Solar radiation shields that are configured like a stack of plates, like the expensive commercial shields, are more effective than shields that are shaped like tubes. Shields that are open on the bottom are less effective because sunlight can be reflected from the ground up into the shield. A solar radiation shield with an acceptable level of erroneous readings (most under 2 F) was built for $5.00 in materials and 45 minutes of construction time. Technical Abstract: The importance of shielding temperature sensors from solar radiation is understood, but there is a lack of prescriptive advice for plant scientists to build inexpensive, effective shields for replicated field experiments. Using general physical principles that govern radiation shielding, a number of low-cost, passively ventilated radiation shields built in-house was assessed for measurement of air temperature against the same type of sensor in a meteorological "standard" Gill radiation shield. The base shield material had high albedo (0.9) and low emissivity (0.03). Differences in air temperature (delta-T) between low-cost shields and the standard Gill were greatest for shields with open bottoms (up to +7.4C) and for those with poorly perforated sidewalls. Open-bottomed shields were prone to heating from reflected radiation. Tube-shaped shields required more than 30% sidewall perforation for convection by ambient wind (up to 4 m/s) to offset the midday radiation load of the shield. The smallest daytime delta-T consistently were produced by a shield that emulated the stacked plate design of the standard Gill, for a total of US $4.00 in materials and 45 min construction time. Eighty-nine percent of all daytime delta-T for the 'homemade Gill' shield were <1.5C. The combination of low ambient wind speed and high global irradiance produced the largest delta-T for all shields, the magnitude of which varied with shield design: stacked plate configurations were more effective than tubes. Cost effective radiation shielding can be achieved by selecting shield materials and a configuration that minimizes daytime radiation loading on the shield while maximizing the potential for convective transfer of that radiation load away from the shield and the sensor it houses. |
