Calibration and tests of commercial wireless infrared thermometers

Authors: Colaizzi, Paul; Oshaughnessy, Susan; Evett, Steven - Steve

Submitted to: Applied Engineering in Agriculture
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 4/18/2018
Publication Date: 8/1/2018
Citation: Colaizzi, P.D., Oshaughnessy, S.A., Evett, S.R. 2018. Calibration and tests of commercial wireless infrared thermometers. Applied Engineering in Agriculture. 34(4):647-658. https://doi.org/10.13031/aea.12577.
DOI: https://doi.org/10.13031/aea.12577

Interpretive Summary: Measurement of crop leaf temperature is important for management of crops, because leaf temperature can indicate water shortages, diseases, and other problems. Leaf temperature has been used to schedule crop irrigations, resulting in reduced water usage and greater crop yields. The easiest way to measure leaf temperature is by an infrared thermometer, which is a small, non-contact sensor that views the top of the crop. However, measuring entire crop fields by infrared thermometers was previously not practical without wireless technology. Scientists at the USDA Agricultural Research Service in Bushland, Texas, developed a wireless infrared thermometer system designed for large crop fields. The commercial version of the wireless sensor system was tested in the field and in the laboratory, and performed better than previous systems that used wires. The new commercially available wireless system offers farmers a better way to manage crops, resulting in improved crop yield while conserving water and other crop inputs, improved farm profitability, and less environmental impact.

Technical Abstract: Applications of infrared thermometers (IRTs) in large agricultural fields require wireless data transmission, and IRT target temperature should have minimal sensitivity to internal detector temperature. To meet these objectives, a prototype wireless IRT system was developed at USDA Agricultural Research Service, Bushland, Texas and commercialized by Dynamax, Inc., Houston, Texas. The objective of this paper was to calibrate and test the Dynamax, Inc. system. Following deployment in an irrigated field during the 2015 crop season, twenty six IRTs were calibrated and tested in a temperature-controlled room. The IRTs measured a black body target temperature controlled at 15 to 55 deg C in 5 deg C increments, and for each range of black body temperatures, ambient room temperatures were controlled at 15, 20, 25, 35, and 45 deg C. Discrepancies between uncalibrated IRT and black body temperatures varied by IRT, and had root mean squared errors (RMSE) between 0.25 and 1.51 deg C, mean absolute errors (MAE) between 0.19 and 1.17 deg C, and mean bias errors (MBE) between -0.66 and 0.16 deg C. A calibration equation was derived from the energy balance of the IRT internal detector, and sensor-specific calibrations reduced discrepancies for all IRTs, with RMSE between 0.16 and 0.28 deg C, MAE between 0.12 and 0.21 deg C, and absolute MBE less than 0.05 deg C. A generic calibration was derived by pooling all sensor-specific calibrations, and reduced discrepancies for all but five IRTs, but these were very marginal compared with no calibration. Therefore, the generic calibration did not appear justified, but sensor-specific calibrations were justified for most IRTs. The IRTs were again deployed in the irrigated field and measured corn canopy temperature in 2016. Crop evapotranspiration (ETc) was calculated using measurements from one IRT and compared to ETc measured by a large weighing lysimeter. The choice of calibration (none, generic, or sensor-specific) had little impact on calculated ETc, which was likely related to a limited range of target and sensor body temperature differences in the field (mostly +/- 10 deg C), in contrast to those in the temperature controlled room (up to +/- 40 deg C).