Comparing a SNOTEL Extended-Range Air Temperature Sensor and Equations to a NIST-Certified Sensor in an Environmental Chamber

In the National Snow Survey program, air temperature is one of the parameters that is measured at a SNOTEL site.  The original YSI air temperature sensor did not adequately measure the temperature range that was found throughout the network.  Therefore, the extended-range YSI air temperature sensors were installed over time at all the telemetered stations.  The new sensor required a different algorithm to convert the voltages (measured by the sensor) to degrees C.  This conversion occurred in the server at the National Water and Climate Center.  At the time, the algorithm used was developed at the center and was not the manufactured-recommended algorithm.  Since the conversion, there has been some concerns that the algorithm or the sensor has overestimated the air temperature.  To test for this, we installed 6 extended-range YSI temperature sensors and one certified Fluke sensor in an environmental chamber.  The chamber was set to start at -50° C and then step 5° increments to +50° C.  We then reversed the stepping from hot to cold in the same increments.  The sensors were connected to a Campbell CR10X data logger and the millivolts from the sensor were recorded every 15 minutes.  The measured millivolts were converted into degrees Celsius by the existing equation and the manufacturer-recommended algorithm.  Both derived temperatures were compared to the certified sensor in the analysis.  It was shown that the existing algorithm showed higher temperatures in the middle of the temperature profile.  The manufacture’s recommended algorithm did match more closely to the certified sensor throughout the profile.  It was recommended that the data be remapped with the manufacturer-recommended algorithm or that a lookup table be developed to correct for the middle ranges of the air temperature profile.  (KEYWORDS:  SNOTEL air temperature, NRCS, air temperature, YSI extended range)