How Fast is Fast: Testing the Response Time of a New Air Temperature Sensor and its Implications

The World Meteorological Organization (WMO), National Oceanic and Atmospheric Administration (NOAA) and other organizations recommend faster-response sensors for measuring air temperature. Quantified as the sensor time constant (the time it takes for a sensor to reflect some percentage of a step change), recommendations differ somewhat depending on the organization. For instance, the WMO states the 63% time constant should be ≤20 seconds in a 1 m/s air flow whereas the NOAA recommends 25 seconds. Recent work has revealed that many of the commercially available PRT sensors do not meet this requirement. I tested a recently released PRT sensor from Campbell Scientific and replicated previously published work on other available PRT sensors against the WMO’s 20 second requirement. Using a bench-top wind tunnel equipped with a hot-wire anemometer and reference PRT, I measured the temperature change in the wind tunnel at 1 m/s air flow after heating to 10-15 C above ambient temperature. I performed this testing on three replicates of each of four sensor models. The results from the three, previously available models of differing diameters are generally consistent with previously published work. The results from the new sensor show a time constant of 11.7 (0.85) seconds (sd). To my knowledge, this is the first commercially available sensor to achieve this specification. There are, however, important implications for long-term data records, measuring extremes, and for inter-site comparisons when using or switching to fast-response sensors.