Differences between the sonic calculated temperature and the air temperature measured by a more traditional sensor do not necessarily indicate a problem. Sonic anemometers, because of their high sensitivities to sonic geometry and transducer response time, do not typically measure absolute temperature as accurately as traditional temperature probes. Even very small changes in geometry can lead to errors in sonic temperature because sonic temperature is proportional to the square of any error in the distance between sonic transducers. As an example, a change of only 200 micrometers in sonic path length at room temperature yields a 1°C change in measured sonic temperature.
Even though sonic anemometers are generally not recommended for measuring absolute temperature, they excel at measuring fast temperature fluctuations, which are needed for eddy-covariance calculations (sensible heat flux). Furthermore, the error in sonic temperature in most cases can be regarded as an offset after sonic temperature has been corrected for humidity, and therefore, it will not have an effect on the covariance calculation. Nevertheless, if a user desires to calibrate the sonic temperature to account for this offset, this can be done using the temperature reading from a collocated temperature probe, such as the EC150 or IRGASON® temperature probe.