Its worth noting that the temperature time constant listed for the HMP155 is at a hefty 3 m/s airflow speed as shown in the photo from its manual. Thus the HMP155 temperature time constant converted to still air will, unfortunately, be significantly higher and hence the HMP155 temperature measurement error due to fluctuating air temperature will NOT be per WMO recommended guidelines as specified in the WMO – The CIMO Guide – Guide to Meteorological Instruments and Methods of Observation, 2014 Edition updated in 2017.
Due to the slow time response of this temperature sensor, an air temperature fluctuation of 2 °C over a period of 2 minutes in 3 m/s airflow will show a measurement error of 0.29 °C (at 3 m/s air speed) on top of the basic sensor accuracy/uncertainty.
Flow obstruction caused by a radiation shield will significantly increase this error for low wind speeds. Despite the status quo, it will be very hard to justify the HMP155’s use for precision climatic measurement. In 1 m/s air, the time constant can rise to 35 seconds and in 0.5 m/s air to 49 seconds, thus producing errors over 0.5 °C due to only the sensor’s slow response time.
To reduce most air temperature errors due to sensor response time below 0.2 °C a sensor time constant of 13 seconds is required for air speeds of < 1 m/s.
To reduce the errors below 0.1 °C a τ63.2% time constant of 7 seconds is required for air speeds of < 1 m/s.
The above values were calculated using the online Web calculator at the following link:
Below results were calculated using the Web calculator for sensor step response at this link: