QUESTION: How can I eliminate or avoid temperature and humidity measurement error that the heater and chemical purge functions produce in the Vaisala HUMICAP HMP155 probe?

QUICK ANSWER: HMP155 heated sensor stabilization time will vary based on measurement precision requirements. As a rule of thumb, all data within a time period of 5 * τ63.2% (see photos) after the end of each heating function should be considered as not usable due to being contaminated by sensor heating error (five times the sensor time constant, tau, assures us that error is reduced by 99.3 %).

DETAILED ANSWER: Based on your maximum acceptable error, wait time for the sensor to stabilize will vary as shown below and calculated with the following Web calculator: Calculation of sensor stabilization time for climatic temperature measurements with heated humidity sensors

Sensor response time effect on measurement

WARNING: Its worth noting that the temperature time constant listed for the HMP155 is for an additional temperature probe and at a hefty 3 m/s airflow speed while the humidity time constant is in still air. Temperature time constant for the HMP155 without the additional temperature probe, as used by most meteorological institutions, is in the range of 120-180 seconds. 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 the temperature sensor for example, an air temperature fluctuation of 2 °C over a period of 2 minutes in 3 m/s airflow will show a maximum measurement error of 0.32 °C on top of basic sensor accuracy/uncertainty. Adding in the flow obstruction of a radiation shield, the error will significantly increase, and it will be very hard to justify the HMP155’s use for precision climatic measurement. In slower moving air, the time constant can rise to 30 seconds and higher, thus producing errors over 0.6 °C due to only the sensor 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:

  • Influence of sensor response time (τ 63.2%) on climatic temperature measurements

• Below results were calculated using the Web calculator for sensor step response at this link:

  • Calculation of sensor stabilization time for climatic temperature measurements with heated humidity sensors

Recommended stabilization times after sensor heating in 3 m/s (11 km/h) air flow

Below 3 m/s (11 km/h), temperature stabilization times are significantly extended and we do not recommend using this sensor for climatic measurements without an additional temperature sensor.

HMP155 HEATER (100 °C) @ ambient temperature of 0 °C:

• Wait 125 seconds to reduce error below 0.2 °C when ambient temperature is 0 °C.

• Wait 140 seconds to reduce error below 0.1 °C when ambient temperature is 0 °C.

HMP155 HEATER (100 °C) @ ambient temperature of 15 °C:

• Wait 122 seconds to reduce error below 0.2 °C when ambient temperature is 15 °C.

• Wait 136 seconds to reduce error below 0.1 °C when ambient temperature is 15 °C.

HMP155 HEATER (100 °C) @ ambient temperature of 30 °C:

• Wait 118 seconds to reduce error below 0.2 °C when ambient temperature is 30 °C.

• Wait 132 seconds to reduce error below 0.1 °C when ambient temperature is 30 °C.

Recommended stabilization times after chemical purge using a heater in 3 m/s (11 km/h) air flow

Below 3 m/s (11 km/h), temperature stabilization times are significantly extended and we do not recommend using this sensor for climatic measurements without an additional temperature sensor.

HMP155 CHEMICAL PURGE (180 °C) @ ambient temperature of 0 °C:

• Wait 137 seconds to reduce error below 0.2 °C when ambient temperature is 0 °C.

• Wait 150 seconds to reduce error below 0.1 °C when ambient temperature is 0 °C.

HMP155 CHEMICAL PURGE (180 °C) @ ambient temperature of 15 °C:

• Wait 134 seconds to reduce error below 0.2 °C when ambient temperature is 15 °C.

• Wait 148 seconds to reduce error below 0.1 °C when ambient temperature is 15 °C.

HMP155 CHEMICAL PURGE (180 °C) @ ambient temperature of 30 °C:

• Wait 132 seconds to reduce error below 0.2 °C when ambient temperature is 30 °C.

• Wait 148 seconds to reduce error below 0.1 °C when ambient temperature is 30 °C.

For the above calculations, heater setting and chemical purge settings were taken from the user manual of the Vaisala HUMICAP HMP155. If you would like the calculations performed differently, please contact us so that we can update this article or create an article to suit your needs.

• HMP155 default Heater settings: Heater on for 30 seconds with 100 °C heating temperature and 10 seconds cooling time after heating cycle finishes.

• HMP155 default Chemical Purge Settings: heater on for 6 minutes (360 seconds) with 180 °C heating temperature and 1 minute (60 seconds) cooling time after chemical purge heating cycle finishes.