How to use the surface compensation factor

Published 2026-09-03

Several of Testo’s refrigeration instruments have a function for surface compensation during temperature measurement – do you know when it should be on or off?

When using a surface temperature probe, for example to measure the temperature of a pipe, heat dissipation can affect the measurement. The surface compensation factor compensates for this effect and provides a more accurate measurement value. To obtain the most reliable measurement result possible, it is therefore important to ensure that the function is correctly set.

The function is available in the smart probes testo 115i and testo 915i, and is configured under the settings in the testo Smart App. The testo 550s, testo 557s, and testo 558s digital manifolds also have a corresponding function called “Auto Tfac”.

A small tip that can make a big difference to the measurement result!

Correct settings for each instrument

The testo 115i pipe temperature probe

By default, the surface compensation factor should be activated. However, the probe can also be used to measure air temperature. In these cases, the surface compensation factor should be switched off.

testo 915i

This thermometer can be used with air, immersion, or surface probes. When measuring on a surface, the surface compensation factor must be activated. With air or immersion probes, it must be switched off.

The testo 550s, testo 557s, and testo 558s digital manifolds

These are typically used together with pipe temperature probes, in which case the surface compensation factor (Auto Tfac) should be activated. If the digital manifold is used with a probe to measure air temperature, the function should be switched off.

If you are using wireless probes, such as the testo 115i, the surface compensation factor is set separately for each probe in the testo Smart App.