For measurements with a probe thermometer, the temperature probe is inserted directly into the measurement object. The measurement is finished when the so-called t99 time is reached. That is the time which a probe needs to achieve 99% of its final value. The 90% value is achieved considerably sooner, but it takes just as long again to get from the 90% value to the 99% value. All data concerning the acclimatization time of temperature probes in Testo documents refer to the t99 time.

A measurement error that frequently occurs with temperature measurement is that the temperature probe and the measurement object have different temperatures. This means either heat is supplied to the measurement object (probe warmer) or taken from it (probe colder) – and therefore energy as well. And the real temperature is no longer measured due to the energy extraction. However, this measurement error can be minimized by choosing a penetration depth of 10 to 15 times the diameter of the probe. For immersion measurements in liquids, the measurement error can also be reduced by the liquid being constantly kept in motion. This measurement error also occurs when using a food probe to measure foods, especially frozen goods, but it can be avoided through a sufficient penetration depth.