A time constant which indicates the response of a thermometer to a change in temperature. A common expression is the time a thermometer takes to reflect 63.2% of a temperature graduation in moving water. A 90% change is referred to as “T90”.

The speed of response depends on the mass of the thermometer and the rate of heat transfer between the external surface of the element and the medium in which it is immersed. A small time constant reduces errors in a system subject to rapid changes in temperature.