What does the "time constant" specification mean

EM21 and EM22 environmental monitors specify a time constant in their datasheets.

A time constant is defined in science and engineering as the time it takes for a physical system to reach 62.3% of its total value. After 5 time constants, the system is considered to be at 100% value.

The actual time constant will vary based on the medium being measured.

For example, the EM datasheet defines the EM22 temperature probe's time constant as 5.00s.

If we imagine that the temperature in the air is 20ºC, the probe reads 20ºC, and then the probe is inserted into a stream of 10ºC water that is flowing at 1 m/s, it will take 5.00s to drop 62.3% of the way from 20 - 10º (in other words, after 5.00s the probe will read 13.77º).

After 5 time constants (25s), the probe will be considered to have reached the 100% value of 10ºC.