Vaisala DM500 - Manual (Page 135)

Appendix A __________________________________________________________ Humidity Theory
VAISALA ______________________________________________________________________ 133
Response Time – The response time of a hygrometer is proportional
to the amount of water vapor in the gas being measured. Thus, at
dewpoint temperatures of –60°C, things take ten times longer than at –
40° because there is ten times less water vapor.
Condensation - It is important to remember that condensation will
form on any surface if the temperature of that surface is at or below
the dewpoint temperature of gas that is in contact with the surface. In
practice, condensation may form in sample lines or on interior or
exterior parts of the hygrometer. Condensation inside the sample line
will cause a measurement error.