ROTRONIC MBW 473 - Manual (Page 25)

MBW473_MANUAL_E_V2.0 21
Why it is Important to Distinguish Between Dew and Frost
For mirror temperatures above 0 °C, water vapor always condenses on the mirror in
its liquid phase (dew). A condensation layer on a mirror above 0°C is therefore al-
ways considered a dew point.
Although ice always starts melting at exactly 0 °C, water will not necessarily freeze at 0 °C. Water may
stay in its liquid phase at temperatures far below 0 °C. This phenomenon is referred to as ‘Super-
Cooled Water’.
The fact that water at subzero temperatures can condense either as dew or as frost makes it some-
what difficult to determine whether the condensate layer on the mirror at temperatures below 0 °C is
liquid or solid. Various factors such as contaminants, time, pressure etc. may cause the condensate
layer to remain liquid at mirror temperatures of
–20 °C and below.
It is furthermore important to understand that the difference in the
temperature at which the liquid or the solid condensate layer stabi-
lizes can be up to 3 ˚C. As shown on the picture to the right, it is
also possible that dew and frost exist concurrently on the mirror
which results in a non-stable value reading somewhere between
the dew and frost point.
Therefore the phase of the condensate must be known in order to
avoid significant errors and to correctly calculate all humidity val-
ues, including vapor pressure, dew point, %RH, volume ratio,
weight ratio, absolute humidity and specific humidity.
It would be desirable for manufacturers and users of humidity instruments to use the term frost point
for temperatures below zero and dew point for temperatures above zero. While not technically correct,
it has been common practice to use dew point for temperatures below 0 °C, although frost point would
be the correct term. As discussed above, dew point can exist below 0 °C in the form of super-cooled
water and is different in value from the equivalent frost point temperature. For the same vapor pres-
sure, the frost point is approximately 10% of reading above the corresponding dew point value (when
expressed in °C). For example, a vapor pressure of 38 Pa corresponds to a frost point
of −30 °C and a
dew point of −33 °C. From a measuring perspective it seems obvious that a clear and consistent dis-
tinction between dew and frost point is important.