Lumidigm M-Series M3xx Modules Mechanical Integration Guide, PLT-02217, Rev. A.0
December 2014
Page 26 of 28
4.7
Condensation Considerations
Any enclosure with an interior air mass containing water vapor must provide controls to
prevent or manage condensation. Condensation will form on any surface with a
temperature at or below the dew point of the air mass. The dew point temperature is a
function of the absolute amount of water present in the air mass and the air mass
temperature. Even a hermetically sealed enclosure contains vapor phase water
representative of the air it was assembled in.
Controlling condensation is important because liquid water inside the enclosure can lead to
premature component and connector failure. In addition, condensation inside the M3xx
sensor head can temporarily degrade optical performance and create shorts within its
electronic components.
Relative humidity (RH) is a term used to describe the amount of water vapor that exists in a
gaseous mixture of air and water vapor. It is very nearly a property of the water vapor
alone, that is, it depends very little on what other gases (nitrogen, oxygen, etc.) are present
and in what amounts. For a given sample of air, relative humidity is the ratio (times 100 to
get percentage) of two partial pressures: the actual partial pressure of water vapor in the
sample and the maximal partial pressure which, at the sample temperature, water vapor
could have without starting to condense into liquid.
The total amount of liquid water present in an air mass is very low with respect to its
volume. For example an enclosure with a 3000 cm
3
air mass at 22°C and 40% RH contains
a total of 0.022 ml of liquid water or approximately one 3.5mm diameter droplet. This is not
the total amount of condensate possible, since as condensation occurs the RH decreases,
which in turn decreases the dew point temperature.
Condensation can manifest itself in two ways. First, the enclosure walls can cool to the dew
point and cause spot condensation at the cooler contact points. Secondly, as the air mass
temperature decreases the RH increases and when it reaches 100% it will condense enough
to keep the RH <100% and this will be localized to the coldest points in the air mass. So, in
the example enclosure, air at 40% RH at 22°C contains 44 grains of water. If the
temperature drops to -1°C, this same air mass can only contain 21 grains at 90% RHAn
enclosure that is rated IP65 will not permit significant (or at least rapid) transfer of humidity
into or out of the enclosure. If the enclosure is sealed, then a desiccant can be used to
lower the RH below that amount present during assembly and thus significantly reduce the
potential of condensation at low temperatures. This is the way, for example, dual pane
windows are constructed. The other approach is actively heating the interior air mass to
prevent a situation where saturation occurs.
If the enclosure is designed to work at near freezing temperatures or below, a combination
of active heating and a desiccant is recommended. The air mass should be heated
sufficiently to prevent saturation and this depends on the environment that the enclosure
was assembled in and the amount and type of desiccant used. Because we also need to
maintain the platen temperature in the comfort range, a set point of 10°C should be
adequate.
And the final management strategy is insuring that the interior platen temperature is
always greater than the dew point of the enclosure air volume. The interior platen surface is
the most vulnerable point since the exterior surface is exposed to the environment.
A rough rule of thumb is Tdp = Tenclosure – (100 – RH) / 5. So for example if the enclosure
interior air mass is at 25°C and the RH is 40%, the interior platen temperature has to fall to
25 – (100 – 40) / 5 or 13C to cause condensation on the interior platen surface.
Here we recommend that an active heating element be applied to near the platen surface.
Содержание Lumidigm M Series
Страница 28: ...hidglobal com...
