Moisture on display when freezing

[elecoil]

I have a fully sealed display that is being used in Canada, it works well until real cold weather comes in, at this time it shows some frozen moisture on the inside of the LCD protective polycarbonate.
I have added two different bags of dessicant (silica gel and molecular sieve); they worked fine during summer time but I am atrting to see the frozen moisture again.
Is there any reason why the moisture appears only when weather is below freezing?
What else can I do to avoid this problem?

Thanks for the help.

 

[MintJulep]

When the inside surface temperature of the poly reaches the dewpoint of the (apparently moist) air inside the display, you will get condensation on the inside surface. When the temperature falls below freezing, you get ice.

If the display is "fully sealed", how do you add desiccant?

Regardless, any amount of desiccant in a truly sealed volume of moist air will reach some equilibrium moisture balance. The balance point will not be air a zero humidity. If you start out with moist air, you will always have moist air. It may be possible to achieve a sufficiently low equilibrium point such that the dewpoint is lower than your lowest temperature.

There is also the possibility that your seal leaks.

If your seal is truly hermetic, you can avoid the problem by charging the unit with true dry air, or dry nitrogen.

 

[zdas04]

The dew point of the air is near the current ambient temperature. As long as temperature is above the dew point, the water will remain in gas form. The desiccants you're using will only remove water down to a certain (non-zero) level. It seems that the level they've achieved is about the same as current ambient temperatures.

If you can apply a tiny amount of heat to the cover, the problem should stop, but it might just migrate to some place where the liquid water will be a hazard instead of an annoyance.

David

 

[IRstuff]

Yes, dewpoint.

However, the issue is that your display is not "fully sealed." If it were, it would hermetic and you wouldn't be asking this question. The problem is that your display is nearly "fully sealed." During the hot times, the air inside expands and overpressures the display, and the air escapes through whatever seams you have. Once the display is turned off or cools down, the pressure inside drops below outside pressure and moist air is sucked in. Do that enough times and the dessicant gets overwhelmed.

Some similar systems in the Navy require internal heaters 24/7, to ensure that there is always more pressure and higher temperature inside at all times.

Another approach is to use a dessicant filter with a low-rate port that has a higher flow rate than the seams to ensure that the pressure differentials are always equalized through the dessicant. If you use an indicating dessicant, i.e., one that turns pink, your user can visuall inspect and replace the dessicant as needed.

TTFN

FAQ731-376

 

[TXiceman]

I have used a small heater strip to keep a display warm in cold weather.

Ken

Ken
KE5DFR