Condensation question

You could use a table like this one
http://www.tis-gdv.de/tis_e/misc/klima.htm


Which shows that the Relative humidity (RH) must be greater than 60% when the external air temperature reaches 40 DegC in the middle of the day, for the dew point to be reached with a drop in external air temperature of 10 DegC. The RH will be a minimum during the hot daytime and a maximum during the cooler nighttime.


Here are some 24 hour time series for a humid place Hong Kong, which shows how the RH varied a couple of days ago.
http://www.hko.gov.hk/wxinfo/ts/display_element_tt_e.htm

I expect you could find similar data for your site.


Heating the product as you are doing should stop condensation occurring in the morning due to the thermal inertia of the device.


In the evening the thermal inertia of the device should make the the above dew point calculation a worst case one since the internal air temp inside your device will be higher than the outside ambient air temp as it begins its fall to the 30 DegC thermostat switch temp set for the heater.


James


 

Thanks I might be saved after all.  The site is in Qatar by the sea.

Doesn't this also depend on the RH when the enclosure was last "sealed"?

I've worked in Middle East places on the coast where the RH can be very, very high, so even if nothing is condensing when you seal the enclosure, you always have some moisture in there.


Depending what it is (taking into account cost, safety etc.), may be worth recommending that a fresh dessicator pack is put in the enclosure each time it is opened and "re-sealed" during commissioning, maintenance and repair?

I've been reviewing this and it seems that Nitrogen cannot hold humidity, nor can Oxygen, so which component of air holds the water?

Not sure exactly what you mean.


Water itself can exist in vapour and gaseous states as well as liquid and solid we are so familiar with.


Otherwise condensation couldn't occur ...


In other words, the "air" contains a proportion of H2O as well as O2, CO2, etc. etc.

I've found that you can use Nitrogen to purge moisture, since it does not hold humidity.  

www.terrauniversal.com/.../nitrogen-purge-desiccators.php

Since Nitrogen is the main component of air, how does air hold moisture.

All it's telling you is that water is heavier than nitrogen.


If you force nitrogen into a space, heavier gasses are forced out of the bottom leaving only the nitrogen.


Since Nitrogen has no "delta charge" across the molecule, there's little way that water can electrostatically or electromagnetically "latch" onto the molecules and stay in the upper (lighter air) portion either, meaning it's really good at keeping "dry" in itself.


However, stop the notrogen flow (or open the enclosure), and the heavier gasses will start to re-enter the upper area ...

The nitrogen purge works because dry nitrogen enters the vessel, with a mixture of nitrogen, oxygen and water vapour leaving. Thus over time the remaining oxygen and water vapour fractions in the vessel atmosphere reduce. The remaining water vapour in the vessel will try to diffuse into the dry nitrogen air in exactly the same way as if dry air (nitrogen/oxygen mixture) were being added.


James