[TriLUG] Re: [OT] ESD issues

Joseph Mack NA3T jmack at wm7d.net
Mon Mar 20 21:41:56 EST 2006


On Mon, 20 Mar 2006, Lance A. Brown wrote:

> We've had an abnormally dry winter....

OK we get dry air from the high pressure system over Canada 
in winter. Winters are normally dry. but it would be the 
same inside if it rained all the time in winter here (except 
for wet clothes drying).  The real problem is taking cold 
air from the outside (which doesn't hold a lot of water) and 
bringing it inside and warming it. Whether the outside air 
was saturated (raining) or dry (clear blue sky) doesn't 
matter a whole lot since there is so little water in the 
cold air.

I couldn't find any graphs of saturated water vapor pressure 
in the range 0-30C on the web. Instead look at the wet-bulb 
depression (which is related to vapor pressure)

http://www.novalynx.com/reference-rh-table.html

Take saturated air at 50degF (10deg C left side) warm the 
air 10degC (to 70F), by going across the table, and you have 
humidity of <10%. If the air had been dry initially, say at 
30% humidity, then you would only have to warm the air 8degC 
(to say 65degF) to get the same low humidity.

(anything below 40% is regarded as dry)

If instead you brought in air saturated at 15deg C (say 
60deg F), (rather that at 10degC) and warmed it 5degC (say 
to 70deg) then the humidity would be 50% rather than <10%.

Air outside at temperatures less than 50degF are going to be 
bone dry once it's brought inside and warmed to anything 
we're likely to be sitting around in.

The main determining factor for humidity inside a building 
in winter is the temperature of the air you're bringing in 
not whether it was dry or wet to start.

Joe

-- 
Joseph Mack NA3T EME(B,D), FM05lw North Carolina
jmack (at) wm7d (dot) net - azimuthal equidistant map
generator at http://www.wm7d.net/azproj.shtml 
Homepage http://www.austintek.com/ It's GNU/Linux!



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