Subject: HUMIDITY

From: [email-masked] (Jack Schmidling)
Date: 1 Aug 1993 23:48:30 -0500

HUMIDITY
 
 I am trying to establish an environment in my basement 
 wherein I will not have to worry about machined steel parts 
 rusting.
 
 I installed a small air conditioner and a relative humidity 
 indicator.  All though the room seems dryer and there is no 
 more water condensing on and dripping from cold water pipes, 
 the RH is frequently higher than outside.  The temperature 
 is fairly constant at about 75F and the humidity hangs 
 around 65%.
 
 I am curious to know if there is some particular RH required 
 to promote rust or if absolute humidity is the determinant.
 
 I would also like to know the formula for Temperature 
 Humidity Index or Comfort Index if they are the same.
 
 Finally, just what the hell good does it do to know what the 
 RH is?  One perfectly delightful morning it was 95% and this 
 evening it is hot and muggy but the RH is only 45%.  Seems 
 like something created to give the "weatherman" something to 
 talk about.
 
 js

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From: [email-masked] (HENRY E SCHAFFER)
Date: Wed, 4 Aug 1993 02:34:43 GMT

> is fairly constant at about 75F and the humidity hangs 

  I had trouble with tools rusting in my basement - the 
humidity was 65% and up - especially when my wife would hang up
clothes to dry during rainy weather.  :-(  (By the way, I have
no idea how accurate my RH indicator is.)  So I put in a 
dehumidifier - from Sear's, about 2' high x 1 1/2' x 1 1/2', and
it easily keeps the humidity 50% - 55% and I no longer have any
problems with rust.  I checked with a knowledgeable friend who
said that I need to keep it under 60% or I would have rusting
problems.  Note that a dehumidifier does a better job of
dehumidification than does the normal air conditioner.

> ...

--henry schaffer

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From: [email-masked] (Vance Morrison)
Date: Wed, 4 Aug 1993 14:05:52 GMT

> wherein I will not have to worry about machined steel parts 

I would be interested in hearing about other peoples experiences
with this too.  My personal experience is that I have kept the 
air at 70% RH (the basement stays in the range 65-75 F year round).
So far I have not had a problem, but I have been watching my jointer
bed closely for the first signs of rust.  

As to the question of whether its is relative or absolute humidity
that counts, I would stronly suspect that it is relative humidity
that matters most.  I suspect that it is free water that causes the
problem.  Since cast iron has a high thermal mass, with the RH
gets above 80% or so, it is likely that temperaturs changes during
the day-night cycle cause a small amount of dew on the surface of
the metal (probably not even visable).  

The above is pure conjecture, but its worked for me so far.  If it
is in deed true, what it implies is that the more stable and uniform
the temperature is in your shop (both uniform in space as well as
time), the closer you can get to 100% RH without a problem.  Conversely
if the temperature varies, then you need a lower average RH to 
keep the rust away.

Vance

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From: [email-masked] (Living in the Outhouse)
Date: 5 Aug 1993 19:29 CST

In article <[email-masked]>, [email-masked] (HENRY E SCHAFFER) writes...

>> the RH is frequently higher than outside.  

>--henry schaffer 

humidity is a relative term, hence relative humidity. if the outside air 
is hot then is is able to hold more water vapor. your cold basement 
cannot hold as great of an amount of water vapor per liter of air, 
so, though your basement is drier than the outside, the RELATIVE 
amount of water vaper per liter of air is higher.

i learn that back in thermogoddammits somewhere in my childhood

[email-masked]

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From: [email-masked] (DICK SANFORD)
Date: 5 Aug 93 15:09:10 EST

>  wherein I will not have to worry about machined steel parts
In his reply Vance Morrison has hit the nail pretty squarely on the head in my
experience.  

The presence of moisture may enable stray current corrosion to
take place.  Attaching some sort of sacrificial anode to the metallic structure
and letting it hang free in the air allows the corrosion to take place on the
anode.  This is fairly common practice as a means of protecting structural
steel, tanks etc.  In these cases the anodes are buried in the earth.  

I have used small gallium arsinide anodes for this purpose in tropical
environments to protect things like air conditioners and automobiles.