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Re: Detectors and humidity
Again - air containing water vapor is NOT denser than dry air - it's the
other way around, assuming your talking about typical and relatively uniform
room temperatures and pressures, or gas/vapor densities at STP.
For air at a given pressure and temperature, a unit volume (say 1 cc) will
contain a fairly uniform number of molecules, whether the molecules are
oxygen, nitrogen, or water in vapor form. For dry air, about 80% of the
molecules are nitrogen, with each molecule comprised of two nitrogen atoms
and about 20% of the molecules are comprised of two oxygen atoms. The
relative masses of these molecules is therefore:
nitrogen: (2 x 14) = 28
oxygen: (2 x 16) = 32
If you go from dry air to air with a high water vapor content AT THE SAME
PRESSURE AND TEMPERATURE (meaning with about the same number of molecules
per volume), then you have displaced some of the heavier nitrogen and oxygen
molecules with lighter water molecules, which have a relative mass of 1 + 1+
16 = 18, less than either nitrogen or oxygen. Since the number of molecules
in the volume is about the same, but now a significant fraction of those
molecules are LIGHTER than the ones you started with, you have less mass per
unit volume (i.e., lower density).
The common misconception is that adding water vapor to dry air adds mass but
doesn't change anything else, like pressure or volume. That's not the way
it works. Something has to give - either the pressure goes up (if you keep
the same volume), or the volume goes up (if you keep the same pressure).
You can't just squeeze those water molecules in there without the other
molecules noticing and making room for them. (The old science class
demonstration of dissolving sugar in a full glass of water probably has a
lot to do with this misconception.)
Vincent King,
Grand Junction, CO
----- Original Message -----
From: <John_Sukosky@DOM.COM>
To: "Rick Orthen" <rorthen@cecinc.com>
Cc: <radsafe@list.vanderbilt.edu>
Sent: Tuesday, June 01, 2004 10:20 AM
Subject: RE: Detectors and humidity
>
>
>
>
> Rick,
>
> The problem is what definition of air we use, dry air or air with water
> vapor. If you look at it in a practical way, since water vapor is part
of
> air, then a cubic centimeter of air containing water vapor is denser than
a
> cubic centimeter of dry air (since water is denser than air). In terms of
> alpha efficiency, it seems intuitive (at least to me) that alphas would
> have less efficiency (and lower count rates) in air containing water vapor
> than dry air. Am I misguided on this?
>
> Here's the definition of air from free-definition.com:
>
>
>
> Air is a name for the mixture of gases present in the Earth's
atmosphere.
>
>
> Dry air is roughly 79% nitrogen, 20% oxygen, and 1% argon. Air may
> contain 0-7 % water vapour (the 79%, 20% and 1% become accordingly a
> little less), and less than 1% carbon dioxide.
>
>
>
>
> John M. Sukosky, CHP
> Dominion
> Surry Power Station
> (757)-365-2594 (Tieline: 8-798-2594)
>
>
>
>
> "Rick Orthen"
> <rorthen@cecinc.c To:
<John_Sukosky@DOM.COM>
> om> cc:
> Subject: RE: Detectors and
humidity
> 06/01/04 09:50 AM
>
>
>
>
>
>
> Maybe I'm not seeing the forest for the trees, but I think it is
> intuitively
> clear why air density decreases with increasing humidity. As humidity
> increases, each cubic centimeter of air contains a larger proportion of
> water molecules, effectively displacing the amount of air in that cc cube.
> With less air molecules in the cube, the air density has to go down.
>
> Rick
>
> Richard F. Orthen, CHMM
> Senior Project Manager
> Civil & Environmental Consultants, Inc.
> Four Triangle Lane, Suite 200
> Export, PA 15632-9255
> 724/327-5200, ext. 231
> www.cecinc.com
>
>
>
>
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