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I'm Convinced: Humid Air is Less Dense than Dry Air!











Now I understand.  Thanks to everyone for correcting me on this:  Joe,

Vince, Michael, Dale, Rick, Wes - hope I didn't leave anyone out.  You all

were very patient with me.  Sure glad this question wasn't on any of the

ABHP certification exams!



Thanks again,



John







                                                                                                                                       

                      "Vincent King"                                                                                                   

                      <slavak@bresnan.n        To:       <John_Sukosky@dom.com>, "Rick Orthen" <rorthen@cecinc.com>                    

                      et>                      cc:       <radsafe@list.vanderbilt.edu>                                                 

                                               Subject:  Re: Detectors and humidity                                                    

                      06/01/04 04:47 PM                                                                                                

                                                                                                                                       

                                                                                                                                       









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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