RE: Detectors and humidity

["Cadanau, Rod" <rod.cadanau@boeing.com>]

Actually, Rick has the correct analogy of water molecules displacing air

molecules.  From the ideal gas law ( PV=nRT ), it's clear that for a

given pressure, volume, and temperature, the number of molecules is

constant.  So if water vapor has been added, an equal number of

molecules of air were displaced.  Then it's just a matter of molecular

weights:  H2O = 18, N2 = 28, O2 = 32 (dry air average about 28.8).  So,

1% (by volume) of water vapor added to dry air decreases the average

molecular weight by about 0.1, or about 0.3%.



Rod



-----Original Message-----

From: John_Sukosky@DOM.COM [mailto:John_Sukosky@DOM.COM] 

Sent: Tuesday, June 01, 2004 11:20 AM

To: Rick Orthen

Cc: radsafe@list.vanderbilt.edu

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









************************************************************************

You are currently subscribed to the Radsafe mailing list. To

unsubscribe, send an e-mail to Majordomo@list.vanderbilt.edu  Put the

text "unsubscribe radsafe" (no quote marks) in the body of the e-mail,

with no subject line. You can view the Radsafe archives at

http://www.vanderbilt.edu/radsafe/



************************************************************************

You are currently subscribed to the Radsafe mailing list. To

unsubscribe, send an e-mail to Majordomo@list.vanderbilt.edu  Put the

text "unsubscribe radsafe" (no quote marks) in the body of the e-mail,

with no subject line. You can view the Radsafe archives at

http://www.vanderbilt.edu/radsafe/