[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index]
RE: Detectors and humidity
John
I know very little about alpha dosimetry, but I have evaluated the effects of relative humidity on air density as it pertains to x-ray beam measurements with ion chambers that are open to the atmosphere. There is a good discussion of this in the dosimetry text by Attix. If one knows the partial pressure of water vapor in air(torr) the density of the ambient air can be calculated with
rho = rho naught (760) * [[273/((273)+T(Celcius))]*[(P-(0.3783Pw))/760]]
where
rho naught (760) = the density of dry air at 0 C and 760 torr and is 0.0012929 g/cm^3 and
Pw is the partial pressure of water vapor in the air.
At 22 C the vapor pressure of water is 19.827 torr and it increases with temperature at a rate of approximately 1.21 torr/degree C. Relative humidity (RH) is the percentage of the saturated value of the vapor pressure that is present. Thus at 22 C and 100%, Pw = 19.827 torr. At 50% RH, Pw = 9.914 torr and so on.
Assuming P=760 torr and Pw = 9.914 torr, the air density using the above equation comes out about 0.5 % LESS than that obtained when the air is assumed to be dry (Pw = 0). A rule of thumb listed in the Attix text is that each 10% increase in RH decreases air density by about 0.1 % at 760 torr, 22 C.
I really haven't followed this thread so I don't know how this may impact alpha measurements, but humid air is indeed less dense than dry air. While liquid water is denser than air, water vapor is less dense than air.
I would not be suprised if I am confused and have misinterpred what you are saying, and if so I apologize. But since i did just recently go through this humidity calculation I thought I would share.
Don Parry
Donald E. Parry, CHP
Health Physicist
Radiation Safety Section
Michigan Department of Community Health
Phone : 517-241-1989 Fax: 517-241-1981
mailto:dparry@michigan.gov
Web Site: www.michigan.gov/rss
-----Original Message-----
From: owner-radsafe@list.vanderbilt.edu
[mailto:owner-radsafe@list.vanderbilt.edu]On Behalf Of
John_Sukosky@DOM.COM
Sent: Tuesday, June 01, 2004 12:20 PM
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/