Re: Altitude correction for Air Samplers

["Otto G. Raabe" <ograabe@ucdavis.edu>]

May 8 1998
Davis, CA

Dear Paul:

When operating an air sampler at high altitude, it is important to
determine the volumetric flow rate under local conditions using your flow
meter. The necessary correction depends on the type of flow meter used. If
you have a mass flow meter then it will read in standard cubit feet per
minute or standard liters per minute corrected to standard condition. You
can correct to local conditions with the gas laws using the temperature and
true absolute barometric pressure (previously discussed on RADSAFE).

If you are using a rotameter or orifice meter or other flow meter than
depends on Bernoulli's Law for its operation, a pressure and temperature
correction is necessary to determine the true meaning of the readings if
the flow meter was calibrated at sea level. It is NOT self correcting
because it depend on the square root of the gas density. You can find this
explained in any good aerosol science book. For example: See pages 352-360
in T.T. Mercer's book, Aerosol Technology in Hazard Evaluation, Academic
Press, 1973.

Alternatively, you can borrow a precision dry gas meter and calibrate you
flow meter under local conditions. Then you will know the local flow rate,
and you can use the gas laws to correct to sea level conditions if you want
to. 

EPA requires pollutant air concentrations be corrected to 25 C and 1
atmosphere, but this is not too useful for air concentrations of
radionuclides which are better expressed in terms of true local air volumes.

I hope this helps.

Otto
		*****************************************************
		Prof. Otto G. Raabe, Ph.D., CHP
                [President, Health Physics Society, 1997-1998]
		Institute of Toxicology & Environmental Health (ITEH)
		     (Street address: Old Davis Road)
		University of California, Davis, CA 95616
		Phone: 530-752-7754  FAX: 530-758-6140 [NEW AREA CODE]
		E-mail ograabe@ucdavis.edu