[ RadSafe ] Fwd: Air monitor question

[Joseph Alvarez]

---------- Forwarded message ----------
From: Joseph Alvarez <jalvabeta at gmail.com>
Date: Fri, Mar 12, 2010 at 11:30 AM
Subject: Re: [ RadSafe ] Air monitor question
To: Wes Van Pelt <WesVanPelt at verizon.net>


Wes
Isokinetic sampling has everything to do with turbulent flow and mixing. The
concept of isokinetic sampling was developed for laminar flow, then extended
by assumption to turbulent flow, i.e., the particles will behave essentially
the same. Isokinetic sampling has not been demonstrated to work and for this
reason the latest version of ANSI N13.1 recommends that the term and
practice not be used. Mixing is important to the isokinetic concept because
multiple probes across a nonlinear flow profile were believed to compensate
for the various flow rates and lack of mixing. ANSI N13.1 recommends single
point non-matched flow sampling because multiple probes matched to the flow
have entry diameters that are too small. Particle loss at the probe entry
increase with decreasing diameter.

ANSI N13.1 recommends demonstrated, representative sampling. In most case,
demonstration is not difficult. If the probe is located downstream of a fan,
the flow volume is well-mixed. The velocity profile may not be regular.
Joe


On Fri, Mar 12, 2010 at 9:25 AM, Wes Van Pelt <WesVanPelt at verizon.net>wrote:

> John,
>
> Isokinetic sampling has nothing to do with turbulent flow or mixing. It is
> simply the matching of the linear velocity of the air in the duct with that
> in the sampling probe so as to not select by particle size.  With a low
> duct
> flow rate just reduce the flow rate in the sampling probe and sample for a
> longer time.
>
> The other issue is representative sampling across the duct which is related
> to turbulence and mixing. For this I would suggest installing a matrix of
> sampling probes across the duct and combine the flows so as to get an
> average concentration.
>
> I would not recommend using a conservative "safety factor" for various
> reasons.
>
> Best regards,  Wes
> Wesley R. Van Pelt, PhD, CIH, CHP
> Wesley R. Van Pelt Associates, Inc.
>
>
>
> -----Original Message-----
> From: radsafe-bounces at health.phys.iit.edu
> [mailto:radsafe-bounces at health.phys.iit.edu] On Behalf Of
> JOHN.RICH at sargentlundy.com
> Sent: Thursday, March 11, 2010 6:53 PM
> To: radsafe at health.phys.iit.edu
> Subject: [ RadSafe ] Air monitor question
>
> Radsafers
>
> we have a plant vent monitor with the following problem:
>
> On occasion, the airflow is reduced to the point were there isn't enough
> to produce turbulent flow, mixing, or a velocity anywhere  near the that
> needed for isokinetic sampling.
>
> If anybody has had a similar situation, I'd like to discuss our proposed
> "fix" with you.  The thinking at the moment is to develop a conservative
> compensation factor that would be applied to the releases measured and
> recorded during these periods.  But there are could be hidden traps in
> this approach that will cause problems down the road.
>
> thanx  in advance
>  - - jmr
>
>
> John Rich
> 312-269-3768
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-- 
Joseph L. Alvarez, PhD, CHP
Alpha Beta Gamut
716 Schenley Rd.
Knoxville, TN 37923
Phone: 865-591-0175



-- 
Joseph L. Alvarez, PhD, CHP
Alpha Beta Gamut
716 Schenley Rd.
Knoxville, TN 37923
Phone: 865-591-0175