[ RadSafe ] FW: Reporter's question about lowerlimitsofdetection

Tom Simpson bullet308 at att.net
Mon Aug 8 12:54:28 CDT 2011

It occurs to me that the analysis of car air filters is an excellent 
qualitative approach, in that it can tell us radioactive materials were 
present in the air at some point during the service life of the filter. 
Of further utility would be dismantling the filters and analyzing the 
contents both spectroscopically and physically.  Its good science within 
its limits.

Extracting quantitative data from the same source strikes me as a lot 
more problematic, as there are so many variables to consider. Just the 
differencing driving habits, engine capacities and relative efficiencies 
of the air intake systems are just a few (a fair bit of stuff settles 
out in portions of the intake system before it reaches the filter, and 
one might think that denser materials such as radionuclides would be 
more prone to this).

Really, I would think to get a qualitative measure out of this, you 
would need a lot of data from the engines EFI as to throttle position, 
etc. for the entire time the engine was on and a bunch of data from the 
cars manufacturer on what that means in terms of air flow through the 
filter. And none of this is a proper substitute for particulate samples 
taken using an apparatus that is properly calibrated as to air flow, 
filter media of a known and fixed type that is changed and analysed at 
regular intervals, etc etc.


On 08/08/2011 01:32 PM, Busby, Chris wrote:
> When a car engine is running it works at varying rpm from 800 (tickover) to 3500 (revving).  If it is in a traffic jam and going slowly the engine is running slowly and taking in less air. When it is travelling fast, the engine is revving and taking more air.  The history of these cars is known and was reported by the owners. The mileage is on the odometer. I assumed 60km/h and 2500rpm as a mean. Actually this is quite a good method for estimating fuel consumption on a test rig and comparing it with average driving in town. So it is also a reliable estimate of the air flow, since the air mix is proportional to the fuel consumtion. In any case, the revs can only vary between 800 and 3500 rpm so that gives an outside limit. You can assume what you like. Do the sums and see what you get. You will always get a great deal more than the 2.7mBq/m3 from the global fallout or the 10mBq/m3 from Chernobyl in the N hemisphere.
> You cannot calibrate an air filter with a vacuum cleaner. What do you mean? How would you measure the air that passed through it?
> Chris
> -----Original Message-----
> From: radsafe-bounces at agni.phys.iit.edu on behalf of Brennan, Mike  (DOH)
> Sent: Mon 08/08/2011 17:03
> To: The International Radiation Protection (Health Physics) MailingList
> Subject: Re: [ RadSafe ] FW: Reporter's question about lowerlimitsofdetection
> The number of km driven does not give you good information about how
> much air was pulled through the filter, for the simple reason that
> distance traveled in an uncontrolled environment does not correlate to
> the amount of air that was pulled through the engine.  I have, over the
> years, made a number of trips from my home to the nearest major airport,
> about 45 miles away.  I have made the trip in as little as 45 minutes,
> when traveling in the early morning when traffic was light.  I have also
> made the trip in almost 2 hour during the middle of the day.  Same
> distance, very different times, gas mileage, and amount of air through
> the filter.  Additionally, the daytime trip probably had more
> particulate in the air to act as sites for contamination (including
> radioactive material) to attach to.
> Given that you could have used a vacuum cleaner with a filter over the
> hose, and calibrated the flow with little trouble, I have to admit to
> the suspicion that you chose the automobile filter specifically BECAUSE
> of the unknowns, which you could manipulate to support your agenda.
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