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Offline captncars

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Observation interval in MDCR calculation
« on: Aug 21, 2013, 04:23 »
I need help determing the value for the obsevation interval "i" when calculating MDCR in the Scan MDC formula. MDCR = si × (60/i) MARSSIM typically uses 1 or 2 seconds.

 This is from MARSSIM:
"The observation interval during scanning is the actual time that the detector can respond to the contamination source— this interval depends on the scan speed, detector size in the direction of the scan, and area of elevated activity. Because the actual dimensions of potential areas of elevated activity in the field cannot be known a priori, MARSSIM recommends postulating a certain area (e.g., perhaps 50 to 200 cm2), and then selecting a scan rate that provides a reasonable observation interval."

NUREG 1507 states:
"an observation interval of 1 second translates into different scan rates, depending on the scan distance covered in that time for each detector type."

We are using this formula to calculate the MDC for 20cm2 gm probes with a scan speed 2 inch per second.

My interpretation is to take the detector width divided by the scan speed.
Which would be 2.7in / 2 in per sec = 1.35 second observation interval.  ??

I am a beginner at this and was hoping some of you pros can help me out.  ;)

Offline GLW

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Re: Observation interval in MDCR calculation
« Reply #1 on: Aug 21, 2013, 04:53 »
The conservative tack for observation interval is that period of time the full detector active area is exposed to the postulated sample area,...

ISO 7503-1 (ref ISO 8769) lays out the calibration principles built upon in 1507,...

So, if you're a beginner, you should read the ISO-7503 series and ISO 8769 so you can have a springboard for the 1507 expostulations you are trying to interpret today,...

Also the Tritch study referenced in the text of 1507 is a good read ( Tritch is one of my favorite colleagues in this business and I stand by his technical acumen),...

If you can get your hands on a copy of the original Somers 1975 from Health Physics (or was it ANS?) that is also an illuminating read for a beginner,...

I have a .pdf copy somewhere, maybe I'll try to find it this week,...

Now with all that said, if you're determining your observation interval for a GM detector you can get a little credit for the saturating pulse as opposed to a proportional detector,...

But you really need to have your TBD bulletproofed to try and take that credit,...

If this is for MARSAME or MARSSIM take the conservative vantage for routine surveys and if you're in a bind for schedule or operational constraints let those who are not beginners deal with it,...

It is never good to think you are all done only to have your regulator tell you they'll get back to you in six months or so because they have issues with your field measurement quality,...

good reading and PM me for further questions,...

that's the best I have without opening the books and being sure,...

almost forgot,...peace,....8)
« Last Edit: Aug 21, 2013, 04:56 by GLW »

been there, dun that,... the doormat to hell does not read "welcome", the doormat to hell reads "it's just business"


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