RE: False alarms on DMC model 90 electronic dosimeters

[Glen.Vickers@ucm.com]

This sounds like the symptoms of RF interference.  This problem is well
documented by millions of dosimeter-hours at numerous power plants.  The
original DMC-90 dosimeters did not have any RF shielding because they were
designed before the wonderful 900 MHz radio came into widespread use.  I
believe, from 7 years ago, the original mods incorporated some tin-like
shielding with a "filter" board.  The board is probably something like a
band pass filter that many of us saw in college.  Later mods moved to having
the tin shell completely enclose the board holding the detector.  I'm sure
the DMC-100 and DMC-2000 have evolved even further in their discrimination
capabilities.

By and large the mods seem to work okay, considering the way in which they
evolved.  I believe it boils down to actual "potential" the dosimeter is
exposed to.  The nearer you get to the antenna on that Motorola 900 MHz
radio, the higher the field strength will be.  From the beginning, MGP was
smart enough to say that their mods were good for a certain "potential" or
"x" number of inches from a certain wattage radio.

The biggest offenders at a power plant will be those who routinely carry
radios such as operators, security, and fireguards.  Personnel may also
experience problems near repeating antennas in the plant.  These antennas
are often times located near stairwells so the transmission and reception
capabilities are maximized.  The problem is that these stairwells must be
accessed by the general population as well.  The best thing is to move such
antennas a few feet further from the path of personnel to drop the field
strength.

We have greatly reduced this problem by transitioning from DMC-90 dosimeters
to "late" model DMC-100 dosimeters.  I believe some of the early model
DMC-100 dosimeters had some teething problems, but the later models seem to
serve us well.  

You must remember there is absolutely nothing manufactured which doesn't
warrant improvements in later models.  This applies to space shuttles, jet
airplanes, artifical hearts, etc.  The DMC-90 was the first electronic
dosimeter that really saw so much widespread use and it has evolved over
time as well.  This dosimeter really helped the industry to take a
significant step away from the pencil dosimeter.  They should put one in the
HPS exhibit in the Smithsonean Institute.

Give the guys as MGP a call for a little more detailed history on the
evolution of RF mods.

Sincerely,
Glen Vickers
glen.vickers@ucm.com

> -----Original Message-----
> From:	Leamon, John A. [SMTP:jaleamon@tva.gov]
> Sent:	Wednesday, October 06, 1999 7:36 PM
> To:	Multiple recipients of list
> Subject:	False alarms on DMC model 90 electronic dosimeters
> 
> Does anyone out there, especially those of you working at a plant site,
> have
> practical experience dealing with false alarms on Merlin Gerin Model 90
> electronic dosimeters.  We are using these dosimeters with the Merlin
> Gerin
> LDM101 reader with 549C firmware and Windows 95 based HIS20 Access Control
> software.  
> 
> Several times each day, a small percentage of workers are exiting the RCA
> where known and confirmed radiation levels are less than 5 mrem/hour but
> the
> ED indicates a much, much higher value (sometimes in the hundreds or
> thousands of mrem/hour).  Needless to say, this is quite disturbing to the
> Rad workers, the Radcon staff, and other interested parties.  
> 
> We have been struggling with this issue for a month without success.  Any
> good ideas are appreciated to help get this behind opportunity behind us
> and
> move on to more worthwhile efforts.
> 
> Contact me, John A. Leamon, at 423-751-7038 or email at jaleamon@tva.gov
> 
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