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Re: INSTRUMENT CALIBRATION
Kenneth,
Please consider that there are two component to calibrating an instrument:
the calibration of the electronics (linearity across decades, high voltage is
what the meter says, etc.,) and the radiometric calibration (what is th
efficiency of the detector).
I am in the field myself right now, so I don't ANSI N323 with me, but I think it
also requires that the calibration facility maintain and document temperature
and humidity, that the equipment used to calibrate the instruments, such as
pulsers, high voltage power supplies, frequency generators, etc., also be
traceable.
Once the instrument is calibrated, then the radiometric calibration can be
performed, in which one gets the instrument efficiency calculations performed.
Usually, I suggest using a jig of some sort such that source to detector
geometry can be controlled and reproduced. As long as the sources are
traceable, and the person is qualified to conduct the radiometric calibration,
and the procedure used is documented, site-specific radiometric calibrations
should be allowed.
Therefore, I don't recommend field calibration of instruments, however, if you
have a site-specific need, a field determination of detector efficiency can be
accomplished and documented.
Just my opinion, and again, I'm out in the field myself right now, so I don't
have any references with me.
Many shops allow for interchangeability of certain detectors, such as pancakes.
N323 uses 20%. However, I think with todays electronics, that number could, and
maybe should be tightened in the next revision.
Being you are working in the FUSRAP arena, you may be working with many NaI
scintillometers. Our experience is that crystals of the same size are
consistant with one another, but often the PMT tubes are not, and this is
critical. The older and more used those PMT tubes are, the more likely they are
to start dropping off in efficiency. The best way to check this (IMHO) is to
plateau the instrument, that is, set it up in a jig with a source at a fixed
geometry and then vary the voltage across the operational range of the
instrument. Plot the data, and set the high voltage in the middle of the
plateau.
If all your detectors/PMT combinations are in the same range, then one can
probably swap them out with confidence.
Do you cross calibrate your scintillometers with a PIC? If you are converting
count rate to exposure rate you can develop a site-specific conversion using
regression.
Good luck,
Phil Egidi
ORNL/GJ
7pe@ornl.gov
Fleming, Kenneth wrote:
> I recently had a peer come up and ask a question regarding calibration of
> portable instruments. A debate began over whether portable instruments
> could be "calibrated" outside of a laboratory environment (e.g., in the
> field). We've all had the opportunity in the past or present of conducting
> source checks and "calibrating" instruments in areas that would most
> definitely not be considered to be a lab. ANSI N323A-1997 defines several
> terms: calibrate, national standard, secondary standard, and tertiary
> standard. The standard provides good information in Section 4 about
> calibration.
>
> The questions I'm providing are how legitimate is it to conduct "field
> calibrations" (typically using one source at on one range) and use the
> subsequent efficiency to determine activity? (This instrument may have been
> calibrated by the manufacturer/vendor to a different source and then field
> calibrated.) If it is legitimate to calibrate in the field, is it necessary
> to send instruments to manufacturers or other calibration vendors to be
> calibrated, we can still calibrate in the field on an annual basis? From my
> read on calibration and a discussion with the ANSI N323A working group
> chair, a calibration must take place in a location that maintains at a
> minimum NIST traceable, tertiary standards and these standards need to be
> part of a managed QA program which are sent off routinely (say once a year)
> to be verified.
>
> Also at issue was the interchangeability of separate detectors onto a
> ratemeter. This subject is discussed in the Standard. Probes are allowed
> to be changed without recalibration if the variation between the new and
> previous baseline reading is within 20%. The calibrated efficiency (from
> the manufacturer) is intended to remain the same unless you can show that
> the standards used to calibrate are part of QA program. This may be news to
> many field hounds, but this is message I was given.
>
> Obviously, I'm more concerned operationally about survey technique because
> this in itself is the largest variable in a survey. I'd be interested in
> how others view this subject regarding calibration. If there are any
> regulators (either DOE, NRC, or Agreement State) looking at this I'd much
> appreciate how you would view this if you came upon a site where
> efficiencies changed on a daily basis. With this I'll call it a week and
> await any responses.
>
> This message is most definitely mine alone.
>
> Kenny Fleming CHP CSP
> FUSRAP Radiological Control Supervisor
> knflemin@bechtel.com
> (423) 220-2306
> (423) 220-2464 FAX
>
> ************************************************************************
> The RADSAFE Frequently Asked Questions list, archives and subscription
> information can be accessed at http://www.ehs.uiuc.edu/~rad/radsafe.html
Fleming, Kenneth wrote:
> I recently had a peer come up and ask a question regarding calibration of
> portable instruments. A debate began over whether portable instruments
> could be "calibrated" outside of a laboratory environment (e.g., in the
> field). We've all had the opportunity in the past or present of conducting
> source checks and "calibrating" instruments in areas that would most
> definitely not be considered to be a lab. ANSI N323A-1997 defines several
> terms: calibrate, national standard, secondary standard, and tertiary
> standard. The standard provides good information in Section 4 about
> calibration.
>
> The questions I'm providing are how legitimate is it to conduct "field
> calibrations" (typically using one source at on one range) and use the
> subsequent efficiency to determine activity? (This instrument may have been
> calibrated by the manufacturer/vendor to a different source and then field
> calibrated.) If it is legitimate to calibrate in the field, is it necessary
> to send instruments to manufacturers or other calibration vendors to be
> calibrated, we can still calibrate in the field on an annual basis? From my
> read on calibration and a discussion with the ANSI N323A working group
> chair, a calibration must take place in a location that maintains at a
> minimum NIST traceable, tertiary standards and these standards need to be
> part of a managed QA program which are sent off routinely (say once a year)
> to be verified.
>
> Also at issue was the interchangeability of separate detectors onto a
> ratemeter. This subject is discussed in the Standard. Probes are allowed
> to be changed without recalibration if the variation between the new and
> previous baseline reading is within 20%. The calibrated efficiency (from
> the manufacturer) is intended to remain the same unless you can show that
> the standards used to calibrate are part of QA program. This may be news to
> many field hounds, but this is message I was given.
>
> Obviously, I'm more concerned operationally about survey technique because
> this in itself is the largest variable in a survey. I'd be interested in
> how others view this subject regarding calibration. If there are any
> regulators (either DOE, NRC, or Agreement State) looking at this I'd much
> appreciate how you would view this if you came upon a site where
> efficiencies changed on a daily basis. With this I'll call it a week and
> await any responses.
>
> This message is most definitely mine alone.
>
> Kenny Fleming CHP CSP
> FUSRAP Radiological Control Supervisor
> knflemin@bechtel.com
> (423) 220-2306
> (423) 220-2464 FAX
>
> ************************************************************************
> The RADSAFE Frequently Asked Questions list, archives and subscription
> information can be accessed at http://www.ehs.uiuc.edu/~rad/radsafe.html
************************************************************************
The RADSAFE Frequently Asked Questions list, archives and subscription
information can be accessed at http://www.ehs.uiuc.edu/~rad/radsafe.html