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6.3. GAMMA
CALIBRATION
If the unit has previously been calibrated with tritium gas, then it is sufficient to use a gamma
radiation source to produce a response when placed at a specified location relative to the
instrument under test. Lead shielding is advised since the compensation chambers will cancel the
measurement. It is best to shield both of the compensation chambers plus one of the
measurement chambers. Use a minimum of ½” thick lead. For the highest value response, the
gamma source should be directed through the bottom of the case to minimize interaction with the
compensation chambers. If the gamma source is long lived, no chronological correction is needed.
To verify calibration at a future date, the original gamma source must be used. Records must be
kept to identify relative location of the source and the expected result. Be sure that temperature
and pressure variations are taken into account.
If calibration by either of these methods is performed, and the instrument response is somewhat
different from the expected value, then small adjustments can be made by turning the calibration
potentiometer with a small screwdriver. The calibration potentiometer is accessed by removing
the small Phillips head screw on the front panel located above the label
CAL
.
Large changes in calibration are evidence of malfunction. The factory should be consulted
immediately.
OVERHOFF TECHNOLOGY CORPORATION
Telephone (513) 248-2400
Facsimile (513) 248-2402
Email:
6.4. GAMMA
CHECK
If a tritium monitor has previously been calibrated by any other method, gas or gamma, a low
intensity gamma radiation source check can be used as a quick verification of monitor
performance. On the left side of the instrument case towards the front which is the defined location
for “
GAMMA CHECK
”. When using the identical gamma check source, at the defined spot, it
should always produce the same instrument response, provided, of course, temperature and
pressure variations are taken into account. This source check may be performed at a frequency
of your choice, it could be daily, weekly or monthly. We recommend a low intensity gamma check
source of the type which is commonly intended for G-M counters or other survey instruments. For
example; a 10 micro Curie, Cesium-137 check source should be sufficient for a monitor reading of
100-200
μ
Sv/hr.
IMPORTANT:
Do not adjust the calibration when performing a gamma check.
