Troxler 4640-B, Operation And Instruction Manual

Page: 35 / 140

STANDARD COUNT
The 4640-B uses a cesium-137 gamma source for taking density
measurements. This low-level radioactive source undergoes a
natural decay process that results in a gradual loss of strength. The
time required for the source strength to diminish by 50% is referred
to as the
half-life . The half-life of cesium-137 is approximately 30
years.
To compensate for the source decay a daily reference
standard
count is performed. It is very important to take a standard count on a
daily basis to ensure the highest accuracy and precision possible
with the gauge.
The 4640-B is equipped with two methods of testing the reference
standard count. The first method compares the new standard count
with the average of the last four standard counts. This method is the
Multi-Standard Mode . The second method compares the new
standard count to the decay corrected factory calibration reference
standard count. The factory standard count is corrected for the decay
over the elapsed time. The new count is compared to the decay
corrected calibration count and is referred to as the
Single-Standard
Mode . Refer to Chapter 6, page 6–12 for information on selecting
Single-Standard or Multi-Standard Mode.
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TAKING THE STANDARD COUNT
The gauge comes with a magnesium (Mg) reference block and an
air gap spacer for taking the standard count. Place the reference
block on a dry, flat surface at least 10 ft (3 m) from any large
vertical surface (for example, a concrete block wall) and at least
33 ft (10 m) from any other radioactive source. The surface should
be asphalt or concrete at least 4 in. (10 cm) thick.
Place the spacer on the Mg reference block and then place the gauge
on top of the spacer. The gauge must be in the
SAFE position. The
handle end of the gauge should rest over the two posts on the
spacer.
Model 4640-B 3–9