to obtain good seating:
a. Take a reading in both normal backscatter (BS)
and in the shallow backscatter (AC) on a smooth
a. Take a backscatter reading prior to any work.
concrete surface. Use the 2 minute count period for this
b. Perform a minimum of surface work, take
b. Raise the tester the thickness of a dime,
approximately 0.050-inch, producing a 100% void under
c. Perform more work, clearing, smoothing, etc,
and take another reading.
c. Repeat the readings in both AC and BS test.
The readings will drop in count rate as the tester is
d. The tester will read approximately 3 pounds per
seated better and better. When readings have ceased to
cubic feet (PCF) low in BS and will read approximately 7
change, the tester is seated as well as it can be. This is
(PCF) low in AC.
then an index of the work required to obtain good
This will vary slightly between testers of the same design
and between testers of different manufacturers.
1-15. SURFACE ROUGHNESS ERROR
The error is an index of the amount of effort required by
the operator to obtain good seating with any specific
Surface roughness error is the error introduced into the
tester readings due to the rough surface obtained during
seating. Such error was exhibited in the work in
The dual position tester backscatter feature provides a
Certain physics limitations present themselves in depth
means of selecting a shallow backscatter position (AC
of reading of nuclear testers.
Position) for best results on thin lift Asphaltic Concrete
and a slightly deeper backscatter position (BS Position)
No matter how the design of the tester is made, soil
for use on soils or thick Asphaltic Concrete.
testers using GM tubes and radioactive gamma sources
of approximately 0.7 MEV energy will not read more than
Unfortunately, the thinner the tester attempts to read, the
5-inches deep for 99% of their response in construction
more sensitive it is to surface roughness.
materials from 100 to 170 PCF.
The following test will illustrate the potential roughness
If the radioactive source is well collimated as in the BS
error and will permit the operator to use the tester to his
position, the tester will tend to pick up 90% of its returned
best advantage with a minimum of error.
radiation from approximately 2.5-inches deep.