TM 5-6635-386-12&P
to obtain good seating:
a.
Take a backscatter reading prior to any work.
b.
Perform a minimum of surface work, take
another reading.
c.
Perform more work, clearing, smoothing, etc,
and take another reading.
The readings will drop in count rate as the tester is
seated better and better. When readings have ceased to
change, the tester is seated as well as it can be. This is
then an index of the work required to obtain good
seating.
1-15. SURFACE ROUGHNESS ERROR
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
paragraph 1-14.
The dual position tester backscatter feature provides a
means of selecting a shallow backscatter position (AC
Position) for best results on thin lift Asphaltic Concrete
and a slightly deeper backscatter position (BS Position)
for use on soils or thick Asphaltic Concrete.
Unfortunately, the thinner the tester attempts to read, the
more sensitive it is to surface roughness.
The following test will illustrate the potential roughness
error and will permit the operator to use the tester to his
best advantage with a minimum of error.
a.
Take a reading in both normal backscatter (BS)
and in the shallow backscatter (AC) on a smooth
concrete surface. Use the 2 minute count period for this
test.
b.
Raise the tester the thickness of a dime,
approximately 0.050-inch, producing a 100% void under
the tester.
c.
Repeat the readings in both AC and BS test.
d.
The tester will read approximately 3 pounds per
cubic feet (PCF) low in BS and will read approximately 7
(PCF) low in AC.
This will vary slightly between testers of the same design
and between testers of different manufacturers.
The error is an index of the amount of effort required by
the operator to obtain good seating with any specific
gauge.
1-16. DEPTH OF READING (Fig. 1-4.)
Certain physics limitations present themselves in depth
of reading of nuclear testers.
No matter how the design of the tester is made, soil
testers using GM tubes and radioactive gamma sources
of approximately 0.7 MEV energy will not read more than
5-inches deep for 99% of their response in construction
materials from 100 to 170 PCF.
If the radioactive source is well collimated as in the BS
position, the tester will tend to pick up 90% of its returned
radiation from approximately 2.5-inches deep.
1-13
