Gauge Errors
Density - There are three error parameters inherent in all gauges
manufactured: Nuclear Precision (P), Surface Roughness (SR),
and Composition Error (CE).
Precision or repeatability of the gauge is defined as the
variation in repetitive density reading on the same test spot for
a given counting time. Precision is calculated based on the
equation (P= √Count/ Slope) which depends on the number of
counts collected during a given period and the volume of the
test material measured by a specific gauge at a given depth.
Precision for a typical gauge for a one minute reading at the
time of manufacture is approximately ±0.5 PCF (8 kg/m3) at
backscatter and ±0.25 PCF (4 kg/m3) at 6” direct transmission
depth. The reduction in precision over the life of the gauge
(approximately 20 years) is insignificant.
Surface Roughness error is caused by streaming of photons
from the source to the detectors in the air space immediately
under the gauge. Surface roughness error at BS depth is much
higher than direct transmission. Surface error is usually
calculated by taking a reading with the gauge directly on
limestone block and repeating the reading on the same block
with 0.05” (1.3 mm) gap under the gauge (100% air voids). The
difference between the flush and raised reading is a measure
of the error expected from the gauge on rough surfaces, such
as open graded or base coarse asphalt. Filling the surface with
Portland cement or fine cement powder helps reduce this error,
when measurements are taken on rough materials.
Composition Error - is caused by the effect of material
characteristics on the gauge measured density. Since
attenuation of photons is effected by chemical composition of
the material, density measurements can be effected
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