Model 3430 Plus & 3440 Plus
MOISTURE
The gauge uses the principle of neutron thermalization to monitor
the moisture content of a material. The gauge includes an
americium-241:beryllium (Am-241:Be) source that is fixed in the
gauge’s base. Fast neutrons emitted by the Am-241:Be source pass
into the test material. Multiple collisions between the fast neutrons
and a similarly sized mass (such as the nuclei of hydrogen atoms)
cause the neutrons to slow to the point where further collisions with
hydrogen or other materials will not continue to reduce the neutron
energy further. These neutrons are said to have been thermalized.
The gauge contains a helium-3 detector that is sensitive only to
thermalized, or “slow,” neutrons. As a result, the moisture counts
relate directly to the amount of hydrogen in the material. Using the
gauge calibration, the gauge software converts the helium-3 detector
counts to a moisture content. (Note that the helium-3 detector is in
the same plane as the Am-241:Be source. Therefore, moisture
measurement is similar to a density measurement taken in
backscatter mode, as described earlier in this chapter.)
The depth of measurement is defined in terms of a maximum depth
beneath the surface of the material being measured. Of the neutrons
counted by the gauge, 98 percent will penetrate no deeper than the
depth of measurement. The depth of measurement is a function of
moisture content. The following equations can be used to determine
the approximate depth of measurement.
Depth (inches) = 11 – (0.17
×
M), where: M = moisture in pcf
or
Depth (mm) = 280 – (0.27
×
M), where: M = moisture in kg/m
3
The normalized curve set shown in Figure 2–1 illustrates the effects
of moisture content on the depth of measurement.
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