General MM8 User Manual page 3

The meter exploits two physical phenomena to make its measurements:
1. The linear relationship between a solid material's moisture level and its
dielectric constant—and therefore its capacitance.
2. The so-called fringing-field effect—the slight spreading of the electric
field produced by current flowing between two electrodes when both
electrodes are on the same side of a material.
Behind the top of the MM8 are two metal plates. When the meter is powered
on, the plates are given small and opposite charges. The potential difference
causes current to flow, creating a three-dimensional electric field.
When the top of the meter is placed against one side of a material with
moisture on or slightly below its surface, the increased capacitance of the
material distorts the electric field to an extent that can be sensed (as a change
in flux over the sensing area) and measured. Displayed readings reflect the
ver ge moisture level of the material between its surface and the electric
field's maximum penetration of 3/4 in. Moisture closer to the surface has a
greater effect on readings than moisture at the maximum penetration depth.
The MM8 has been calibrated at the factory for use with wallboard, masonry,
hardwood and softwood. The capacitance of wood and the capacitance of
building materials are affected differently by moisture because they have
different densities. The meter compensates for density by adjusting the gain of
its internal sensing circuitry.
The MODE button on the front of the meter provides a convenient way to
switch among the four materials. The name of the material selected appears on
the LCD. Below the LCD is a bank of colored LEDs that roughly mirrors the
digital reading above it in bar graph format, with green indicating "dry", red
indicating "wet", and yellow indicating an intermediate moisture level. An
audible out-of-range alarm (beeper) sounds whenever wood is tested and
found to have a moisture content above 17%, or when drywall or masonry is
found to be more than 70% wet.
Although the LCD displays measurements of drywall and masonry moisture
content as % readings, these are actually relative readings with no accuracy
specification. Despite that shortcoming, relative readings are useful for quickly
comparing the moisture levels of materials, or the wetness of different areas of
the same material. For example, you can use the meter to locate the source of
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