General Tools and Instruments MMD7NP - Manuel (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 back cover of the MMD7NP 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 back 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
average 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 MMD7NP 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.
Two buttons on the front of the MMD7NP provide 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 18%WME, 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 %WME 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
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