General Tools and Instruments MMD900 - Manual (Page 3)

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Thank you for purchasing General Tools & Instruments’ MMD900 Pin/Pinless
Deep Sensing Moisture Meter with Remote Probe. Please read this user’s
manual carefully and thoroughly before using the meter.
The MMD900 is designed for use in woodworking, water damage restoration,
building construction and home renovation. Examples include:
• Checking for moisture on or below the surface of carpets and
sub-flooring
• Measuring the surface moisture content of wood, drywall or concrete
before painting, wallpapering, sealing or treating
• Locating water leaks above ceilings, below floors or behind walls
• Selecting dry lumber
The meter senses the moisture level of a material using either of two
techniques:
1. Inferring the material’s electrical conductivity from the current flow
induced between a pair of steel pins placed on or into the material. The
wetter a material, the higher its conductivity. The MMD900 is equipped
with two pairs of test pins—one pair on the top of the instrument and the
other at the end of a cable and remote probe that can fit in tighter spaces.
2. Measuring the change in the material’s capacitance produced by the
slight spreading of a local electric field generated by the meter. When
the meter is powered on, two plates inside the back of the MMD900’s
housing are charged with opposite polarity and pulsed at a high
frequency. This causes current to flow, creating a three-dimensional
electromagnetic wave at radio frequency that extends about 2 in.
(50mm) perpendicular to the meter. When the back of the meter is
placed against a wet material, the increased capacitance of the material
distorts the wave’s electric field to an extent that can be measured. The
distortion—called the fringing field effect—is proportional to the
material’s moisture level.
WHICH SENSOR SHOULD YOU USE?
It depends on the application. The pinless (capacitive) sensor is the only choice
for estimating the surface or internal moisture level of materials like finished
wood, paint or wallpaper that cannot be marred by pinholes. If accurate
readings are important, the pin sensor is the only option because the pinless
sensor can only provide relative readings.