General Tools and Instruments MMD900 - Manual (Page 4)

The distinction between absolute and relative readings is important to
understand. The moisture level measurements made by either pair of test
pins—on the instrument or on the remote probe—are displayed in units of
%WME (Wood Moisture Equivalent) simultaneously on a 0-99.9 count digital
LCD and a 40-LED analog bar graph. Pin measurements are accurate within
3% of the reading plus 5 digits.
By contrast, readings made by the pinless sensor are simultaneously displayed
with no units on the LCD and on a second scale (called REL) of the bar graph.
Although the pinless readings have no accuracy specification, they are
nonetheless useful for quickly comparing the moisture levels of materials, or
the wetness of different areas of the same material. Two possible uses for the
relative readings are:
1. Determining whether two pieces of wood to be joined have roughly the
same moisture content (which means they will dry out at the same rate
without warping).
2. Locating the source of a water leak above a ceiling, by comparing the
REL readings at various points on it. If the ceiling is level, the point with
the highest REL reading is below the source of the leak.
The biggest advantage of the pinless sensor is its ability to sense moisture up
to 2 in. below or behind the surface of a material. The stainless steel test pins
of the MMD900 can also sense moisture just below the surface of some
materials. However, for hard materials like wood or concrete, measurements
made by the meter’s test pins mostly represent surface moisture content
because: 1) Moisture close to the surface has a greater effect on a reading than
moisture deep below it; and 2) The pins of the MMD900 are too short to be
driven deep into a hard material. The pins at the top of the meter are 15mm
(0.6 in.) long and the pins on the remote probe are 10mm (0.4 in.) long.
For softer materials like soil, paper or powders, pin measurements are more
likely to reflect the average moisture level of the material between its surface
and the penetration depth of the pins (usually far less than 0.6 or 0.4 in.).
Other features of the MMD900 include the following:
• Any reading (%WME or REL) can be held by pushing a front-panel button.
Doing so—called locking or “freezing” the reading—makes it possible to make
a measurement in a dark place, hold it, and display it later in a better-lighted
area.
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