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Think of a moisture meter
as cheap insurance.
Spend $70 and you’ll
never have to wonder
whether that lumber you
bought is too wet or too dry.
You can tell if the “kiln-dried”
pine you bought from the
home center was dried to 9-
percent moisture content
(about what you need for
indoor projects) or 19-percent
(what most constructiongrade
pine is kiln dried to).
Knowing the moisture content
(MC) of your wood helps
you determine when the
wood is stable enough to use.
Pin vs. Pinless Meters
How They Work
There are two types of meters on the market,
pin and pinless.Both types of meters measure
the effect of moisture on an electric current
(pin type) or an electromagnetic field (pinless)
to determine the moisture content (MC)
of the wood (Photo 1).The beauty of a pinless
meter is that it can quickly scan an entire
board without putting holes in the wood.You
can even take it to the lumberyard to test the
wood before you buy; try that with a pin
meter! One concern about pinless meters is
that the sensor pad must be in good contact
with the wood for accurate readings. Very
rough or warped stock may leave too many air
pockets under the sensor pad. I’ve found a few
swipes with a block plane creates a nice flat
spot to take your readings.
Pin meters can take readings in wood no
matter what the shape, size or degree of roughness.
All that’s required is that the two pins
make contact with the wood.Pin meters also
allow you to use remote probes (Photo 2).
Nails or probes can be driven to the center of
thick lumber for core readings that are out of
reach for pinless meters. If you dry your own
wood, the probes can be left in a sample board
in the stack to monitor the wood as it dries.
Plus,pin meters can take readings on the edge
of a board stacked for drying (Photo 3).
Species and Temperature Correction
Temperature and wood density affect the
readings given by moisture meters.All meters
are calibrated to read the MC of Douglas fir at
about 68 degrees F. (The Timber Check is
the only exception; it is calibrated for red
oak). That means if you’re using a meter on
something other than Douglas fir and the
temperature is above or below 68 degrees F,
you’ll need to make adjustments to the meter
reading. Manufacturers include charts that
adjust for species and temperature variations.
More expensive meters have built-in species
correction and a couple have built-in temperature
correction as well (see chart below). Just set the meter to the desired
species and the meter automatically corrects
the readings.This is a huge benefit when you
have a lot of wood to test.
Pin meters are more sensitive to temperature
variations than pinless meters. That’s why pin meters always come with temperature
correction charts. Some manufacturers
include corrections for pinless
meters should you need a very
precise reading.
Pinless meters,on the other hand,are
more sensitive to differences in density,or
“specific gravity”of different species than
pin meters. That’s why pin meters with
built-in species correction can get away
with grouping species into a handful of
settings while pinless meters generally
require you to set the specific gravity of
each species into the meter.
Should I Buy a Pin or
Pinless Meter?
That’s the first question everyone asks
when looking to buy a moisture meter.
The question is best answered by identifying
what you want a meter for and
comparing that need to the advantages
unique to each type of meter.
If you tend to buy surfaced stock and
can’t bear the thought of poking holes in
expensive lumber, then a pinless meter
is probably your best bet.
If you buy rough stock,dry your own
wood, use wood thicker than 2 in. or
have a weakness for piles of rough lumber
discovered in some old barn, a pin
meter is for you.
Important Features
Pin Length
A rule of thumb states that the average
MC of a board can be found at a depth
equal to 1/5 to 1/4 the thickness of the
board. For example, 5/16-in. pins are
long enough to get an average MC reading
on a 1-1/2-in.-thick board and
1/2-in. pins will work for 2-in. stock.
Remember,however, that this rule works
only when the board has an even moisture
gradient where the surface is drier
than the core.
It’s tempting to think that a pin meter
measures the MC of the wood at the
ends of the pins. In reality, the uninsulated
pins measure the wettest layer of wood they come in contact with. Wood that’s been stored in a shed or shop can have a higher MC on the surface than the core. In this case, the reading only reflects the MC of the wetter outer surface, regardless of how deep the pins penetrate. To get an accurate core reading with uninsulated pins you can crosscut the board and take a reading of the core on the freshly exposed end grain.
Insulated pins only measure the MC
of the wood at the tips of the pins.They
come with the external probe accessory
that’s available with some meters (see
the chart, below.
Minimum Sample Size
Pinless meters have a minimum sample
size that’s dictated by the size of the
sensor plate. The entire plate must be
touching the wood you’re testing. So, a
meter with a 2 in. x 2 in. sensor pad
can’t be used on a board that’s only
1-1/2-in. wide. This precludes using
most pinless meters to scan the edges of
4/4 boards stacked in a pile.
Moisture Content Range
A range of 7 to 20 percent is all you
need to check air-dried or kiln-dried
wood. You can pay extra for a meter
with a range that exceeds 30 percent,but
keep in mind that accurate readings
higher than 30 percent are impossible
because there is just too much water in
the wood. People who dry their own
wood use the higher readings to get a relative sense of how wet the wood is to
start and how fast it’s drying. Turners
and carvers who work with green wood
may benefit from a meter with an
extended range.
At the low end of the MC scale, pin
meters are accurate down to 7 percent
and pinless,down to 5 percent.Readings
below these levels are unreliable because
there is just too little water in the wood.
Displays
Both types of meters come in one of
four types of displays (Photo 4): analog,
LED (light emitting diode), digital LED
and digital LCD (liquid crystal display).
We like the digital LED and digital LCD
best.Analog displays are inconvenient.
A “hold”feature on the display is nice
to have. Sometimes readings have to be
taken in an awkward position or in poor
light where it’s difficult to read the display.
Being able to hold the reading until
you can actually see the display can be
quite handy.
Some of the more expensive meters
give MC readings with a resolution of
1/10 percent.The less expensive meters
generally read out larger increments.
But, that may be all you need for a
go/no-go decision on your wood.
Built-In Species and
Temperature Correction
We think that built-in species correction
is a feature you can live without unless you
typically need to take readings on a large
quantity of wood.A chart can be a bit of
a hassle,but it’s no big deal if you’re dealing
with just a few boards. Even with
built-in correction, you may have to use
a chart to find the right setting.
Carrying Cases
Sensor pads and pins need protection
when they’re being carried around.
That’s why we liked Delmhorst’s toolbox
type of carrying case best. It also
gives you a place to store charts and
manuals that need to travel with your
meter.Second best are the ballistic nylon
pouches on the Wagner MMC210 and
220. Electrophysics and Moisture Register
do not come with carrying cases.
Sources
(Note: Product availability and costs are subject to change since original publication date.)
Moisture Meters Evaluation Chart
This story originally appeared in American Woodworker June 2002, issue #94.
June 2002, issue #94
Purchase this back issue.
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Click any image to view a larger version.
1. Pin and pinless meters measure moisture differently.
Pin meters have a pair of nail-like probes that are inserted into the wood.An
electric current is sent between the two pins. Because water is a good conductor
of electricity and wood is a poor conductor, the meter can tell how much water
is in the wood by how much current travels between the pins.
A pinless meter has a sensor plate that’s held against the surface of the wood.
The plate projects an electrical field into the wood.The meter can sense changes
in the field caused by moisture and wood.The meter then converts the change to
a moisture content reading.
2. External probes extend the reach of your meter. External probes driven
to the center of a board allow you to get a core reading in stock that’s too thick
for the pins built into the meter.The probes can also be left in a stack of green
wood where readings can be taken to monitor the wood as it dries. Some
meters have built-in jacks for aftermarket probes, but a pair of nails and alligator
clips are an effective, low-cost alternative for all pin-type meters.
3. Taking readings
from the edges of
boards in a stack is
a task better suited
to pin meters. Most
pinless meters have
sensing plates that are
too big to read the
edge of a 4/4 board.
4. Four types of displays are available on moisture meters.We liked the digital
LED and LCD displays the best. Analog displays are the hardest to read. LCD models
show the moisture content value on a little screen.This type of display is easy to read
in full sun but hard to read in dim light. LED models turn on when the right moisture
setting is dialed in on the meter.With a digital LED, the numbers themselves light up.
A digital LED is easy to read in the dim light of a storage shed, but difficult to see in full sun.
Recommendations
The good news is that all of these meters will do a great job for you. But for most
of us, there’s no need to spend more than $90 for a pin meter or $140 for a pinless.
Meters in this price range can tell you all you need to know about the moisture content
of wood that’s been kiln or air-dried.That’s why all of our picks are Best Buys.
Our Best Buys are simply the least expensive pin and pinless meters. If you want
built-in convenience features that the low-cost meters don’t offer, check the chart
for features and prices that best suit your needs. If you dry your own wood, you may
want to spend a little more for a meter that reads above 30-percent MC.
Best Buy, Pin Meters
Electrophysics MT90; $69
This no-nonsense meter is simplicity itself. Insert the pins and turn the
dial until the LED turns from red to green.At that point the dial points
to the moisture content of your wood. This meter is not limited to
1-percent increments but is capable of fractional readings like 6-1/2
percent.The meter comes with complete, full-size charts and a pair of
alligator-clip leads to use with external nail probes.Our only complaint
is the lack of a carrying case that can hold the meter,manual and charts.
Moisture Register DC2000; $88
For those who want a little more than a barebones
meter, the DC 2000 offers the most features
for the least money. For $88 you get a
meter with built-in species correction and
the largest MC range (6 percent to 65 percent)
of any meter under $150.Wood species
are grouped into three different categories,
A, B and C. If you really want precise readings,
the DC2000 also comes with species correction charts.
The Moisture Register also features an easy-toread
digital LCD display. Unfortunately, a
carrying case is $20 extra.
Timber Check; $65
Rugged and simple are the operative words
for this meter. It can tell you all you really
need to know about air-dried or kiln-dried
wood. It’s the only meter out there that’s calibrated
to read red oak instead of Douglas fir.
The Timber Check works when you insert
the pins and turn the knob on the base until the LED light goes on.Each click of the knob represents 1-percent
intervals from 6 percent to 12 percent and 4-percent
intervals from 14 percent to 25 percent. The readings are
printed clearly on the body of the meter.
Best Buy, Pinless Meters
Wagner L-609; $140
Easy to use and compact, the L-609 has been in the Wagner
stable for many years.What we really liked about this meter,
besides the price, is that it comes with an extensive species
correction chart with over 170 species,
including tropical exotics. If you can’t find
your wood on this list, then you’ve really got
a rare specimen.We also liked the fact that the
sensing pad is small enough to allow for
readings on the edge of 4/4 boards.
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