By Dave Munkittrick
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.
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.
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.
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.
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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.
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.
This story originally appeared in American Woodworker June 2002, issue #94.