There are two basic types of meters – pin and pinless. Pin meters have pins, usually about ½” long, that are pressed or driven into the wood to determine what the moisture content of the wood is. Pinless meters are placed on the surface of the wood to obtain a reading. Although they both are accurate, they take readings using different principals and have certain pros and cons.
Moisture conducts electricity very well. Dry wood, on the other hand, is a good insulator. A pin moisture meter uses these two facts to determine how much water is in a piece of wood. It sends an electrical current through the wood sample and measures its resistance to electricity. The electrical resistance of wood increases as the moisture content decreases. The meter will then turn that information into an accurate moisture content value. Pinless moisture meters send radio waves into the wood to obtain a moisture reading. When the waves return the meter measures how the waves react to the moisture in the wood.
Both types of meters give you accurate readings only for the area that the meter actually contacts. Since wood isn’t a homogeneous material, it’s a good idea to do a number of readings from the piece of lumber to determine whether or not the wood has dried evenly.
The moisture content of a piece of wood can theoretically range from 0% to well over 100%. Most moisture meters will only give you accurate readings between about 6% and 25-30% (just below fiber saturation point). For most of us this is acceptable, since an appropriate moisture content for most situations is between 7% and 11%.
There are pros and cons to both types of meters. One of the main advantages of a moisture meter with a pin is that it can be used on a piece of wood with a rough surface. The pins can be inserted into a board whether it has been dressed or not – unlike pinless meters, which need a fairly smooth surface (a planed/jointed surface is best) to obtain accurate readings. If you’re air-drying wood it’s much easier to use a meter with pins than to create a smooth surface for a pinless meter.
A major advantage of a pinless meter is the fact that it doesn’t leave any unsightly holes in the wood, potentially damaging furniture parts that will eventually be taken from the lumber. While a pin meter takes a reading from a relatively small area, the scan area of a pinless meter is much larger, typically 1 ½” x 2 ½”.
Each make of moisture meter is slightly different. Read and understand the meter you have purchased before making any conclusions. Practice on a number of different pieces of wood to get a good idea of how they work. To get some widely varying moisture values, test different types of wood: a 4 x 4 post that you know is very wet and a piece of wood that has been sitting around for ages in a dry location. Avoid taking readings close to the ends of boards. The end grain will allow moisture transfer to take place faster than the rest of the board, causing readings that are not typical of the entire board.
When using a pin meter, insert the pins ¼ of the depth of the board that you are measuring to get an accurate reading. On ¾” stock insert the pins about 3⁄16″ deep. You’ll be able to take accurate readings on boards up to 4″ thick with ½” long pins by taking readings on both sides of the board, fully inserting the pins. For thicker stock you’ll need to drive nails or screws into the stock and use extension leads. Some meters have a hammer attachment that makes pin insertion easier. If you are pushing the meter into the wood with your own strength do your best to apply even, straight pressure to force the pins into the wood without bending them. The pins can be replaced fairly easily, but a new set of pins for each reading will get expensive. The pins should also be inserted parallel to the grain.
Pinless meters are much easier to use. There is a sensor plate on the bottom of the meter. This plate needs to be firmly in contact with the wood surface. Pressing over the center of the meter with about three pounds of pressure will give an accurate reading. You also need to ensure that there is nothing under the board you are measuring; this is especially important for thin stock. You can simply hold the piece above your workbench, or place it over support blocks. Pinless meters can take accurate measurements from ¾” and upward. As with pin meters, take measurements on both sides of thick stock.
Once a reading from the meter has been obtained there are two adjustments you may have to make in order to account for variables. The first variable is temperature. Most meters are calibrated for the industry standard Douglas fir at 20ºC. Your meter will come with a chart to make this adjustment, so keep it handy. Take the reading from your meter and adjust it accordingly. After adjusting for temperature you have to take the species of wood into account. Once again, use the species adjustment table supplied with your meter to account for differences in the specific gravity between species.
It may seem like a lot of hassle: spending your hard earned money, taking the time to learn about your meter, obtaining a reading and then doing the adjustments. But, consider the time and energy that you put into a project. Imagine watching a finished piece of furniture develop weak joints due to gluing problems or expansion forces, and eventually fall apart. Learning a bit about the moisture content of the wood you’re working with will go a long way in preventing these problems from happening.
The MT270, measuring 1 ⅞” x 3 ⅛” x 4 ⅜”, features ½” pins and an analog dial with a 4% to 30% scale. It comes with a cork-like pin protector, a set of spare pins, calibration/battery checker, two 16″ leads and a manual containing a species correction chart. The calibration/battery checker is simply a small round plastic ‘cup’ with two metal screws protruding from one side. The cup has an effective moisture content of 16%. Touching the meter pins against the screws enables you to simultaneously test the battery and calibrate the meter. The leads have alligator clips on either ends, enabling you to take readings on stock of any thickness via nails or screws inserted into the stock. The analog dial is easy to read, and you can take readings in approximate ½º increments; not as precise as a digital readout, but reliable enough for most tasks. This ‘pin and read’ meter is easy to use, and particularly suited for rough stock, or milled stock that is oversized, so that when you do the final milling the pin holes will be largely removed. I found it quite easy to push the pins to a full ½” depth in softwood. On oak, hard maple and ash I could easily push the pins ¼” to ⅜” deep. Inserting them to the full depth was difficult, and best done by pre-drilling with a 3⁄64″ twist drill bit. You’ll need to refer to the accompanying manual to correct for the specific gravity variation among different wood species; easy enough to do, but a bit inconvenient. An optional storage case is available. Electrophysics also manufactures pinless meters. $138.00 with a two year warranty. Available direct from electrophysics.on.ca.
Pin-type meters require and individual species-by-species adjustment of the raw meter reading. The necessary information to do so, for over 200 wood species, is included with all Electrophysics pin-type meters.
The MMC220 pinless meter measures a compact 1″ x 2 ¾” x 4 ½” and weighs around six oz. (168 grams). The large LCD screen is very easy to read, and a low battery indicator appears when it’s time to re-charge or replace the battery. An automatic shut-off conserves battery life by turning the unit off after one minute of inactivity. To take an effective measurement the scanning plate on the back of the unit has to be firmly in contact with the wood surface. The manual lists the scanning area as 1 ½ x 2 ½ and to a depth of ¾”. The ¾” depth of scan doesn’t mean that you are limited to using it on 1 ½” stock by taking reading on both sides. I took readings on both sides of 4” lumber that were almost identical to readings taken by the Electrophysics pin meter via leads on embedded screws. There are two steps to using a ‘scan-and-read’ meter. You begin by setting the meters specific gravity value, via the Species button, for the species of wood you are going to measure. This takes about 60 seconds. Once the setting is made, you only need to change it if you are measuring wood with a different specific gravity. Fortunately the clever people at Wagner have put an erasable pad on the front of the meter where you can jot down the specific gravity for up to 10 species of wood – ample room for me as I regularly use only about six different species. A small booklet that lists settings for most common domestic and imported species fits neatly into the carrying case, which has a handy belt clip. The second step is simply to firmly press the meter onto the surface being measured. As with a pin meter, you really need to take readings at several locations on the work piece, particularly if it’s a wide, long piece. The MMC220 is very ‘retailer friendly’; you won’t be leaving pinhole marks when you check how dry the wood is at your local lumber dealer. Sometimes you need to take readings on lumber that is in a hard to reach place or where the lighting is not very good. Pressing the On/Hold key when taking a reading will freeze the display so you can move the unit without losing the reading.
Pressing the key a second time turns the hold feature off. The MMC220 gives very precise readings in .1% increments. I tried it on a wide range of softwood and hardwood; it’s very quick to use, and adjusting for species is a cinch. $419 with a one year warranty, moisturemeters.com. Available from leevalley.com.