Q: When we receive lumber, we also check it carefully with a moisture meter before we unload it. Several of our customers indicate that they think we are not using the meter correctly. What is the correct procedure?
A: I think that the user of the wood that checks the MC with a meter probably has figured out, from experience, what MC readings are unacceptable, even if they are not using the meter "properly." So, I would almost always accept the customer's conclusions that the wood is too wet or too dry as being true - true for that customer. The supplier needs to meet the customer's requirements and not try to change the customer's measuring technique. Having said this, however, it is possible that the customer's technique can be improved. Customer and seller should both be on the same page! So here is my "top 10" MC meter operating guide for kiln-dried furniture-grade lumber.
First, there are several different, popular brands of moisture meters today. It may be surprising that the MC indicated will vary depending on the meter used. As an example, I measured the MC of several hundred pieces of kiln-dried lumber with three different brands of meters and then oven-dried the lumber to get the correct MC. For ash, there was a maximum of 2.1 percent MC difference between brands; for basswood, 2.3 percent MC; for hard maple, 0.6 percent MC; for red oak, 0.7 percent MC. Overall, however, pin-type meters and non-pin meters worked well and quite accurately for the species tested.
Let me expand slightly. Electric moisture meters get that name because they measure an electrical property of a piece of wood and convert that measurement to a corresponding MC value, usually read directly on the meter. Depending on the type and manufacturer of the meter, the electrical property measured may be resistance, conductance, dielectric constant or power loss factor. Because meters read different electrical properties and electrical properties are not perfectly aligned with MC, it is not surprising that the actual MC readings vary.
Second, many meters are calibrated for Douglas fir. That is the tradition. Make sure that species corrections are made; the correction for pin meters is usually quite small for most North American species.
Third, you need to specify what MC you will be using as the basis for measurement: shell, core or average. The non-pin meter will respond heavily to the MC at or near the surface of the piece where the meter is placed. These meters give you an average MC for the wood. The pin meter with uninsulated needles will measure the wettest spot, even if the spot is very small, along the length of the pins. With uninsulated needles and with slight surface moisture regain, perhaps due to dew, the readings will be very high, even though the piece is quite dry. This surface moisture will also be a problem for the non-pin meters. A meter with insulated needles will measure the MC of the wood in contact with the tip of the pins. Therefore, with insulated pins (the most common), the MC measured depends on how deeply the pins are driven. If driven about 1/5 of the thickness, the measurement is typically an average for the entire piece. If driven to the core, the wettest spot is typically measured.
Fourth, pin meters require substantial correction for temperature. For example, wood at 30 F that reads 7 percent MC has a true value closer to 9 percent MC.
Fifth, meters only work well under 25 percent MC. Pin meters have trouble under 6.5 percent MC; non-pin meters can go several percent lower.
Sixth, select a location to make the MC measurement. If you want the average MC of lumber, the location should be at least 2 feet from the end of piece and about the middle of the wide face of the board. If measuring pieces of moulding, turnings or other products less than 4 feet in length, select a location in about the middle of the piece. For piled lumber, readings with a pin meter may be taken on the edge or narrow face of the board. However, remember the edge pieces may not represent the MC of the interior pieces.
Readings taken from the end grain of boards will not usually represent the average MC of the piece. For the non-pin meter, often there is a minimum contact area required to get a reliable reading on a particular board; in other words, the sensor plate must be entirely covered; edge readings are probably not reliable.
Seventh, with pin meters using insulated needles, readings taken at different depths from surface to core will provide the moisture gradient. A shell wetter than the core would indicate moisture regain after kiln drying. To determine core MC of the piece, drive the pin tips to the center of the piece. Non-pin meters can be moved over the surface of the lumber to provide a profile of MC along the length and across the width.
Eighth, if meter readings drift, use the reading taken immediately after the electrode reaches desired depth in the piece. In a very dry environment (under about 30 percent RH) or when very dry lumber is planed, a static charge can develop on the lumber or on the cable of a pin meter. This static charge will result in erroneous readings. Often the meter will show erratic behavior of the MC readout.
Ninth, when wood is wet on surface due to rain, snow or ice, it is difficult to obtain accurate readings in this situation. Liquid moisture on the surface of the lumber can be dragged down with the probe on pin-type meters, and may give incorrect (too high) readings even with insulated pins. Do not use non-pin meters when surface moisture is present as readings will not be accurate.
Tenth, if a meter or probe is brought from a cold into a warm environment and the equipment is colder than the dew point temperature of the warm air, then moisture will condense on the cold equipment. The condensation may give an extremely high reading or may just give a reading of 10 percent MC. Low MCs cannot be measured until the moisture is evaporated from the equipment, and this may take several hours. It is preferred if the meter is at room temperature.
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