Methods of Moisture Content Measurement

in the Lumber and Furniture Industries

by

Phil Mitchell

Assistant Professor and Wood Products Extension Specialist

North Carolina State University

The moisture content of wood is a basic property that can impact subsequent machining, gluing, finishing, and product performance. It is the job of wood product producers to establish a moisture content in the wood that will be similar to what the wood will experience in service. To accomplish that, manufacturers must develop a standard procedure that will measure wood moisture content consistently and accurately. This brief note discusses methods of moisture content measurement commonly found in the lumber and furniture industries.

Ovendry Moisture Measurement

The "true" moisture content (MC) of wood is defined by the ovendry method. According to the Wood Handbook, ovendry wood is defined as "wood dried to a relatively constant weight in a ventilated oven at 102 to 105 oC (218 oF +3 oF)." An oven with circulating fans is preferred over a convection oven, but it is more important to have a working thermometer inserted in the hole in the top of the oven.

In the kiln drying of lumber, moisture sections cut from sample boards are recommended to be 1" along the grain. For weighing these moisture sections, the balance should have a capacity of at least 1,000 grams and weigh to at least 0.1 gram (0.01 gram is preferable). For most sample boards, the balance should have a maximum capacity of 15,000 grams (33 pounds) and weigh to the nearest 1 gram. For operations that process wide, high density hardwoods the maximum capacity should be 20,000 to 30,000 grams (44 to 66 pounds).

As mentioned, the ovendry MC is assumed to be the true or actual MC. In reality, there are several sources of error in practical ovendry moisture content determination. Three of the most common errors include: incorrect oven temperature; incomplete drying of small moisture sections; and moisture pickup by dry sections from newly introduced green sections. Letís consider each error in more detail. The oven should be set at 218oF + 3 oF. An oven that is 35 oF too cool will have an equilibrium moisture content (EMC) approaching 1% on a warm, foggy morning. As a result, the ovendry weight may be overestimated by 1%, resulting in underestimating the final MC by 1% MC. In turn, this will cause: schedules being advanced too early (possibly causing additional degrade); equalizing and conditioning that are less effective because they are begun too soon; and a final MC that is on average 1% too high. A working thermometer in the oven will help insure that the correct temperature is maintained. The second error listed is incomplete drying of small moisture sections. This might happen when kiln operators establish a time based routine for drying sections, and do not check to see that ovendrying continues until a constant weight is reached. With low density species, failure to remove even just 0.5 grams of moisture can underestimate the MC of sample boards by 1.5%. Lastly, adding green moisture sections to an oven with nearly dry moisture sections can temporarily raise the ovenís EMC (especially in a convection oven). If you cannot avoid this situation, the solution is to have two or more drying ovens.

Electric Moisture Meters

Electric moisture meters allow the user to rapidly and accurately estimate wood moisture contents less than 30%. Most hand held moisture meters are typically either resistance (pin type) or dielectric (flat plate) meters. In the past, meter readings needed to be corrected for species and temperature by hand using printed tables. Today, commercially available state-of-the-art moisture meters have species and temperature corrections built into the digital circuitry. In addition, the newer meters are capable of taking multiple measurements and storing them internally, providing the ability to calculate and display mean and standard deviation statistics, and the option of downloading the MC data to a computer. The ability to transfer data to a computer is a significant plus if you want to plug moisture measurement into a quality control program.

Both types of meters offer the same accuracy over about the same range of moisture contents. None of these meters provide accurate readings above 25 to 30% MC. Your decision on which type of meter to obtain depends on your needs. Further details about resistance and dielectric meters are provided below.

Resistance Moisture Meters

The operating range for resistance moisture meters is from 7 to 25% MC. This type of meter uses pin type electrodes that penetrate the wood up to depths of 2Ĺ inches. To determine the average MC, the depth of penetration should be ľ the thickness of rough lumber, and 1/5 the thickness of planed lumber. Resistance meters have an average accuracy of + 1% MC over their operating range.

Resistance meters made and marketed in the US have pins that must be inserted parallel to the grain so the current will run along the grain rather than across the grain. If the pins are inserted across the grain, the meter will read too low by 1 to 2% MC. Caution: Because meters manufactured in other countries may differ in this regard, it is important to follow the instructions provided by the manufacturer.

Resistance moisture meters are sensitive to the temperature of the wood. New meters on the market today allow the user to specify the wood temperature, and the meter automatically makes the correction. A very rough estimate of the temperature correction can be obtained by subtracting 1% MC from the reading for every 20oF the temperature of the wood is above 70oF, and adding 1% MC for every 20oF below 70oF.

Resistance meters were originally based on Douglas-fir, and the resistance characteristics of many domestic woods are similar to those of Douglas-fir. At 8% MC, most domestic woods are within 1.5% MC of the Douglas-fir reading; at 16% MC domestic woods are within 3% MC of the Douglas-fir reading. Differences among species occur mainly due to differences in the extractives between woods. Tropical woods often require much larger corrections due to higher amounts of extractives than domestic woods. Fortunately, todayís resistance meters typically include built-in corrections for most domestic and many foreign woods. It should be noted that if these corrections are being done by hand using printed tables, the temperature correction should be made first, followed by the species correction.

Moisture gradients can be measured with resistance meters. When using pins that are not insulated, the meter will record the highest wood MC with which it comes into contact. The measurement of moisture gradients requires the use of pins coated with insulation (the pin tip is not coated). With insulated pins, the MC is measured at the furthest depth that the pins penetrate into the wood. When measuring kiln dried lumber with a normal moisture gradient, driving the insulated pins deeper into the wood will encounter wetter wood resulting in higher MC readings. This allows the kiln operator to get a rapid estimate of shell to core MC differences. Insulated pins also help avoid false high readings if the lumber has been surface wetted with rain or dew.

It is very important that the calibration of your resistance meter be checked periodically. This can be done with either the built in calibration function, or with the use of a calibration bar that provides a known resistance equal to a specified MC when placed across the pins.

Pocket sized meters typically have the pins built into the housing rather than having a separate electrode connected by a cable. These short pins generally do not penetrate as deep, and are limited to spot checking as they are not rugged enough to withstand production quality control work, and usually cannot fit insulated pins to check moisture gradients.

Dielectric Moisture Meters

The operating range for dielectric moisture meters is generally considered to be greater than that for resistance moisture meters, and is often stated to range from 4Ĺ to 25% MC. Dielectric meters use surface contact, flat plate electrodes that do not penetrate the wood. The depth of penetration by the measuring field ranges from 0.5" to 1.0" depending on the model. Field penetration should be half the thickness of the wood being tested. In situations where the field penetration is greater than the thickness of the wood, care should be taken as the reading will be affected by the material beneath. Suggestions to avoid this error include: taking measurements on top of a stack of similar material with similar moisture content; using rigid polystyrene foam as a backing; or making the measurement with nothing but air beneath the wood being metered.

Dielectric meters read the average MC of the zone penetrated by the electric field. Similar to resistance meters, the accuracy of dielectric meters in measuring average MC is + 1% moisture content. The readings are reportedly most influenced by the wood nearest the electrode, and are consequently more reliable on wood with a fairly uniform MC than on wood with substantial moisture gradients. Dielectric moisture meters are not useful in determining moisture gradients.

Commercially available dielectric moisture meters do not require a temperature correction between 32 oF to 250 oF. Kiln operators sometimes use these meters to quickly look for "wet pockets" in the lumber before they pull the kiln.

Corrections for wood species, however, must be performed due to density differences between species. Many dielectric meters marketed today have species corrections built into the digital circuitry of the meter. For species corrections not provided with the program, it will be necessary to determine the representative average specific gravity of the wood, and adjust the meter accordingly. The user can then evaluate the species adjustment by comparing meter readings (using the measured specific gravity factor) from representative samples with the MC determined by ovendrying.

For dense lumber of a given species, the MC readings will be erroneously high. The considerable normal density variation within many species limits the accuracy of dielectric meters. An idea of the accuracy achievable with a species can be obtained by using wood samples representative of the density variation typical of the species and comparing meter readings with the MC determined by ovendrying.

The surface contacting electrode can be placed with any grain orientation on the side grain of wood with little effect on the meter reading. It should not, however, be used to measure MC on the end grain. It is also important that the electrode is in firm contact with the wood surface; hence moisture measurement in cupped lumber may be erroneous if the electrode is not in complete contact with the wood.

Comparing Resistance and Dielectric Meters

Both types of meters measure wood MC with the same accuracy over about the same moisture range. Although one type of meter is not clearly better than the other, one meter type might better meet your needs. Letís compare some of the advantages and disadvantages of these two types of meters.

_ With pins that penetrate the lumber, resistance meters leave holes in the wood. Dielectric meters use surface contact electrodes that are non-invasive and leave no marks.

_ The use of insulated pins with resistance meters allows the measurement of lumber moisture gradients which is useful to kiln operators. Dielectric meters cannot provide gradient information but instead give the average MC of a slightly larger area. In addition, dielectric meters are more reliable on wood having a fairly uniform MC than on wood with substantial moisture gradients (early morning surface dew can cause reading errors).

_ Dielectric meters can read to slightly lower MC than resistance meters.

_ Resistance meters are not sensitive to differences in wood density. Although dielectric meters provide species corrections that are based on average species density, the MC readings are sensitive to within species variation in density.

_ Resistance meters typically are used with a separate hammer/electrode probe, making them more bulky to carry around on the factory floor. In addition, these electrodes require frequent attention due to broken or loose pins or broken cables, and can be sensitive to static electricity in dry, cold weather. Because the moisture sensing element is built into the case of the dielectric meter, it is more compact and easier to carry around the plant.

Which is the best portable meter? The answer, of course, is the meter that meets your needs. If you dry and sell lumber, you should consider using the same meter that your customer is using. If you use lumber in a manufacturing process, the best meter is the one that will be used! For many plants it makes sense to use both types of meters.

In-Line Moisture Meters

The resistance and dielectric technologies used in portable meters has been applied in the development of in-line moisture meters. In-line moisture meters are commonly used to measure the moisture content of each piece of lumber. The meters are typically located on the lumber chain in front of the planer, or at the planer outfeed, and are capable of either painting or dropping wet lumber out at a tipple gate. Some in-line meters can be purchased with an information and statistical package, which allows MC data collected from a kiln or package to be stored in a computer along with the mean and other statistics.

Resistance in-line moisture meters use wire brushes that resemble drum snares to contact the woodís surface and measure MC. Resistance in-line meters are primarily used to measure the MC of dry veneer, but are sometimes used to monitor the MC of lumber. These meters are very sensitive to surface moisture. Because their most appropriate use is on veneer, the remainder of the discussion on in-line moisture meters will examine dielectric meters.

The technology used in dielectric meters has been applied in the development of the in-line moisture meter. With the in-line meter configuration, however, the sensor electrodes do not contact the wood. Two types of sensors are available. The transverse sensor measures the MC as the lumber travels sideways down the lumber chain. This arrangement typically uses multiple heads that scan the lumber from the bottom side. The longitudinal sensor, or end to end sensor, uses a two-sided head to measure the MC at the planer outfeed.

In-line moisture meters are used extensively in the softwood lumber industry to detect wet lumber. They are usually installed in the package breakdown area, often immediately behind the planer. These meters measure every piece of lumber without contacting it. With softwood lumber destined primarily for construction, these meters are used principally to detect wet lumber which would be off grade. The meters are capable of storing data for whole kiln charges by lumber package. This allows kiln troubleshooting by knowing the location of each package within the kiln.

Although these meters have not found widespread application in furniture rough mills, they do offer the potential to reduce the introduction of off-spec MC lumber to further processing. A furniture rough mill operation that allows a high MC piece of lumber to be cut and processed into perhaps as many as a dozen or more parts will potentially allow that number of finished furniture pieces to become defective with parts that will likely shrink and prove dimensionally unstable. An in-line moisture meter can also be used to determine and record the MC of each piece of purchased lumber.

The reaction of the dry kiln operator may be to resist this technology since the MC of every piece of lumber will be checked; but with time the kiln operator will recognize this tool as an ally. For example, if management insists that the lumber be pulled too early, the effect of inadequate equalization on MC variability will be documented by the in-line moisture meter. Over time, the kiln operator will be able to use the meter to identify areas of the kiln that exhibit unbalanced drying conditions, and make maintenance recommendations based on MC information generated from the in-line moisture meter.

Summary

This note has discussed the practical aspects of wood MC measurement. The ovendrying method defines the wood moisture content, while measurements obtained with electric meters are estimates of the "true" ovendry moisture content. Portable resistance and dielectric meters available today can accurately read moisture contents below 30% to + 1% moisture content. In-line moisture meters are capable of determining the moisture content of every piece of lumber. Because wood shrinks and swells as it either loses or gains moisture, it is important that the manufacturer produce wood having a MC close to the moisture content it will reach in service. Wood whose MC changes significantly in-use will be prone to checking, splitting, warping, opening of glue joints, and cracking of finishes. Knowledge and control of wood moisture are critical in the manufacturing of quality products made from wood.