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METER METHOD. There are two different types of moisture meters used by the wood industry. Each method has limitations on the range at which they are accurate, but regardless, they are both very useful! They are much better suited to boards rather than to small samples used to monitor drying progress however. RESISTANCE METERS. Resistance meters work by measuring the electrical resistance of moist wood, measured by inserting insulated steel pins with uninsulated tips into wood at the appropriate depth. There is a relationship between wood resistance and moisture content and these meters display moisture contents directly, but the meter readings have to be adjusted for both the species and the wood temperature (not the air temperature). Different North American species have different electrical resistance at the same moisture contents due to their extractive content, and without adjustments the moisture contents readings could be off by one or two percent. Adjustments are provided by the meter manufacturers. It’s particularly important to use the recommended adjustments for tropical species because the extractive contents in tropical woods are often higher than in most North American species and can affect moisture content readings significantly. Heartwood and sapwood often contain different amounts of extractives so these chemical differences might affect the accuracy of the meter reading. This is true for sapwood in particular since calibrations are based on heartwood specimens. Without correction, resistance meters will read up to 1% different from the true moisture content for each 20°F difference from room temperature. (Cold wood is a little wetter than the meter reading at the room temperature setting and vice-versa.) Resistance moisture meters are only accurate between 7% MC and 28% MC. Drying lumber is wetter at its core than at the surface, so insulated meter pins (with uninsulated tips) must be driven to about 20–25% of the board thickness to measure the average moisture content for boards with drier surfaces/wetter cores (as normally happens in drying lumber). About 1/16” penetration will work well if you want to measure the surface MC. Surface condensation can become an issue if you aren’t using insulated pins in cold weather; you might put samples into a plastic bag if you have to move them into a colder environment to take a moisture meter reading with a pin-type meter. There are several reputable manufacturers of resistance-type meters. Many instrument makers (e.g., Delmhorst) calibrate their meters for pins placed parallel to the grain, while the meters made by Lignomat are calibrated for pin placement perpendicular to the grain. The pin alignment is not critical below 15% MC for any brand. The accuracy of resistance meters will vary according to the moisture content. Delmhorst states that their resistance meters are accurate to ±0.5% MC for samples between 6–12% MC, to ±1% for samples from 12-20% MC, and to ±2% MC for samples between 20–30% MC. DIELECTRIC METERS. Dielectric meters don’t need pins to be inserted into the lumber (no holes!) because these meters work by measuring the capacitance properties of wood. For these meters, all you need to do is to put the plate-like sensor on the bottom of the meter onto a wood surface. The boards must also be flat so the entire sensor plate can make good contact with the board. The moisture measurement will be the same regardless of whether the meter is aligned parallel or perpendicular to the grain, but the meter has to be set for the species being examined because the readings are affected by wood density. Dielectric meters are accurate between about 5% and 25% MC, but they can’t differentiate the moisture at different depths inside the wood like you can with a pin-type meter. The space beneath the board being measured 31PDF Image | HARDWOOD DRY KILN OPERATION A MANUAL FOR OPERATORS OF SMALL DRY KILNS
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