Right Tool, Wrong Data: Moisture Meter Mistakes
When trying to diagnose or prevent moisture problems in wood flooring and other wood projects, the obvious tool to use is a wood moisture meter; and, it follows, that the first obvious mistake is not to use one. But there are a number of factors to consider when both choosing the right meter and using the meter you’ve chosen. Your choices can mean the difference between an accurate moisture content (MC) reading, and a potential problem down the road.
There are two basic types of wood moisture meters: pin-style and pin-less.
Pin-style meters make use of two narrow metal probes that are inserted into the wood product you are testing. An electrical signal is passed between the tips of the two probes and the resistance measurement is then interpreted into a moisture level content that is read on the meter. Large hammer probes can be attached to a hand meter for deeper measurements as the probes are driven in by force, or shorter pins are most commonly pushed in by hand.
A pinless meter, as is suggested by this common name, does not use pins or probes but instead makes use of either a radio or electromagnetic signal that will change characteristics depending on the moisture level in the wood being tested.
So, while the meter types are fairly easy to understand, there are still several mistakes that prevent these tools from being an asset on the job site. Some are problems that are inherent to certain meter types, but others can be easily avoided with a little knowledge. What should you watch for?
- For a pin-less type of meter, not knowing the meter’s reading depth. For example, if the meter you’ve chosen takes readings at up to 1 ½” deep but you’re taking readings on already-installed ¾” thick flooring, obviously the reading you get will be somewhat affected by the MC of the subfloor. Not necessarily a helpful set of moisture readings from which to make decisions.
- Using too little pressure. Pinless meters can produce improper readings if the correct pressure is not applied. Too little, and the air gap under the meter’s sensor can impact the reading it takes. Some lower end meters are even skewed by the position of the user’s hand and can vary depending on the position of the hand. It is not necessary to apply more than about three pounds of pressure for most pinless meters.
- The obvious risk of over-stressing a pin-style meter is that pins can bend or break. Most pin-style meter manufacturers are clear that pounding or banging on the meter is not the correct way to insert the pins into a pin-style meter. Hammer probes are obviously built to withstand more force, but over time, even these more rugged pins can break, especially in hardwood applications.
- Improperly driven pins. Pin-style meters (and even the hammer probe type) take a considerable amount of pressure to reach the proper pin depth. In softer woods, the effort exerted is often minimal but when you are working with many hardwoods, especially some very dense tropical species, the effort needed to properly insert the pins to depth can become quite a challenge – if not virtually impossible – particularly if your meter is not constructed to take that type of force. Bent or broken pins or even cracked meter housings become more and more common with increased density of the wood types. The temptation is to not completely insert the pins, but obviously then, the reading will not be taken at the correct depth. A reading taken with pins close to the surface will never accurately reflect the moisture conditions deeper in the wood.
- Trying to take a shortcut. Because of the effort to insert pin-style meters, or because of their tendency to bend or break, it can be tempting to shortcut the testing process. Obviously, this will speed up the testing, but it will not give any sense of the real moisture conditions of the wood or wood flooring that will be used. When testing large quantities of wood, or multiple parcels, it can be tempting to only test a few representative boards from the edges of the bundle. But even in acclimated wood, moisture conditions can change in areas with more restricted airflow. Handheld moisture meters, like the Wagner MMC220 let you “scan” lumber or wood stock to quickly gauge the moisture conditions in much of the wood. But if a representative number of boards aren’t tested, the potential for trouble will still be high.
- Overlooking general maintenance. Any meter will suffer when exposed to high temperatures, overly wet conditions, being dropped or other circumstances that compromise the integrity of the housing and the circuitry. But even something as seemingly minor as peeling insulation on the insulated pins of hammer style probes can skew the readings of the meter. Because the signal is not restricted to a certain depth of the wood (at the tips of the probes), readings will not be precise depthwise and may represent a sort of composite reading of the signal along the length of the probe, or simply read too shallow based on where the signal is strongest. Pinless meters may also give skewed readings if their sensors become scratched or otherwise damaged. Lack of regular maintenance can also upset the calibration of the meter. Many manufacturers, like Wagner Meters, provide a calibration block to confirm the meter is still within factory calibration even after repeated use.
- Not using the correct species settings or species adjustment. Most moisture meters use either user-programmed species settings or species adjustment tables. Obviously, a piece of oak will have different properties than Douglas fir, so different species settings would be needed for these two. But if your meter doesn’t allow for those differences, the readings it takes will mean poor decisions about applied finishes, or its readiness to be installed. Be sure you’ve read and understood the settings for your meter.
A moisture meter can be an invaluable tool for advising finish and adhesive options or for avoiding moisture-related problems in a finished project. But if improperly used, a moisture meter will not do the job it is designed to do. Know your meter, follow the manufacturer’s instructions and be sure you are choosing the meter that best suits your purposes. Then you’ll easily avoid those costly mistakes.
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