Moisture Meters for Wood: A Second Opinion
As a hardwood flooring installer, I have been using wood moisture meters professionally for over 30 years. To use these meters properly, you need to understand some fundamental properties about wood moisture content and how the various meters actually function.First, let’s look at what defines the concept of moisture content (MC) in wood. Next, we’ll talk about how a moisture meter works. And, finally, I’ll share some of the fine points of using a moisture meter—along with a few tricks of the trade that improve results.
What is Moisture Content?
Moisture content may sound like a simple thing to understand and measure, but it’s actually a little complicated—and it’s not what many people think it is. Moisture content is the weight of the water contained in a piece of wood compared to its oven-dry weight. Therefore, the best method for determining MC is to perform an “Oven Test” by weighing the piece of wood and then placing it in the oven at 212F or 100C for approximately 24 hours. When the piece of wood stops losing weight, it is oven dry. Oven-dry wood is at zero moisture content. In the best-case scenario, the oven test takes 24 hours to determine the moisture content—and the piece of wood is destroyed. Not an option if you’re working on a jobsite.
How Moisture Meters Work
Pin-type moisture meters measure the electrical resistance between two electrodes. When water is present in wood, electricity is more easily conducted, but as wood becomes dryer it resists electrical flow. This resistance is measured in OHMS. You can buy an ohm-meter from Radio Shack that measures the same property. About 20 years ago, Fine Woodworking published a set of plans with details on making your own electrical resistance moisture meter, but with the cost and availability of moisture meters today, making your own isn’t necessary.
Using Moisture Meters
Here are a few tips that should make it much easier for you to use a moisture meter accurately:
- The pins should line up with the longitudinal axis of the wood—in other words, with the grain, not against the grain.
- Touching the pins to the surface of the wood doesn’t work. You have to insert the pins to a depth of 1/4 in. to get the MC of a piece of 3/4-in. thick lumber.
- The pins don’t give accurate MC readings in end grain.
- The surface of the wood needs to be dry. A wet surface will give an erroneously high MC reading.
- In a pinch, you can use finish nails as electrodes. Insert the nails and touch the pins of the meter to them.
- Use a drill to prepare holes for inserting pins into very hard species like Ipe, Cumaru, Brazilian Cherry, Lignum Vitae. Regular pins will break or bend.
- Select a moisture meter with simple controls and an easy to read display. I use the Delmhorst J-4 and BD-10 models. These models are scaled to reflect the moisture content of Douglas Fir, which is the reference species used by the wood science industry. Over 50 years ago, wood scientists studied the relationship between electrical resistance and moisture content and published data for many different species of wood. It is this data that is used to calibrate current pin-type moisture meters.
Keep It Simple 1: The analog models I use are less expensive than Delmhorst digital models. But whatever meter you chose, take my advice, rather than spending money on additional features, invest in a copy Understanding Wood, by Bruce Hoadley, or the Wood Handbook, published by the USDA, Forest Products Laboratory.
Keep It Simple 2: Adjustable moisture meters, which can be set for different species, add another potential source for errors: it’s easy to set the meter for the wrong species, and it’s even easier to forget to reset the meter.
Keep It Simple 3: Because most moisture meter scales are set for Douglas Fir, corrections are necessary for other species. I make corrections manually, but those corrections are usually under 2%—not enough for most carpenters or woodworkers to worry about.
One last bit of advice: keep a spare battery with you.
Howard has operated Brickman Consulting, a consulting and wood floor contracting business in the Boston area, since 1984. Though his background is wood science—he spent three years as the graduate teaching assistant in the Wood Anatomy Lab at the University of Massachusetts, Amherst—Howard has always preferred the hands-on part of the business—installing wood floors and helping the industry understand wood-floor failures. Howard designed and manufactures the Slab-Safe concrete moisture meter. When he’s not working on a floor, Howard is particularly fond of Speyside Scotches and Welsh Corgis.
Thanks for writing. You’ve inspired me to learn more about wood movement and to start keeping a meter in the truck. – Rob