Understanding Moisture Content in Wood, Part 2
In last month's newsletter, we reviewed the importance of understanding moisture content in wood and how that moisture content affects woodworkers. This month, we are following up to this initial tutorial to describe to you the methods which can be used for determining the moisture content in a piece of wood.
There are a few things to keep in mind before we begin. Wood will only become as dry as the environment in which it is placed will allow. In covered areas outdoors, or in a non-climate controlled shop, the wood will usually only dry to about 14% - 18% moisture content. In a climate controlled home or shop, the wood will dry to a lower moisture content, perhaps 6% to 8%. In order to properly dry a piece of wood, it needs to be placed within an environment which will be similar to the one in which it will eventually be placed after finishing. This will reduce the likelihood that the piece will deform or check later down the road.
Determining Approximate Moisture Content by Weight -
As a piece of wood dries, it will slowly lose weight as water is removed. Once a wood is no longer able to dry further, it will stop losing weight, or the weight will fluctuate up and down slightly. This fact is useful in determining if a piece of wood has completed the drying process and is ready for use. To check the dryness of a piece, use an accurate scale capable of measurements in ounces or grams (we prefer digital scales). Check the weight of the piece of wood every 2-3 days. Once the weight stabilizes for a period of at least 2 weeks, the wood should be sufficiently dry for final use. You may also notice the weight increasing or decreasing slightly over this time period, as humidity and temperature levels fluctuate and cause the moisture content in the wood to increase or decrease.
Determining Exact Moisture Content by Weight -
In order to be more precise when determining moisture content by weight, you will need to use an oven-dried sample block cut from the same piece of lumber for comparison. Here's a step-by-step breakdown of the entire process:
- Cut a small sample block (perhaps a 1" cube") from the same piece of wood for which the moisture content will be determined, and measure its weight. Be as accurate as possible (to within at least 1/10th ounce or gram), as this weight is critical for use in determining the correct moisture content.
- In an oven, dry this sample block at approximately 210-220 degrees Fahrenheit. Remove and weigh the block periodically, until it no longer loses any weight. Record this final weight, which is called "oven dry" weight.
- Now, use these two recorded weights to determine the moisture content of the wood using the following equation:
Moisture Content (MC) = ((Initial Weight - Oven Dry Weight) x 100) / Oven Dry Weight
And there you have it...an accurate moisture content measurement. This measurement can also be used in order to determine what weight your larger block should weigh at any given moisture content. This will allow you to pre-determine the moisture content you wish to have, then know how much the wood will weigh when it reaches the needed degree of dryness. To determine the final necessary weight after dry, use the following equation:
Final Weight = (Initial Weight x (100 + Final Moisture Content)) / (100 + Initial Moisture Content)
Determining Moisture Content Using Moisture Meters -
Moisture meters make determining moisture content a fairly simple process, using electrical current to calculate moisture content. As the meter applies an electrical current to the wood, it reads the electrical resistance or the dielectric properties that the wood provides. This value is then converted by the meter and shown as a readable moisture content number. Let's take a look at the process, step-by-step:
- Insert probes of meter into wood, making sure that probes reach into the center of the lumber's thickness.
- Use meter to check moisture content of lumber.
- Adjust this reading to compensate for the temperature and species of the wood being checked. This is typically done using charts provided by the meter's manufacturer.
And that's it! Moisture meters make measuring moisture content much easier. The process of choosing and buying a moisture meter, however, is much more complex. Originally, we had planned to include more detailed information concerning meters in this month's tutorials, but felt that the amount of information was a bit too much to include with what we had for you so far this month. We'll go into full detail next month, explaining the types of moisture meters available, and what options to look for or avoid when making your decision to purchase.