Accuracy is the most important test of our entire review and the core of all our objective testing. If a meter doesn’t consistently give accurate readings under a variety of conditions, it’s practically useless.
For instance, if a meter is affected by changes in either wood or meter temperature, it can throw off the accuracy of the reading.
If a meter is sensitive to surface moisture, it can interfere with accuracy. Or, if a meter is affected by cable interference, it too can jeopardize accuracy.
How do we know whether or not a moisture meter is accurate? How do we calibrate it?
Basically, you weigh a board, then put it in an oven to get it bone dry. Then you weigh it again to see how much water was lost. Because you measure the volume, you can calculate that as a percentage.
That’s how percentage moisture content is determined – a standard all meters are calibrated to.
Moisture Meter Accuracy in the Real-World
To ensure each test is truly indicative of a meter’s accuracy, we obtained a variety of wood species from a variety of trees. Why is this necessary? If you have 50 boards all coming from one tree in the forests, you’re not going to get an accurate representation of a meter’s accuracy due to the fact that the chemical composition of the wood will be the same.
So during our testing we needed to make sure that we had boards that came from a variety of trees and not the same tree. The reason is that pin meters are highly sensitive to the chemical composition of wood. It can throw off a meter’s accuracy. The chemical composition within a tree is very similar, but it’s very different from tree to tree. Using boards all from one tree makes a pin meter look highly accurate, where in the real world, most boards come from multiple locations.
Pin meters, then, will not be as accurate. So, again, to make the test truly indicative of a meter’s accuracy, we used a variety of boards from multiple trees and not just one.
Accuracy Test Procedures
Note: It’s important to weigh and measure one board at a time (as instructed below) to prevent drying between weighing and readings.
Taking moisture content readings of wood
- Take the first piece of wood and unwrap it if it’s in the (11%-20%) or (20% -25%) sample group.
- With each meter, take three readings along the length of the top and three more along the bottom. Record the readings and the SG setting used. Immediately re-wrap board if necessary.
- Repeat steps 1 and 2 with each piece of wood.
Recording weight and dimensions of wood
- Take the first piece of wood and unwrap if necessary.
- Cut 1” from both ends of the sample.
- Record the weight in ounces.
- With calipers:
- Take two measurements of the length of sample and record.
- Take two measurements of the width of sample and record.
- Take two measurements of the height of sample and record.
- Repeat steps 4-7 with each sample.
- Place each sample in the oven.
- After three days, take out each board and record the weight. Each successive day, record the weight until it ceases to drop significantly, per ASTM D 4442-92 section 5.4.5 (“Assume that the endpoint has been reached when the mass loss in a 3-h interval is equal to or less than twice the selected balance sensitivity. For example, with a 10-g (oven-dry) specimen, the balance sensitivity for 0.01% MC precision is 0.1 mg (see 5.1.2), therefore, dry to 0.2 mg or less mass loss in a 3-h period.”).
Note: The oven should be set at 220 degrees F (average). The boards in the oven are on edge with a minimum 1” clearance between each board, and stacked with stickers.
This test should utilize a minimum of 40 boards from each MC range, for a minimum of 120 boards total. This should be distributed as evenly as possible across the various species.