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 assess a meter’s accuracy? Basically, the idea is to compare a given meter’s readings with the actual moisture content of each wood sample. Weigh each board beforehand, then put it in an oven to get it completely dry. Then weigh it again to see how much water has been lost. By obtaining before and after measurements of weight, one can use a formula to calculate the percentage moisture content and compare this to the meter readings.
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? The chemical composition of the wood within a single tree is very similar, but it can vary significantly from tree to tree. The readings of some meters are highly sensitive to this variation in chemical composition, and this affects their real-world accuracy.
If all of our wood samples were to come from the same tree, then our accuracy test would not take this factor into account. So during our testing, we made sure that we had boards that came from a variety of trees. This helped us test for real-world accuracy, since one does not usually obtain a supply of wood products from a single tree.
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.
Labeling Each Wood Sample
- Each sample is labeled by species, sample number, and moisture group (high, medium, low) using a marking pen, in order to facilitate easy identification and tracking.
- 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 2 and 3 with each piece of wood.
- Take the first piece of wood and unwrap if necessary.
- Cut 1” from both ends of the sample.
- 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. It is to be assumed that the oven-dry weight has been reached when the mass loss is equal to or less than twice the sensitivity of the balance.
- 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.”).
Taking moisture content readings of wood
Recording weight and dimensions of wood
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.