Reconciling Contradictory Moisture Readings

A pin-type moisture meter and a pinless moisture meter disagree during a quality-control check. What's going on? June 16, 2014

Question (WOODWEB Member) :
We are getting readings from our RF meter that differs from what our commercial lumber vendor is getting from his conventional pin type meter. I understand his need to measure core and shell differential, but our concern is simply reading anything that is out of spec for grade (above 8%). He claims 7 1/2% and we're seeing readings as high 11 and 12%. What gives? Do we need a new meter?

Forum Responses
(Sawing and Drying Forum)
From contributor L:
What thickness is the wood, and how deep does your pinless read? Have you made side by side comparisons with the dealer's meter? It is normal to have a variation in readings from a stack, but that seems like a good bit. How many places is it out? I assume you just got the wood and that it hasn't been sitting around in the rain! The acid test is to weigh, oven dry, and weigh again. You need a very accurate scale and a low temp oven.

From Gene Wengert, forum technical advisor:
RF meters are designed for veneer. If used on lumber, they typically will be way off. The RF meter is also quite rare. Perhaps you have a pinless meter, like the Wagner that is a dielectric meter. If so, it will give you a higher reading if the surface of the lumber has been exposed to high humidity even when the rest of the lumber is dry. It will give you a high reading if the piece of lumber being tested is higher in density than normal or above the density, called specific gravity, than you put in for a calibration. This meter will also read high if you do not have an air space under the piece of lumber you are testing.

The pin meter will read lower than the true value if the wood's temperature is much under 70 F, unless the meter is adjusted. Seldom will a species correction make much difference at low MC. However, non-USA made meters sometimes are not too accurate. The core moisture is almost always the MC when the wood left the kiln. So, a pin meter with insulated needles is very useful in determining if the lumber regained moisture after it left the kiln. I look forward to hearing more about the meters and their use, as that will likely tell us which reading is closest to the truth.

From the original questioner:
The meter in question (ours) is a Wagner, calibrated to the material and I assumed it was an RF meter (my bad). The wood that we use almost exclusively is 6/4 American white ash all FAS. We meter each board every two feet, both sides with the board elevated so we're not reading adjacent material. The wood is delivered in curtain side trucks and is stored in our heated shop that is also humidified to around 45% (measured with a sling psychrometer). Our readings are usually taken after about a week in our facility as the boards rise from the pile. The material simply doesn't see an environment where there could be moisture gain.

This may seem a bit excessive, but our product has an extremely high end user value and we've seen some movement before items go out the door. Your mention of differences in density rings a bell as this material is worked by hand and there appears to be a substantial range in density. I trust that this may only have a bearing on meter readings and not moisture retention. At the end of the day, I'm simply looking for an element of control that should help reduce the likelihood of problems in the field. If that dictates a different meter, we can do that, but I'll need a recommendation.

From Gene Wengert, forum technical advisor:
If the pin meter being used is a Delmhorst and has good batteries, calibration, etc., then I suspect that your meter is likely off, for whatever reason. As moisture is so critical, purchase a pin meter costing over $200 that you can use yourself to check any MC values that seem to be high. Also, check the core values using insulated needles. Note that 45% RH seems high as it is over 8.0% MC. Most homes are not that humid in the winter, so some shrinkage is likely when the product reaches the consumer. I do hear people say that at higher RH values, they notice fewer manufacturing defects, but what they are doing is delaying the shrinkage until the product reaches the customer. I think 35-38% RH is better.