All about moisture meters
January 5, 2023 | 3:46 pm CST
22WPELan gene-wengert DB

Q: We use a moisture meter with pins to check the MC of our incoming lumber. If the temperature is under 32F, does the water freeze and the meter not work? If it does not work, how do we check incoming lumber to make sure the supplier has furnished the correct MC?

A: The moisture of lumber under 30% MC has no liquid water, so the water cannot freeze. (Technically, the water is actually held, molecule by molecule, to the cellulose or hemicellulose, using hydrogen bonds.) As a result, you can use the pin meter on dry lumber that is under 32F. 

The only issue is the temperature correction that must be applied, as the original data is based on 70F.  If your meter does not go under 32 F with an automatic correction, then set the temperature at 70F and then add 1% MC for every 20F under 70 F. A species correction is also a good idea. 

Of course, the meter is only an estimate based on measuring resistance between the two uninsulated pins, or between the tips of insulated pins.

Also, when trying to obtain the average MC, use insulated needles driven to a depth of about 1/4 of the lumber’s thickness. The core moisture at 1/2 depth could also be measured, but most operations dry lumber based on the average and not the core. Nevertheless, good drying operations use an equalization step at the end of drying to minimize shell and core MC variation.

I have a suggestion you could consider. Use a pinless meter to check the MC, as it is quicker. you can also quickly check several spots on an individual piece of lumber, if desired. However, any wet readings would then have to be double-checked with the pin meter. 

The pinless does give a higher MC reading when the wood density is higher than average, which means false “wet” readings. The pinless meter requires air underneath the piece being measured.

Neither meter is highly accurate if the lumber has been subject to moisture gain, so the surface is a lot wetter than the rest of the piece. In your case, a truck driving through foggy, rainey weather could potentially result in false “wet” readings.

One word of caution. Use the same brand of moisture meter that your supplier also uses. 

Different brands, especially foreign made meters, can create incorrect readings.

One word of praise. About 75% of the “problems” that I am involved with as a consultant are moisture related. By ensuring the correct MC, you will have a better product.

Q: We use the pinless meter because it is so fast and allows scanning of any spot on a board. But, we tend to have the MC readings being slightly higher than with a pin-type meter and the oven test. Can you explain this?

A: The pinless meter is based on measuring the dielectric property of wood and then relating that to MC. (The pin meter measures resistance.). The dielectric properties of wood vary with density, so a denser piece of oak will read higher in MC than a lighter weight piece, even though the two have identical MC. 

Further, because there are maybe 30 different species of oak that are lumped into two groups — red oak and white oak — the density varies quite a bit within these two groups. 

Finally, as you already know, wood shrinks as it dries, which means the density of wood increases perhaps 10% to 15%, from green to dry. 

So, with changing density naturally within the tree and with wood depending on MC, there is some uncertainty on the correct density setting with the pinless meter. Should you use 0.56 for red oak or 0.57? This small difference can make at least a 1/4% MC difference.

It is important to appreciate this effect of density. Although the tree density variation is hard to correct for, we can “custom calibrate” the pinless meter. 

First, take a small, flat, kiln-dried piece of lumber of the thickness and species that you normally use. 

Determine the MC by a) using a small adjacent piece of wood and get the MC using the standard oven test; b) if the MC is 7.0% MC or higher when using insulated pins driven 1/4 of the lumber’s thickness, with appropriate species and temperature corrections, make the assumption that the pin meter is 100% accurate. (It should be with plus or minus 1/2% MC almost all the time.). 

Then adjust the pinless meter’s density setting so that the pinless meter agrees with the oven test or the pin meter. Repeat this test for different loads of lumber and use an average value for the pinless meter. 

Overall, this correction should work for a year or more.

Incidentally, this variation you note is one reason that anyone buying and/or selling lumber should agree in advance the type of meter, the manufacturer, and perhaps even the model number. I have seen many foreign made meters with large variations, so this is one case where “Buy American” is prudent.

One other comment. The pin meter readings under 6/5% MC cannot be trusted as the wood could be much drier. 
The pinless meter, however, goes to 4.0% MC. Over-dried lumber, under 6.5% MC, can be as troublesome as wet lumber.

Q: We have learned over many months of careful examination of defects in manufacturing and in returns that moisture content is a critical issue. What about an inline moisture detector and where in the production line should it go? Thanks in advance.

A: Indeed, I agree that incorrect moisture content of incoming lumber can cause many avoidable issues with cabinets and furniture, as well as millwork and flooring. This column in December 2021 discussed some technical details about moisture content (MC).

There are some basic questions to ask yourself.

METER LOCATION. If you detect wet lumber, too dry lumber, what will you do with it? 

If you expect to return it, remember that if you begin to process the wood, returns will be difficult. So, the inline meter has to be at the unstacker. The meter can detect individual pieces and mark them with ink so you can sort these pieces out.

If you plan to accumulate the wetter pieces and then dry them for a few days to obtain the correct MC, again you would want to sort them out before planing. Drying planed lumber just has too many pitfalls.

COST TO BENEFIT. The inline meter is expensive. However, a large operation will reduce defects and call-back repairs, so they can easily justify the expenses. 

Further, because you measure every piece, you know the moisture content at the time of processing, which will help should someone file a complaint. It is surprising how much damage can result from poor storage prior to sale or installation.

IMPROVE MC. The inline meter, by measuring every piece, allows you to determine the average MC of a load or even individual packages, of lumber. In addition, the meter will tabulate the variation of moisture in a load or package. 

You can analyze how your kiln operation is doing, or you can analyze how an individual supplier is doing. A key point is that you do not want to attack a supplier, but rather would work with the supplier that has poor moisture to improve their operation. 

It is surprising how many kilns operations do not consider carefully the needs of their individual customers; they can change if they know what is not acceptable.

CALIBRATION. The common inline meter is measuring an electrical property of the lumber as it goes through an electric field. When you set up the meter for a run, you need to tell the meter what the density of the wood is. The meter and its computer then figure out how much disturbance the electric field has due to the wood and knows that additional disturbance is due to moisture. 

The good news is that the temperature of the wood is not an issue with the meter, but wood density differences do create an “error” that must be appreciated.

Any wet pieces should be double checked with a pin-type moisture meter, as they could actually have more wood (higher density) than more water. Also, there is a natural variation in wood density from species to species, so it is necessary to recalibrate for each species.

My suggestion is to assume that a pin meter with insulated needles drive 1/4 of the lumber’s thickness gives an accurate reading — an assumption that is very close to the truth. Maybe measure 10 pieces and then send the pieces through the inline pinless meter and teach the inline meter what reading gives what MC. You might also check calibration for 4/4 versus 6/4 to 8/4. Most of the time, if the meter probes are kept clean, this initial calibration will be good for years unless the wood changes. 

Appreciate that the pin meter also measures an electrical property which is related to MC, so meters give an estimate. The pin meter is not sensitive to species or thickness, but the wood temperature is a factor.

CASE HISTORY. I had one client making doors. In the wintertime, they average about 100 recalls. They put in an inline meter and were able to reduce the wintertime recall to three. They then had me work with their main supplier that had kilns to improve their kiln drying.

BRAND. There are several inline meters in use. My suggestion is to ask around and then visit or talk in detail to the correct user to find out how it works, improvements, maintenance, and so on. I have seen some meters that were inexpensive and junk. The meter manufacturer should be able to give you some referrals.

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About the author
Gene Wengert

Gene Wengert, “The Wood Doctor” has been training people in efficient use of wood for 35 years. He is extension specialist emeritus at the University of Wisconsin-Madison.