MD918 Moisture meter

Crookedkut · April 20, 2019, 02:01:35 AM

I just purchased an MD918 moisture meter. This meter had density levels for different types of wood. I work mainly with mesquite and oak. Oak is on the chart that came with the meter but mesquite is not on the list. What level of wood density should I use when checking for moisture in mesquite wood

GeneWengert-WoodDoc · April 20, 2019, 08:41:32 AM

This meter is confusing. Their instructions say

Optional 10 different grades of timber density. 4%~80% Rh. Maximum Error (1% Rh + 0.5)

I am confused that they talk about the Rh range throughout their literature and 1% Rh accuracy, but not the wood MC range. If they can measure to a maximum error of 1% RH (whatever the +0.5 means, I am not sure), and because 5% RH is about 1% MC, their meter is over 5 times more accurate than any other pinless meter.

I am not sure what units they are using to measure "timber density"...grams per cubic centimeter, pounds per cubic foot, or maybe specific gravity (which are the widely published units). Do they want the green density or 12% MC density, as these are the two standard values? As an example, red oak density green is 35 pcf and 12% MC is 39 In terms of metric, this is 0.56 and 0.63 grams per cubic cm.

Texas A&M says that mesquite averages 45 pounds per cubic foot. I do not know for certain but I believe this is green and not 12% MC. Mesquite is close to the density of mockernut hickory and a bit more dense than southern red oak and pecan hickory. They also state that the density has a wide range, sometimes as low as 39.9 pounds per cubic foot and as high as 61.6 pounds per cubic foot. This wide variation is unusual for other wood species. It does mean you will have some uncertainty in you MC readings with mesquite as density has a big effect of the pinless meter.

Crookedkut · April 20, 2019, 09:06:35 AM

WoodDoc, thank you for the reply. You are correct, it is confusing to read. I figured I would just read the directions later and try to understand it but I guess not. I really did not want to spend $200 for a moisture meter right now but I will eventually. I was trying to get something not too pricey but still dependable.

Tom the Sawyer · April 20, 2019, 11:22:37 AM

Quote from: GeneWengert-WoodDoc on April 20, 2019, 08:41:32 AMAs an example, red oak density green is 35 pcf and 12% MC is 39 In terms of metric, this is 0.56 and 0.63 grams per cubic cm.

Gene, does that mean that green oak weighs less than oak at EMC of 12% ?

GeneWengert-WoodDoc · April 20, 2019, 05:10:35 PM

No, but the wood shrinks as it dries and that increases the Specific Gravity. To convert SG to pounds per cubic foot, you need to know the MC. So, the SG at 30% MC is the same as at 75% MC (0.56). But when calculating the density at 30% MC, it is 45.4 pounds per cubic foot, and at 75% MC it is 60.8 pounds per cubic foot. Now, if the wood is at 12% MC with the SG = .63, then the density is 46.8 pounds per cubic foot. Under 30% MC, there is a loss of water decreasing density, but there is also shrinkage that increases density. In the case of oak, the US Forest Service SG was based on different samples which can also confuse the issue slightly.

To confuse things a bit more, the US Forest Service uses the oven-dry weight and the volume when green or when at 12%MC.

GeneWengert-WoodDoc · April 20, 2019, 05:20:20 PM

You will read that you need to recalibrate whenever you change batteries...not sure why.

All is not lost...

So, here is what you can do...get a small piece of typical mesquite lumber (maybe 10" long) that is close to the target MC you want. Then have someone measure the MC at 1/4 of the depth with a pin meter using insulated needles. Then measure THE SAME SPOT with your meter and try different DENSITY settings until you get agreement. Remember the setting. Now wrap the sample in plastic wrap real well with three wraps, and then store it in the refrigerator. Pull it out, let it warm up, and unwrap to get a meter check...you may find that you do not even have to unwrap it if the wrapping is tight. With the plastic wrap, you know that the MC cannot change for years. When you measure the MC, your meter must have an air gap under the lumber, not a table, more wood, etc.