Moisture Meter

I have a professional model for sale

Bill Tindall, E.Tn.

I have it for sale because I never had a use for it once we quit running a kiln. We sold lumber for years to all sorts of professional shops. I never saw a moisture meter in any of them. I build as much as most of you and I never saw a need. Personally I think they are no more than a security blanket for the insecure. I will qualify that statement some. If you are using lumber of unknown provenance, that is to say lumber that might have been properly kiln dried or lumber that is more or less air dried a meter would have a use in inexperienced hands. With experience you can feel shavings and know if lumber is dry enough.

If you are buying lumber whose origin is a typical manufacturer of hardwood lumber there is no need for a meter. These sources will have 100,000 bdft in a kiln and you can bet that all necessary steps are taken to ensure it is dried right. This lumber will never be unsuitable for use so long as it has been stored out of the weather. Richard tells us that in England they build from lumber at 12%. Lumber dried in the US to under 7 and brought back to 7 will never get to 12 % under any typical storage condition.

In all the years we sold lumber we only had one incidence of "wet" lumber. We bought some oak from a new source, that unknown to me was drying their product and new at it. We delivered a load that was unplaned or I would have caught it from the feel of the shavings. We made a follow up delivery a few days later and I noticed a wide board on the top of the previous delivery had cupped up(it was summer) because the top had dried some. We confiscated the lumber and replace it with lumber from a trusted supplier.

Re: Moisture Meter

Bill Howatt

I don't think you need a super fancy, expensive meter. You likely have a lot of dry stock in your shop that you can use as a reference for any moisture meter. If the unknown gives a higher reading then it isn't as dry as it could be.

I think you could gain a lot of good info on the meters listed on Amazon from the customer reviews.

Bill

Re: Moisture Meter

Larry Barrett

If you request use of your kitchen oven for a couple of days (and request is granted), and if you have access to a reasonably accurate scale that measures in ounces or grams, you can determine the moisture content (MC) of the wood you intend to use for panels in the bookcase (or for any wood in you shop).

Just take an off cut of the maple you plan to use, weigh it and record the weight. Call this W1. Then put it in your oven set a a low temp, say around 140 or less. Leave the oven door cracked open so moisture can escape. Weigh it again twice a day and record the weight each time. You will see that the weight drops fast the first couple of times, then slowly settles down. When there is no more change in the recorded weights, label this final weight W2. This is what is known as the oven dry weight. The sample has lost all the moisture it can, and is near zero MC. This process may take a couple of days, maybe longer depending on the size of the sample, so patience in using the oven this way is needed.

To determine the MC of the sample when it was first put in the oven, use this formula: (W1-W2)/W2. Multiply this by 100 to convert to percent.

For example, if the original weight was 1130 grams, and the oven dry weight was 920 grams, then (1130-920)/920 = .228. Multiply by 100. The original sample had a MC of 22.8%.

If this was a representative sample, then the rest of your stock is also at or close to 22% (or was when you started this experiment).

Chairmakers use a dedicated kiln to dry rungs and tenons. I used our oven for this for my first few chairs.

NHLA Reference Method

Bill Tindall, E.Tn.

What you describe is the National Hardwood Lumber Association Reference Method for determining moisture content of wood, except drying is at 220 F and there is no need to leave the door open.

Microwave is risky. I had a mineral pocket in a piece of poplar and it caught fire and smoked up the kitchen.

Oven Dry method, post and pics

John K Jordan

I use a toaster oven I keep in the shop. No need to heat the big oven in the house. The sample does not need to be large.

I wrote about this here, pictures and everything:

http://www.woodcentral.com/woodworking/forum/turning.pl/page/11/md/read/id/503549/sbj/moisture-content-by-oven-dry-method/

I used a scale bought from Amazon, very inexpensive but accurate.

https://www.amazon.com/gp/product/B002SC3LLS

It can take a long time to dry a sample. I monitor the temperature with a thermocouple reader. Gene Wengert recommends 215 to 217F. http://www.woodweb.com/knowledge_base/OvenDrying_Wood_for_Moisture.html

JKJ

Temperature observations

Larry Barrett

I did know (at least in general terms) that the NHLA used this method to determine MC. I think I read about it in Hoadley several years ago, but did not remember the recommended temperature. In thinking about it, if the only objective is to determine MC, which is what Joe is interested in for his bookcase panels and what John was interested in for turning, then a temperature high enough to remove moisture quickly but not so high that it damages the wood fibers would make sense. I suppose that 217 F is that value.

For chairmaking purposes, the main objective is to end up with bone-dry wood (rungs or tenons) with no checking or other damage. Determining the initial MC, or the MC of the remaining pile of wood from which the chair post will be made, is usually of secondary importance. My experience is that a temperature around 120 - 140F works for this purpose. A higher temp would be faster, but runs the risk of creating checks in the wood. If you are aware of any published studies along these lines, please let me know. I use a dedicated kiln for this purpose. It is essentially an insulated box with a heating element, a thermostat to control the temp, and a muffin fan to draw in room air and exhaust warm, moist air.

I do use a microwave for heating slats for the chair. I have found that with fairly thin slats, say 3/16" or so, 30 seconds in the microwave is sufficient to heat the slats to the point where you almost need gloves to handle them. They become very flexible and easy to pop into an assembled chair.

Re: Temperature observations

Bill Tindall, E.Tn.

120 - 140F works for this purpose

This temperature is in the ball park of where a commercial kiln would operate to accomplish what you are wanting to accomplish.

I don't know what bone dry is but for your purposes I would think that a moisture content of 5% +/- would suffice.

Re: Temperature observations

Larry Barrett

Thank you. Bone dry, as in dry as a bone, is shorthand for 'close to 0%', which is what you suggest.

ambient moisture

Jerry Gibbs

Curious why kiln dried wood would not assume the ambient moisture of storage, say in garage with no climate control?

reminds me...

Tony - Memphis

My grandfather and my great uncle worked their whole career at Kemp's furniture in Goldsboro, NC. When the place shut down, my uncle bought some stuff. One thing I inherited is the scale that they used to measure moisture content. Been meaning to do some research on how exactly it worked.

On a side note, a Kemp descendant has started back up with the Kemp name I believe.

Thanks for all the info guys.

KD wood and storage

Bill Tindall, E.Tn.

"Curious why kiln dried wood would not assume the ambient moisture of storage"

Wood will equilibrate to its surroundings +/-, what ever they are. The relevant question is whether commercially KD wood is likely to be stored in a location where it will not be ready to use without further drying or lengthy equilibration ( or needless fretting or needing a moisture meter to know what its moisture level is).

Data: A great deal of commercially prepared wood gets sold to distributors that store it in locations that are not climate controlled. This wood in turn gets sold to all sorts of places that use it without further worry. We purchased lumber that left the mill at 7% and came to us by truck, arriving at who knows what moisture content. We stored it in a shed that was under roof but otherwise not climate controlled. Not once in 30 years did we receive a complaint that the wood was too high a moisture content for whatever use our customers made of it.

Given all that is "known" about wood and moisture it is reasonable to ask why moisture content of commercially prepared KD wood is not something I claim the home shop needs to fret over. (I make no claims for boutique lumber operations and air drying)

Some more data: The published shrinkage tables lead one to believe we should be concerned with changes in wood dimension with moisture content. Without going into the details these tables were never meant to predict the dimension response of KD wood to humidity(moisture content). Anyone that has done measurements on KD wood knows that the tables vastly overstate the problem.

Other tables predict the moisture content of wood as a function of humidity. These tables were created to guide the drying process. Once wood is dry- in commercial kilns the wood is over-dried a bit- the wood never comes back to the moisture levels predicted from the tables. Graphically, the curve of moisture content vs humidity for KD wood lies well below the curve for wood being initially dried. The result is that storage of KD wood under most conditions will never result in wood above 12% moisture content. Except when it came to fitting drawers there is nothing I would do different if I was building from wood at 12% vs 8%. Wood will never get below 8% in my house and I don't ship furniture to Arizona.

That said, shape changes are something to fret over. Glue up a panel from 12% wood and lay it flat on the bench overnight at 40% humidity and it will be cupped in the morning. Slight shrinkage of one side will result in a measureable cup. This issue is something an experienced woodworker always accounts for and no moisture meter is necessary to warn of the potential problem.

Re: Temperature observations

William Duffield

I suppose it should be obvious that 215° to 217° F is just above the boiling point of water near sea level where we are. If we were in Denver, a few degrees lower would be just as effective.

https://en.wikipedia.org/wiki/High-altitude_cooking

bone-dry trivia

John K Jordan

>>>Bone dry, as in dry as a bone, is shorthand for 'close to 0%', which is what you suggest.

I asked a medical student. Living bones are wet, 10-30% water, depending.

I suspect bones left in the desert sun might get as dry as wood left in the desert sun.

JKJ

Re: Temperature observations

Larry Barrett

I thought about that, too, and wondered how that factored into the recommended temp. Water molecules on the surface won’t turn to steam any faster with a higher temp, but a higher temp might hasten the transition of interior molecules to the surface.

nothing to do with the boiling point of water

Bill Tindall, E.Tn.

In fact, probably decided by someone that thought it had something to do with the boiling point of water.

The effect of temperature is to lower the humidity surrounding the wood and increase, probably by a huge amount, the diffusion rate of water from inside the wood to the surface. This diffusion rate is very sensitive to temperature.