Thread: Acceptable moisture content

Originally Posted by Bodgy

Just got a moisture meter, the sort with the two prongs at the end.
I tried it out and was a bit surprised by the small range of readings.
For example, some freshly felled and milled tallow wood I've been soaking in the water butt reads 34% and some well dried, years old black butt reads 12%.

This may be a bit simplistic but at what water content can one safely start to use the timber? Without checks , warps and splits that is.

That depends on what the timber is and the location you are using it.
12% is a common equilibrium moisture content for air dried timber - ie the natural moisture content of dried wood - and one at which you can expect to work on wood. In more humid environments it might be a little more while in drier climes it will be less.

BTW your example of the tallow under water is not a good comparison (I presumed you dried the outside first?). It sounds crazy but storing wood under water can actually remove water from the wood - my guess this is because the water in the container leaches out stuff from the tree and the wood cells shrink so the water content drops. This is actually a good way to dry rounds or cookies to reduce splitting!

If the timber is susceptible to movement it should be stored and worked on/under similar humidity conditions in which it is going to be used. Our builder placed our jarrah floor boards in the rooms for 3 weeks before he put them down.

As far as MC meters (MCMc) go there are MCMs and there are toys and stuff in between. Different types of timbers with the same MC will register the same reading on MCMs that use resistivity as measured by physical metallic probes. The MCMs that use electromagnetic scanning are generally more accurate, but all MCMs need calibration if you wish to know the MC absolutely or accurately. Some scanning MCMs have a set of switches to recalibrate the meter for different timbers but I have not seen one for a decent range of Aussie timbers. They typically have a switche to account for varying density but I note the most expensive Carbatec MC meter only goes to 0.95 g/cc which means a large number of Aussie hardwoods are not covered

A metal probe type MCM also has to be used on a freshly cut surface but ideally one should take a piece of wood and weigh it green and then put it through a series of drying steps eg 1 hour in an oven, and then measure the MC as per the meter and weigh the wood at each stage, repeat this step until there is no change. Then back calculate the absolute MC and compare that to the measured MC and then plot a calibration chart. This needs to be done for every different type of timber you work with.

I also have a probe type MCMs and it is OK for comparative measurements on the same timber ie this piece of timber is drier than this one, but otherwise calibration and fresh surface are essential in making an absolute measurement with probe meters.

Well bodgy, there ya go, no point reading down here

Just a thing of track, where is Turramurra near? My house at high school was Turramurra, never seen/heard of it since

Sigidi

Turramarra is on Sydney's Nth Shore, about 15k nth of the CBD adjacent to Pymble and Wahroongah 4k south of Hornsby.

We are quite high here. Rumour has it that the name derives from the Aboriginal for Big Hill.

BobL

Thanks, I think you about cover it.

I quicken up the drying process by turning green timber and then soaking in water for a few weeks then drying in the open in an onion sack.

My MM is in the toy category. Cost $10 on Ebay, but it is better than nothing and from what you say it is fairly accurate.

Originally Posted by Bodgy

My MM is in the toy category. Cost $10 on Ebay, but it is better than nothing and from what you say it is fairly accurate.

Most meters, even the cheapest ones are usually are pre caibrated to be reasonably accurate around the 8 - 15% MC mark. Above and below this range is where they fall off in accuracy, my understanding is they will read very high moisture contents lower and very low moisture contents higher. The extent to which they do this varies for different timbers

They will all provide relative readings in the same timber so you can compare two pieces of the same timber.

Bob, I've been fiddling with my carbatec sourced MCM (non intrusive) and found a way to estimate the MC of some of the greybox I've got stack around here.
I've found that when the Meter is pressed against a suitable piece of timber with the switch set on 1 and the switch is slowly progressed to position 9, the readings on the meter increase equally for each postion. To calculate the MC of a timber that is 1200kg/m3 (1.20g/cc) you add the equivalent amounts of value per switch increments as if there were, in this case, 3 extra switch positions.
Hope that makes sense.
Although this is not 100% accurate it still gives a pretty good estimate.

If that doesn't make sense I'll give some figures.....if I can find them.

Originally Posted by WillBrook

Bob, I've been fiddling with my carbatec sourced MCM (non intrusive) and found a way to estimate the MC of some of the greybox I've got stack around here.
I've found that when the Meter is pressed against a suitable piece of timber with the switch set on 1 and the switch is slowly progressed to position 9, the readings on the meter increase equally for each postion. To calculate the MC of a timber that is 1200kg/m3 (1.20g/cc) you add the equivalent amounts of value per switch increments as if there were, in this case, 3 extra switch positions.
Hope that makes sense.
Although this is not 100% accurate it still gives a pretty good estimate.

If that doesn't make sense I'll give some figures.....if I can find them.

Yes it does make sense - ingenious - well done! How much of a difference in MC is 3 switch positions?

Turramurra eh, actually its next to Warrawee. Its changed a bit since I grew up there

Originally Posted by BobL

Yes it does make sense - ingenious - well done! How much of a difference in MC is 3 switch positions?

BobL,
well it would be three times the increment between each of the other switch positions eg if the reading changed 4% between switch positions 5-6 (position 6 was for 950kg/m3) and the timber had a ADD of 1250kg/m3 then the approximate MC would be different by 12%.......I think???? I'll get some figures tomorrow and let you know mate.

Originally Posted by WillBrook

BobL,
well it would be three times the increment between each of the other switch positions eg if the reading changed 4% between switch positions 5-6 (position 6 was for 950kg/m3) and the timber had a ADD of 1250kg/m3 then the approximate MC would be different by 12%.......I think???? I'll get some figures tomorrow and let you know mate.

I think it would be more like this;

if the 950 kg/m3 reading was say 12% and the 650 kg/m3 was 10% then 1250 kg/m3 is probably 14%? I doubt the trend is linear and there is probably a function involved.

It would be good if you could find a bit of green red gum and post the readings for all the densities and then I can work out the function.

Cheers

But ya gotta love oven dry testing....

Are you measuring with the grain or across it? Those "toy" moisture meters are going to be very sensitive to simple things such as that. In fact, all pin-type meters are affected by that. If you really want an accurate reading, you'll probably have to cough up the extra dough for something like Wagner's L609 moisture meter. That basically does a three-dimensional scan of the area beneath it and averages the results, which will both eliminate the grain problems and depth problems. One thing I never knew until recently is that it also has a cancellation circuit so surface moisture isn't a problem.

I understand a hobbyist who only would use a meter once ever two years not wanting to pay for a quality item. What do you plan on using this for in the future? You may need to upgrade your equipment.

Good luck in all your projects.

Originally Posted by realitybend

Are you measuring with the grain or across it? Those "toy" moisture meters are going to be very sensitive to simple things such as that.

I understand a hobbyist who only would use a meter once ever two years not wanting to pay for a quality item. What do you plan on using this for in the future? You may need to upgrade your equipment.

Good luck in all your projects.

I basically stick the pins in all surfaces of the wood, generally disregard any outer sap wood readings, then average the lot.

I don't place any great reliance on the exact reading but it allows me to compare between readings from what I know is aged dried timber and more recently felled stuff

Whilst I could be described as a hobbyist, I like to use timber that I've sourced and milled myself. I've got by without a MM until I recently started rough turning green timber and shoving it in a water vat for a while, then air drying.

I trying to work up a time regime based on what I've read then interpolating re size/ surface area/time immersed/time drying to get down to < 15% moisture.

I know there are a 100 other variables, but for me I'm afraid close enough is generally good enough.

I'm not predominately a wood turner, but I'm trying to teach myself ATM.

Thanks for all the help.

G'day. Wagner L612, Delmhorst J2000 with slide hammer.
These are the tool of the trade for serious moisture detection.

Originally Posted by BobL

I think it would be more like this;

if the 950 kg/m3 reading was say 12% and the 650 kg/m3 was 10% then 1250 kg/m3 is probably 14%? I doubt the trend is linear and there is probably a function involved.

It would be good if you could find a bit of green red gum and post the readings for all the densities and then I can work out the function.

Cheers

Hey Bob, I've got some figures although they're not for green redgum. They're for somewhat dry greybox. The following groups of figures are the corresponding readings from the same timber with the meter in held in the same position and the 6 position switch starting in the 0.45g/cm3 and moving through to the 0.95g/cm3.
Greybox (1.12g/cm3) .......0.45g/cm3 - 21.4%, 0.55g/cm3 - 20.0%, 0.65g/cm3 - 18.7%, 0.75g/cm3 - 17.2%, 0.85g/cm3 - 15.3%, 0.95g/cm3 - 13.5

The reason I didn't include Redgum is that the meter checks timber up to 0.95g/cm3 and according to the Keith Bootle Wood book Redgum's ADD is .9g/cm3.

Hope this all make sense