DIYbanter - OT Wood moisture meter, can I make one?

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Got a microwave and a scale?
The old method was to dry a sample in a tin can stove above a light
bulb, the new method uses a microwave oven.
weigh the sample before and after drying and compare weights to get a
good guesstimate of moisture content.

OT Wood moisture meter, can I make one?

I am in the market for a wood moisture meter to check the wood I have in
storage before I make them into furniture. The ones I have seen for
sale seem to be expensive for what they are. Could I make my own by
making a little handle that holds the two steel pins, and reading the
moisture by using an an ohm meter? I have access to a friend's moisture
meter to calibrate my homemade one with. I was thinking of making a
rough table in case the correlation between moisture and resistance is
not linear.

What do you guys think about this plan (other than that I am too goddamn
cheap).

Hey Stan,

If it is just an ohmmeter, then you should be OK. I do have a friend
with a "moisture meter" that is NOT an ohmmeter though. His is for
checking for moisture in marine hulls, and specifically fibreglassed
layups, looking for what is called osmosis delaminating. I understood
it to be "sonic", as you can read right through the plastic gel coat.
or even a sheet of dry something (paper, plastic, dry wood).

Take care.

Brian Lawson,
Bothwell, Ontario.
XXXXXXXXXXXXXXXXx

On Fri, 18 Mar 2005 19:23:30 -0600, stanley baer
wrote:

I am in the market for a wood moisture meter to check the wood I have in
storage before I make them into furniture. The ones I have seen for
sale seem to be expensive for what they are. Could I make my own by
making a little handle that holds the two steel pins, and reading the
moisture by using an an ohm meter? I have access to a friend's moisture
meter to calibrate my homemade one with. I was thinking of making a
rough table in case the correlation between moisture and resistance is
not linear.

What do you guys think about this plan (other than that I am too goddamn
cheap).

Certainly worth a shot. There is some possibility that a wood moisture
meter uses AC to measure the resistance. You might use your multimeter
to find out how your friends meter works. You could easily make an AC
resistance meter using a wall wart transformer , a resister, and your
multimeter.

Dan

"stanley baer" wrote: (clip) Could I make my own by making a little handle
that holds the two steel pins, and reading the moisture by using an an ohm
meter?(clip)
^^^^^^^^^^^^^^^^^
I tried it once, and it didn't work well at all. The needle didn't settle
down to a fixed reading. This may be due to the fact that ohmmeters use
DC--something is changing due to the flow of current, possibly formation of
gas at the wood/metal interfaces. Or there may be voltages generated by the
fluid in the wood acting on the metal probe. Or both.

On Fri, 18 Mar 2005 19:23:30 -0600, stanley baer
wrote:

I am in the market for a wood moisture meter to check the wood I have in
storage before I make them into furniture. The ones I have seen for
sale seem to be expensive for what they are. Could I make my own by
making a little handle that holds the two steel pins, and reading the
moisture by using an an ohm meter? I have access to a friend's moisture
meter to calibrate my homemade one with. I was thinking of making a
rough table in case the correlation between moisture and resistance is
not linear.

What do you guys think about this plan (other than that I am too goddamn
cheap).

The Fine Woodworking mag had a plan in it for making one, if you are
into electronics. IIRC there was an error in the original article,
corrected the following month. It has also been reprinted in one of
the FW books.
I believe the resistance of wood is higher than an ordinary multimeter
will handle.
I have a FW one, that someone else made. It didn't work for me and I
never got around tuit to fix it. I came across a "real" one in a
second hand shop and bought that instead. Must do something with the
old one...
Geoff
New Zealand

Hi Stanley,
I doubt that a normal ohm meter can read moisture levels
in timber reliably. There is more to it than just resistance. What you need
to do is sort out how much of the resistance is due to moisture, and how
much is due to the material and structure in the wood itself.

I don't know what principle the commercial moisture meters aer working on,
but an instrument that I have used to measure the moisture content in high
voltage busbar insulation is called dispersion meter. It was originally
developed to measure moisture content during sugar processing. It works on
the principle that some molecules polarise when placed in a DC electric
field, and some don't.

Moisture molecules do, and most other things in wood (which was one of the
main insulation structures in most of our 11,000 Volt busbars) doesn't. When
I say polarise, I simply mean that when the electric field is applied, they
all turn and line up in the same direction in the field. This turning around
and lining up takes time, and energy. When the field is removed, they go
back to where they were, but that takes time as well.

The way the dispersion meter worked, it applied the dc field to the sample
for a minute or so, then it shorted the sample out momentarily to discharge
it.It then removed the short, and applied the sample to a sensitive volt
meter so that you could see the voltage rising again. The voltage was rising
again because as the molecules returned to their normal state, they released
the energy that polarised them, causing the voltage to be generated. The
instrument would continue to short and measure until the voltage recovery
dropped away. The combination of how high the voltage rose and how long it
took to completely discharge was used to derive the moisture content.

Having said all that, the first response I saw to this thread of using the
microwave is an excellent one, provided the moisture is given time to fully
escape from the sample. In your situation, you can use a sample. We needed
the dispersion meter because we couldn't get a sample.

regards,

John

"stanley baer" wrote in message
...
I am in the market for a wood moisture meter to check the wood I have in
storage before I make them into furniture. The ones I have seen for sale
seem to be expensive for what they are. Could I make my own by making a
little handle that holds the two steel pins, and reading the moisture by
using an an ohm meter? I have access to a friend's moisture meter to
calibrate my homemade one with. I was thinking of making a rough table in
case the correlation between moisture and resistance is not linear.

What do you guys think about this plan (other than that I am too goddamn
cheap).

On Fri, 18 Mar 2005 19:23:30 -0600, stanley baer
wrote:

I am in the market for a wood moisture meter to check the wood I have in
storage before I make them into furniture. The ones I have seen for
sale seem to be expensive for what they are. Could I make my own by
making a little handle that holds the two steel pins, and reading the
moisture by using an an ohm meter? I have access to a friend's moisture
meter to calibrate my homemade one with. I was thinking of making a
rough table in case the correlation between moisture and resistance is
not linear.

What do you guys think about this plan (other than that I am too goddamn
cheap).

I think if you find a moisture meter an interesting project, go for it.
If you want to start in on your wood instead, measure the moisture
content by weighing a piece, baking it in the oven until the weight
stops changing, and weighing again.

Steve

geoff m wrote:

The Fine Woodworking mag had a plan in it for making one,
I have a FW one, that someone else made. It didn't work for me

The design that was published in FW simply won't work. What was
published was a circuit that primarily measured the input bias current
of the opamp used. The needle will wave around, but the moisture content
of the test piece won't have a lot to do with it.

I do see that Horrible Freight is now selling pin type moisture meters
for 25 bucks. The ad says "highly accurate". They wouldn't lie.

Kevin Gallimore

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Don Foreman wrote:
See
http://www.fpl.fs.fed.us/documnts/fplgtr/fplgtr06.pdf
=20

Thanks for the link. Really helpful. Had some of the same questions.

--=20
Will R.
Jewel Boxes and Wood Art
http://woodwork.pmccl.com
The power of accurate observation is commonly called cynicism by those=20
who have not got it.=94 George Bernard Shaw

This document is very informative. Since I don't need super precision,
the table that charts resistance vs moisture content for several species
of wood should do the trick for me. Thanks for pointing me in the right
direction again Don.

stan