Moisture testing with a multi meter.

Discussion in 'All Things Boats & Boating' started by Frosty, Jan 17, 2011.

  1. Frosty

    Frosty Previous Member

    There are one or 2 places on the web that tells you that a moisture meter simply passes current through a material and measure the resistance. It will vary for many reasons and is inacurate.

    The simple ubiquitous multi meter can aslo read a resistance, it wont give you a reading in moisture content in % or have little led's light up but it will let you know if your balsa core is wet or not using long probes (old bicycle spokes) to get a rough idea.

    Wet is wet and dry is dry and that maybe all you need to know.

    Set on 20M I get a reading of under 1 on a damp towel, and that is a yard stick I am using to keep an eye on some balsa I am drying.

    At what reading would you think Im ready for glassing over?
  2. CatBuilder

    CatBuilder Previous Member

    This measurement will depend on the distance the probes are separated, so you will need to be accurate with that distance using your damp towel and the balsa.

    What you're after is the resistivity of wood, in Ohms. Here is a handy graph you can use, which covers a range of woods (they all lie between the two curves). So you're getting what... about 1 mega Ohm?

    Roughly it looks like you're in a 14%-22% moisture range, based on the graph, which is not for balsa, but for a variety of woods. Triple that resistance would be better. If want to do a really good job, look for about 4MOhm. Resistance of wood also changes based on temperature, but I doubt any cheap off the shelf store probes bother account for this. It also changes if you measure along the grain or across it (as you are doing with endgrain balsa).

    Also, I believe the probe separation distance is 1cm for this graph.


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  3. Frosty

    Frosty Previous Member

    Thats a great bit of information thanks. Im using roughly a 1 inch gap between the probes.

    I was expecting an outcry of "you cant do that" I guess a lot of people here do it this way too.
  4. CDK
    Joined: Aug 2007
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    CDK retired engineer

    Stick two nickel plated needles, pins or nails in the wood and measure the resistance that way. You want to know the humidity in the wood, not just at the surface.
    Leave them in the wood and check the resistance from time to time until you get a reading of >5 Meg ohms, then the wood is dry enough for your purposes.
  5. Frosty

    Frosty Previous Member

    Its actually foam core, I am using 2 old bicycle spokes sharpened and pushing in about 3 to 4 inches. In some places I am getting infinity and in one spot I am getting less that 1 so I have dug that bit out and it is drying better. I did learn quickly to not hold the spokes as I took a measurement.

    It really does work well actually now Ive got used to it.
  6. cthippo
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    cthippo Senior Member

    Can you post a "how to" article, maybe we can include it in the wiki.
  7. Frosty

    Frosty Previous Member

    I have noticed that any mold growth is cunductive, therefore any mould on the surface will conduct. In my case this was foam and although very dry did still read low untill I tested the foam from the opposite side that did not have mold growth.
  8. michael pierzga
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    michael pierzga Senior Member

  9. DianneB
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    DianneB Junior Member

    What you will read on an Ohmmeter is dependent on a large number of factors aside from the moisture itself:

    1 - the area of the probe in contact with the material
    2- the pressure with which the probe is in contact with the material
    3 - the type of probe material (whether galvanic current is being created)
    4 - contaminants present (effecting conductivity of the water)

    That is why, for laboratory quality moisture measurements, the radiation absorption method is used. Resistive measurements are okay for "dry" or "sort of wet" assessments but anyone buying/selling commodities will be using absorption.
  10. WickedGood

    WickedGood Guest

    What you need is called a MegOhm Meter. Not a Ohm Meter.

    you can not get a high enough voltage from a regular old Ohm meter to calculate the insulation conductivity and derive any usefull info.

    So. Now that you know this. How do you plan on drying out a core of Balsa that is sanwiched between fiberglass?
  11. michael pierzga
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    michael pierzga Senior Member

    Im sure your correct Dianne. I would think that with a home made multimeter setup you would not be actually measuring the moisture content, but rather observing a resistance to compare against a piece of scientifically measured timber . Could be handy around a small shop . Normally I only have my eyes and the behavior , smell of the saw blade as a guide to moisture content.

    Oh and Wicked the shipyard is a waterlogged balsa cored superyacht...40meters......BIG problem. Id mention the builder but lawsuites might fly.
  12. Fanie
    Joined: Oct 2007
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    Fanie Fanie

    You can easily use a 1.5V battery in series with the multimeter to make it a lot more sensitive.

    There is something else you have to consider Frosty. The process making the wood will have a certain acidy or alcaline or conductive chemical content in the wood. The consentration of this will affect the reading.

    Pure clean mineral salt free water does not conduct electricity, neither does a completely dry chunk of salt. It is the combination of the two that causes conductivity.

    So I guess if you know the content of the drying out material you could get a better idea of how much moisture is in it.

    The instrument used to measure resistance with is called an ohm meter. The range you measure can span from micro ohm to mega ohm, it is still an ohm meter. The low ohm scales you use when the wood is still floating, then kohm when the wood rinsed up on the beach. When Frosty found it it was a week later in the sun it was a couple of hundred kohm. He's in the meg ohms now.

    Heat Frosty, heat.

  13. Frosty

    Frosty Previous Member

    Yes Im in the mega ohm now as it dries, just to clear a point the damage has been cut out and I can access under the fibre glass with long probes made from bicycle spokes inserted up to 4 inches at approx 1 cm.

    I have only 1 small area that is not reading infinity now, so I have tried a hair dryer and I think I will be sealing up today.

    Thats good info there Micheal from Spain.
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