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Old 03-04-2008, 13:34   #1
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Moisture Meters

Curious if people have generally used, or not used, moisture meters in evaluation of boats they are looking to purchase.

From my brief reading on moisture meters is seems to quite the art form maybe more than a science. I guess on a basic level, knowing there is *more* moisture here than there would be of interest. Seems an actual reading would be hard to obtain/vary on so many differnt factors?

Did you use one on your pre-survey inspections (IE bought one)? Would you if had to do over again?

Did your survey include one?

Do they work when a boat has been on the hard for a month or longer?

Do they work when the boat is in a sub-zero environment (IE winter!!!).
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Old 03-04-2008, 13:41   #2
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A quality moisture meter is expensive and if you don't have a good understanding and some experience with it you will have little idea of what the reading mean and how to interpret them. Even in the hands of a surveyor that is not experienced with them they are pretty useless to a purchaser.
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Old 03-04-2008, 14:24   #3
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There are 2 main types of moisture meters. The ones with a couple of pins are most often based on measuring the electric resistance using some kind of alternating/pulsed current. This is pretty useless unless the water is at the gelcoat/paint surface, or else you want to puncture the surface.

The ones that measure using a flat surface are most often capacitive/HF based, that rely on the fact that water has a dielectric constant of around 81, air has 1, and dry wood/fibreglass is 3-5. Readings will vary depending on how close you are to the surface, but this sytem is generally the better one to use.

Do not attempt to use your standard multimeter resistance measurement as this is probably a Dc based system, and the reading will drop very quickly due to something known as polarisation.

Both systems have limitations, and unless the user knows what he is doing, can give erroneus readings. (if your paint has metal in it, they are useless)

The latest tool, is to use an Infra Red thermal vision camera, like they use to survey buildings or industrial stuff for hot/cold spots. Each temperature range comes out in a different colour.

You let the sun heat the boat, or else heat it up inside for a couple of hours, then take the pictures. Any areas containing water will show up, as the thermal transfer coefficient for water is different than the dry hull, so wet areas show up with a different temperature.

regards

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Old 03-04-2008, 14:29   #4
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Guessing the $200 buck jobber not that great of one?
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Old 03-04-2008, 14:31   #5
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Alan, Good information and I would bet the cost to get that test done on a good size boat would be about the cost of the boat again.
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Old 03-04-2008, 14:32   #6
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Quote:
Originally Posted by marty9876 View Post
... From my brief reading on moisture meters is seems to quite the art form maybe more than a science. I guess on a basic level, knowing there is *more* moisture here than there would be of interest. Seems an actual reading would be hard to obtain/vary on so many differnt factors? ...
EXACTLY RIGHT.
They can be fine for relative readings, even in inexperienced hands.
I've written (somewhere here on CF) about moisture meters, and their use - but don't have time to search it out right now.
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Old 03-04-2008, 16:07   #7
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Gord, Only problem with that also is that any metals in paints or on the other side of bulkheads and some other factors would show up as wet areas so if someone does not know what is happening they will assume water damage or problems when none exist. In additions hulls just out of the water will show moisture, so without experience how much is too much. And boats in the water all year round will have different readings than boats hauled out part of the year or that have been out of the water for a while. You need to be able to interpret those things.
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Old 03-04-2008, 16:37   #8
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Alan, Good information and I would bet the cost to get that test done on a good size boat would be about the cost of the boat again.
I heard a price of about 500 to 1000 USD from the UK. Some marine surveyors over there have started using this technology.

If you know what is on the inside of the hull, empty water tanks, then it apparently doesn't take that much of an expert to figure it out. Of course the boat has to be dry on the surface. Important is to heat only one side, either in or out, not both. A good time can be at dusk when the hull has been heated by the sun all day, as the air temperature drops, the wet areas will retain the heat alot better than the dry.

These cameras are available for around 5000 USD so not too bad for a surveyor to invest in. They look like a digital SLR camera, and take digital pictures, so cost of use is minimal. You look for temperature differences, not absolute values.
Your local electricity utility probably has one for finding hot/bad connections on high voltage power systems and switchgear. Lots of industries use them to check the power systems in factories for possible fire hazards from electric systems.

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Old 03-04-2008, 17:56   #9
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Have not encountered any surveyors here in the US using this technology or too many ready to put out over $5,000.00 for the equipment Add 500 to 1,000 dollars to a survey that will already cost you 400 or more dollars plus haul out fees, etc and you start getting into some serious bucks just to decide whether you want to buy a boat. The old fashion way still works quite well and there are those that know how to use a moisture meter as a tool in the survey process. Just don't see this catching on for the majority of us. I believe the original poster was looking for an inexpensive way to make some of the determinations.
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Old 04-04-2008, 03:13   #10
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For the inexpensive way, I would suggest the use of a capacitive/HF flat surface style unit. Run it over the surface, and check out the possible reason for any high readings.

If you can't find a reason, then run the area over with one of theose 20$ metal detectors used to find cables behind walls. Also see if you get the same level of reading on both the inside and outside, when you find a high reading.

So if there is no metal, and a high reading, then get suspicious. Common sense will take you the rest of the way.

Good luck

Alan
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Old 04-04-2008, 03:25   #11
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David Pascoe on Moisture Meters: Moisture Meters on Boat Hulls - Do They Produce Reliable Results?
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Old 04-04-2008, 03:57   #12
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Originally Posted by GordMay View Post

Thanks Gord, a good article.

The author however, doesn't have the full understanding of how a capacitance type meter works,
His statement "The principle here is that entrapped air space blocks the instrumentís signal in the very same way it does with ultra sound."
shows this.

A capacitance type moisture meter has 2 plates side by side but not connected. A high frequency voltage is applied to one of these. Depending on the relative dielectric constant of the material between these 2 plates, a current will be picked up by the second plate. In fact there is a "field" a bit like a magnetic field between these plates. So depending on the power used and the shape of the plates etc. there will be different "penetration depths" Like a magnetic field, if there is a good "conductor" the penetration will be lower.

His comparison to ultrasound is not correct.

Hope this clarifies it a bit.

Regards

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Old 04-04-2008, 04:41   #13
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Moisture Meters for Wood ~ by William L. James (USDA)
http://www.fpl.fs.fed.us/documnts/fplgtr/fplgtr06.pdf

Moisture Meters ~ by Captain Wallace Gouk (Port Credit Marine Surveys)
moisture meters
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Old 04-04-2008, 09:37   #14
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Thermal Image

I was involved with a survey that used the technology in question Thermal imaging. The surveyor used a heat device to warm up the hull of the boat after it had dried on blocks for 24 hours. The technology gave real time images that where saved onto a lap top. When viewing it was very obvious if there where wet areas. This did not come cheap at $45.00 per foot or $1800.00 for the Olson 40.

This all came about because of the use of a moisture meter. The hull of the Olson 40's are balsa core. The first survey made a note that there was the possibility of a wet core due to a high moisture content. This turned out to be totally incorrect, the only moisture found was in the fairing compound around the hull keel joint.

Seems to me (my opinion) is that the moisture metering should be used in places like on deck to test moisture under teak or other such places. If the boats hull is solid laminate not cored what is the value of a moisture meter a experienced surveyor will sound for trouble. A cored boat is a crap shoot and I think the thermal technology will be the most reliable indicator of a wet hull. A boat sitting in water year after year will absorb water, with epoxy to a lesser extent. So if you are buying a cored hull it may be wet. Thats where the problems begin.

Jack
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Old 04-04-2008, 10:09   #15
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thermal imaging

I'm surprised to hear a cost of $1800 to thermal image a boat. I just sold my house, and the guys that did the $300 home inspection for the buyer were using a thermal imaging camera to look for bad spots in the insulation, wet roof, electrical hotspots, etc.
If a $5000 capital investment can yield me $1800 a day, I think I just found myself a new business!!!
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