Moisture vs Resistance

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Lenp
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Moisture vs Resistance

Post by Lenp » Sat Apr 30, 2011 3:54 am

I recently had a flooded shop/office. It will be survivable with minimal damage but does raise a question.

The company that did the water removal and dry out used commercial dehumidifiers and fans to quickly remove the moisture. They had a moisture tester to monitor the process, a hand held meter with two needles that obviously read the resistance of various materials. They probed drywall, wood behind baseboards and carpet edges. Several times they visited to test the moisture at various places until they were satisfied that the level was low enough to remove the expensive rental equipment.

So rises the question...
Does anyone have a feel for the moisture/resistance relationship? I don't think distilled water would be valid for any comparison since flood water is contaminated with anything imaginable. Is this really a definable measurement or is it a relative decision. I think moisture meters for woodworking are more understandable since it can probably be assumed that the moisture would be absorbed atmospheric humidity rather than contaminated fluids.

As a side note, they used 3 dehumidifiers, for 3 days. Each can remove 125+ pints a day and rent for $100 per day each excluding labor. The internet cost for one of these dehumidifiers is only $2K. The carped drying fans, 10 of them went for $10/day with a cost of around $200 Ea. The crew said they just move the equipment from job to job, so they don't need a big shop for storage!

Guys, we are in the wrong business! What capital investment equipment do we have that pays itself off in about 20 days?

Len
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Re: Moisture vs Resistance

Post by dyarker » Sat Apr 30, 2011 5:10 am

... I don't think distilled water would be valid for any comparison since flood water is contaminated with anything imaginable. ...
No, it wouldn't; distilled water is an insulator. :mrgreen:
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MrAl
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Re: Moisture vs Resistance

Post by MrAl » Sat Apr 30, 2011 10:47 am

Hi Len,

There are equations and constants for different materials, but the main thing i think (im not an expert on this) they look for is a decrease in slope rather than an absolute value measurement. There are a lot of factors which could affect the conductance measurement, and even distilled water for example might cause a release of chemicals in the material that could at to increase conduction, so a relative reading would be better. Actually, a relative reading over time taking data measurements over time.
In other words, when something is wet and at a certain temperature it has a certain conductance, and as it dries the conductance decreases. It reaches some point where the conductance starts dropping faster, but then the conductance starts dropping slower again. It would be considered "dry" when the slope does not change much with time under the same environmental conditions (or adjusted as necessary).
This would be almost like watching a capacitor discharge through a resistor, where voltage would be the moisture. As the cap discharges, it discharges at some rate but as time goes on that rate decreases (the cap discharges slower) and at some point it gets somewhat low but does not change much over the same time period. That's near the 5th time constant and because of the exponential curve 1-e^-t/RC we know it wont decrease much more after that even if we wait a year or two. That 5th time constant period would be what we would look for if we wanted to find the point where we could not reduce the voltage of the cap much more.
Note that we dont look for a particular voltage like 10 volts or 5 volts, but rather how much that voltage changes from the last time we measured it and how it changed for each sample period.
That's what we would look for in the moisture too, where we would equate moisture to conductance.

For example, we take measurements at time periods spaced equally, and we see readings of 5, 5, 5, 5, and 5. We know it did not dry out much yet. Several samples later, we see 4,4,4,3, and we see it is drying out. Then the next samples we see 3,3,3,3,3,3,3,3,3,2,2,2,2,2,2,2,2,2,2,2,2,2,2,2,2,2,2,2,2,2,2,2,2,2,2,2,2,2,2,2,2,2,2,2,2,2,2,2,2,2,2,2,2,2,2,2,2,2,2
and we notice that it's not changing much anymore even though we didnt get to zero.
If however if when we started out we saw 2,2,2,2,2,2,2,2,2,2,2,2,2, etc., we'd have to wait for lower readings.
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Re: Moisture vs Resistance

Post by Robert Reed » Sat Apr 30, 2011 5:22 pm

MrAl
I like your thoughts on the relativity of measurements being the key indicator and not the absolute value. But----"In other words, when something is wet and at a certain temperature it has a certain resistance, and as it dries the resistance DECREASES"----don't you mean increasing resistance?

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Re: Moisture vs Resistance

Post by jimmy101 » Sun May 01, 2011 9:47 am

I would suspect that the meter they are using is similar, if not identical, to a wood moisture meter like http://www.homedepot.com/webapp/wcs/sto ... reNum=2011

Yes, flood water has stuff dissolved in it so it is more conductive than pure water but the conductivity of a solid is probably largely due to ions that are already present in the material. Adding pure water to wood will increase the conductivity because there are already ions, salts etc. in the wood (or concrete, or drywall or ...). Those ions were left behind when the wood was originally dried. Drying involves the removal of pure water so adding pure water back will restore the conductivity.

So... for most materials there are the ions needed for conductivity, the question then becomes is there enough water to make those ions mobile? Doesn't matter to much if the water added was pure or not, pure or unpure water water will make those preexisting ions mobile again. You can therefore use conductivity as a rough measure of the water content even if you don't know the concentration of the salts in the material. No water means (essentially) zero conductivity regardless of the material. Some water, in most building materials including wood, drywall and concrete, will increase the conductivity. To use the moisture meter to estimate how dry a material is you need to know the normal reading for that material when it is dry. For wood, what is the conductivity of standard kiln dried construction lumber? For drywall the normal conductivity is whatever you measure in a piece of drywall that has been sitting dry for several weeks, same for concrete.

Once you know what the normal dry reading is you can estimate reasonably well what the moisture content of the material is as it dries out. For example, a sample of dry framing lumber reads as "4% water content". That flood soaked wall initially reads at "50% water content" but after drying for 3 days the reading is down to "5% water content". At that point you probably say the wood has dried out.

I suspect that an expert in this area know things like the minimum water content of wood (drywall, concrete, ...) that will support mold formation. If you know that number then you can target the most noticeable affect of a wet building, mold formation, during the drying process. Once the moisture in the building materials are below the mold levels it is adequately dry.

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MrAl
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Re: Moisture vs Resistance

Post by MrAl » Sun May 01, 2011 4:01 pm

Hello again,


Robert:
Yes i meant to say "conductance" not "resistance". I'll have to fix that, thanks.

Later: ok, fixed that.


I should also add that there are two slopes, not just one. Starting with a dry material, as moisture is added the conductance goes up, up, up, but then at some point it levels off and stops rising as fast. This would mean we might see a slow decrease as it dries but then a much steeper decent after it passes that knee.
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Re: Moisture vs Resistance

Post by haklesup » Mon May 02, 2011 5:50 pm

Guys, we are in the wrong business! What capital investment equipment do we have that pays itself off in about 20 days?
Well, that is if you don't pay your employees, don't pay taxes, and don't need a truck to haul that stuff around. However, I do get your point, that kind of business has a relatively low capital investment and apparently low labor cost. A perfect business for a guy who (for instance) has a brother who is a fireman and a sister in law who is a Realtor and Mom is an insurance adjuster (for inside referrals). :wink:

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Re: Moisture vs Resistance

Post by Lenp » Tue May 03, 2011 5:37 am

Thanks all for the feedback. I guess I was just considering making a meaningful measurement with a common meter with some correlation to the percent of moisture.
Yes, the rental rates for the equipment are high, but so are their service charges. A few hours of work, equipment rental for a couple of days and 2 visits to check the moisture and remove the equipment topped $1K! And that does not include their replacement of the damaged carpet, removed baseboards, refinishing several tiled floors and a installing a new counter top.

Thanks due the insurance deductible, they will foot the bulk of the bill, until they get their money back with interest in long term increased premiums!

Len
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Re: Moisture vs Resistance

Post by ringo47stars » Sat May 07, 2011 11:35 pm

Water could be an insulator if it is high in oil content and the other way if it is highly acidic. If you want to check the quality of work they did it might mean testing the place for heat, resistance, and humidity. If it is hot and low in resistance without being humid then it could be contaminated with acid for example.

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Re: Moisture vs Resistance

Post by Bygar » Mon May 30, 2011 8:19 pm

Have you looked at Deionized water characteristics. It has decreasing conductance until
its purity is high enough for the water to become highly reactive.

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Lenp
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Re: Moisture vs Resistance

Post by Lenp » Tue Aug 09, 2011 4:09 pm

Well, that is if you don't pay your employees, don't pay taxes, and don't need a truck to haul that stuff around.
Just to add a footnote!
Those were only the rental charges. There was more than $2000 in labor, disposal, removal, truck charges, call out charges, after hours fees testing.. whatever that were also involved. A crew of 4 were on site 3 hours on the first call then one call later to test the drying process then a crew of 2 to remove the equipment. The total claim was $7000 to put it all back together. I still think we are way underpaid; just consider the relevant skill levels!
Len

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Re: Moisture vs Resistance

Post by Dean Huster » Mon Aug 15, 2011 5:31 am

I've been through the moisture thing with one of my customers. Any moisture in drywall will cause the typical electronic stud finder to be worthless, acting like the entire moist wall is a stud. Therefore, that's a pretty cheap instrument for seeing if the moisture has been completely removed, although it doesn't indicate any percentage.

I suspect that your guy was using a meter designed to check the moisture content of wood when drying it. And I'd agree that the "percentage" will vary all over the place depending upon the material being probed.

To my knowledge, table salt is an insulator. But add water to it and it becomes a relatively good conductor. The same amount of water added to gypsum would not have as great a conductivity as salt, so material does matter.
Dean Huster, Electronics Curmudgeon
Contributing Editor emeritus, "Q & A", of the former "Poptronics" magazine (formerly "Popular Electronics" and "Electronics Now" magazines).

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