Moisture Meters

[jviss]

RC, I know you like the CT-33, but I couldn't help myself and bought a $40 General pinless moisture meter at Home Depot. I'd like to establish a regular inspection protocol with my new-to-me boat. I thought this might be worth a shot.


I read your excellent article, Understanding the Moisture Meter / Electrophysics CT-33.

http://www.pbase.com/mainecruising/moisture_meter

The meter I bought has a pretty good manual:

https://www.generaltools.com/produc...f/type/manuals/pdf/MM8ManualFINALGT083016.pdf

(Is there a way of uploading a pdf to one's media albums?)

RC, I would appreciate your thoughts on this tool, and in particular, thoughts on calibration, and what scale would correspond most closely to cored fiberglass decks? The available modes are drywall, masonry, softwood and hardwood.

For convenience I've reproduced some of the manual here:

The MM8 is designed for use in woodworking, water damage restoration,

building construction and home renovation. Examples include:

• Checking for moisture on or below the surface of carpets and subflooring

• Measuring the moisture content of wood, drywall or masonry before

painting, wallpapering, sealing or treating

• Locating water leaks above ceilings, below floors or behind walls

• Selecting dry lumber

The meter is a non-invasive (pinless) instrument that can detect moisture up to

3/4 in. (19mm) below the surface of the following materials: wallboard,

masonry, hardwood and softwood. It infers the level of moisture from the

material’s capacitance, which the meter measures by gauging its effect on an

electric field that the meter generates each time it is powered on.


The meter exploits two physical phenomena to make its measurements:

1. The linear relationship between a solid material’s moisture level and its

dielectric constant—and therefore its capacitance.

2. The so-called fringing-field effect—the slight spreading of the electric

field produced by current flowing between two electrodes when both

electrodes are on the same side of a material.

Behind the top of the MM8 are two metal plates. When the meter is powered

on, the plates are given small and opposite charges. The potential difference

causes current to flow, creating a three-dimensional electric field.

When the top of the meter is placed against one side of a material with

moisture on or slightly below its surface, the increased capacitance of the

material distorts the electric field to an extent that can be sensed (as a change

in flux over the sensing area) and measured. Displayed readings reflect the

average moisture level of the material between its surface and the electric

field’s maximum penetration of 3/4 in. Moisture closer to the surface has a

greater effect on readings than moisture at the maximum penetration depth.

The MM8 has been calibrated at the factory for use with wallboard, masonry,

hardwood and softwood. The capacitance of wood and the capacitance of

building materials are affected differently by moisture because they have

different densities. The meter compensates for density by adjusting the gain of

its internal sensing circuitry.

The MODE button on the front of the meter provides a convenient way to

switch among the four materials. The name of the material selected appears on

the LCD. Below the LCD is a bank of colored LEDs that roughly mirrors the

digital reading above it in bar graph format, with green indicating “dry”, red

indicating “wet”, and yellow indicating an intermediate moisture level. An

audible out-of-range alarm (beeper) sounds whenever wood is tested and

found to have a moisture content above 17%, or when drywall or masonry is

found to be more than 70% wet.

Although the LCD displays measurements of drywall and masonry moisture

content as % readings, these are actually relative readings with no accuracy

specification. Despite that shortcoming, relative readings are useful for quickly

comparing the moisture levels of materials, or the wetness of different areas of

the same material. For example, you can use the meter to locate the source of

a water leak above a ceiling by comparing readings at various points on it. If

the ceiling is level, the point that produces the highest reading is below the

source of the leak.

Any digital reading can be held by pressing the HOLD button on the front of the

meter. This button makes it possible to make a measurement in a dark place

and display it after bringing the meter into a lighted area.

To extend battery life, the MM8 automatically powers off after two minutes of

inactivity. The instrument is powered by a “9V” battery included in the blister

pack.

KEY FEATURES

• Separate settings for drywall, masonry, softwood and hardwood

• 2 in. diagonal high-contrast, white-on-black backlit LCD + 3-color LED

bar graph

• Non-invasive, non-marring detection technology with 3/4 in. (19mm)

maximum sensing depth

• ±4% accuracy on wood

• Calibrates automatically or manually

• Reading HOLD button

• 2-minute Auto Power Off trigger

• Separate out-of-range alarms for wood and building materials

• Low battery indication

• Powered by “9V” battery (included)

Here's a snippet of the specs:

SPECIFICATIONS

Measurement Ranges 0 to 53% for softwood

0 to 35% for hardwood

Relative readings for wallboard and masonry

Measurement Accuracy ±4% for hardwood and softwood

Maximum Sensing Depth 3/4 in. (19mm)

(in wood)

 

[jviss]

Just for grins, I measured the countertop where I'm sitting, a thin laminate of some kind on both sides, about an inch thick, with probably a particle board core. I got the following readings for the four modes. I presume the piece is stable, and probably pretty dry.

Wall: 10%
Masonry: 15%
Softwood: 9%
Hardwood: 3%

 

[jssailem]

How about using relative results rather than actual. If you get 10% It is low be happy. 50% means you likely have a problem. 90% the meter is in water, get it out.

 

[jviss]

That's funny, John, but relative readings are what this tool is all about, in all seriousness. Tough to calibrate, unless you have, as RC does. a test chunk of boat to measure. But, I know the only spot my surveyor, with his two, expensive moisture meters, detected a little moisture, that he said is readily remedied by re-bedding a particular stanchion-like base. I will take a base-line measurement all over the deck, paying attention to likely areas of moisture ingress, so that I can go back periodically and re-check. I'm thinking the soft wood mode, or range, will be the most useful for a balsa-cored deck.

I will take a copy of one of the deck layout drawings and mark it up, numbering the locations I want to monitor, and keep a log.

I mean, think of this: does it ever occur to an owner to re-bed deck hardware, like teak handrails, port lights, chain plates, etc.? Doe people do this on a schedule, like every five years? Or only when they are leaking into the cabin? How would one know?

On mine, with few exceptions, I expect no issues, since all fasteners that penetrate the deck are in solid glass. However, that are a few things that were added, and I don't know if they went through solid glass, and if not, if the hole was made correctly, i.e., the core edges filled with resin.

Best to check.

 

[LeslieTroyer]

I worry about long term stability. I would think that cheap sensors would degrade over time. Probably a good idea to measure a chunk of wood noting orientation and surface measured then toss it in a food saver bag at low vacuum and pull it out for checking as a baseline.

 

[jviss]

cheap sensors

That's a good idea, Les, to have a calibration standard. Not sure vacuum packaging it is necessary, or would not, indeed, alter its moisture content. And, I don' think inexpensive necessarily correlates with low quality, this is a brutally simple device, the sensors being capacitor plates. The instrument self-calibrates, too, and can be manually recalibrated, or apparently forced to self-calibrate, 'though I don't know "to what."