Moisture Meter Mythology

OK, right up front .... this article is not designed to discourage you from buying a moisture meter so that you have to hire me instead. Rather my point is that 90% of the time you do not need a moisture meter or a surveyor to detemine if your decks are wet so save that $300.00 meter cost for something else.

In the case of cored hulls I believe you are better off hiring a surveyor with long term meter experience as for many reasons determining the moisture content of cored bottoms can be a tricky excercise. The biggest advantage I have over amateur meter users is that I have seen hundreds of structures opened up so that I could see exactly what the meter was telling. I have found that meters are often correct but not often enough for me to put my faith or my reputation on them.

FACT: Moisture meters do not measure moisture.
FACT: Fiberglass is not waterproof.
FACT: No meter can tell you a percentage of moisture in fiberglass or cored structure.

There are a couple of different types of meters and several brands and detailed discussion would include terms like impedance, dielectric constant, capacitance, resistance and conductance but basically they measure how much electricity a material can store or conduct, so for the purposes of this article we will say they measure something most of us understand.........."conductivity". Of the two basic types of meters, "capacitance" and "resistance" , we will concentrate on the former as resistance type meters have pin sensors ( O.K. for wood, not a great idea for FRP) that must be inserted into the material being tested

A simple analogy would be to suggest that they send out a signal and measure the difference in the sent signal and the return signal, thereby measuring the conductivity of the material (boat surface) between the sending and receiving units. Wet fiber glass or core would be more conductive in theory than dry materials and therefore show a higher reading on the meter.

There are many things that can confuse a moisture meter and considerable experience is required to make proper use of them. Given that they actually measure capacitance rather than moisture, highly conductive materials will show higher readings on the meter whether or not they are wet. Without seeing a large number of reportedly elevated moisture areas opened up it is virtually impossible to understand the relative readings that you are seeing on your meter except in the most extreme cases. In these extreme cases you certainly don't need a moisture meter to tell you the core is rotten.

I re-typed the following from The Masthead - Journal of the Westlawn Instition of Marine Technology, December 2009 issue.

Know it All Contest Solution to the September 2009 Question
Wet Moisture Readings
By Dave Gerr, © 2009 Dave Gerr

The Know It All questions and correct answers are important design tips for students as well as other marine professionals. We suggest that you file them away for future reference.

The question from the September issue was :
It's early spring on a cool clear day. The weather has been dry for a couple of weeks and you are surveying Dancing Daisy, a 1992, 38 foot,cruising sailboat with a moderate fin keel. The hull, deck and cabinare all cored fiberglass except for the solid glass region at the centerline (in the keel and stem area). Dancing Daisy has been hauled out all winter and is well ventilated under an open shed.

You have taken moisture meter readings high on Dancing Daisy's topsides as a baseline and find that - in comparison - most of the deck and cabin top, as well as the hullbottom are reading wet (though not the cabin sides). There are no signs of water penetration problems. Are the meter readings an indication that these regions are suffering from water saturation or is there some other explanation ? If so, what is it ?

The correct Answer Is :

The answer is no. Though a number of "no" answers were submitted, the explanations were based on the presence of condensation which is not the likely cause. In fact condenstation on the hull surface shouldn't through readings too far off. The most likely explanation for the apparent wet readings is that the builderhas used a layer of CoreMat on the topsides and on the cabin sides where the smoothest possible surface with no print-thru was desired. (There are better ways to achieve this end but using CoreMat as a print blocker is common and acceptable.) CoreMat wasn't used on the deck or cabin top where print through is less of a concern. (The hull bottom is out of sight. The decks have non-skid as well as fittings which break up the surface so print through doesn't show as easily.)

CoreMat reads notably drier than standard glass so taking the baseline readings on the dry topsides is giving a poor reference in comparison with other laminate areas without CoreMat.

Ed. Note. CoreMat is a non-woven (looks like chopped glass) polyester fabric that looks like a very thick paper towel.

The "Code of Practice for the Measurement and Analysis of the Wetness of FRP Hulls" ( * International Institute of Marine Surveyors (1998) Witherby & Co., London, 17p. ) specifies the correct use of moisture meters and their standard includes the following :

"The surface must be carefully cleaned. A large number of random 4" x 4" areas must have paint or other coating removed down to the gel coat. The vessel should be out of the water at least 24 hours. Minimum number of measurements must be = approx. one per sq. meter (3.3 feet) or 50-100 on the average 35-40 foot boat".

Few sellers will allow bottom coatings to be scraped as necessary for an accurate determination. Generally buyers are also unwilling to pay the costs necessary for the yard to first block the boat and scrape the paint, then re-coat after analysis. If there are reasons to suspect a serious moisture problem, such as water intrusion in a cored hull, you may have to take core samples to be sure of the condition and many sellers also resist this.

Some examples of meter confusion.........

if there is heavy condensation on the underside of an otherwise sound and dry structure the meter may show high moisture levels.

if your boats gelcoat contains a lot of titanium dioxide (a common white pigment made of metal) the meter may read high depending on the pigment concentration.

if the non-skid on your deck is very deeply moulded or if the non-skid paint is very rough, meter contact may be limited so that a false low reading is shown on the meter. Some non-skid paints are rubber based and as this is an insulator again false low readings may show.

if you place a meter on the outside of a hull and there are metal fittings, anchor chain, water hoses or fuel tanks in contact with the inside, the meter may read high.

if a large saturated blister is deep in a very thick laminate the meter may show "dry" as the moisture is too deep for the weak signal of the meter to reach..

if water saturated core has separated from the FRP skin, the meter may read "dry" as there is no contact for conductivity.

if your bottom is epoxy coated, the meter may read low depending on the insulating properties of the epoxy.

if the core is frozen: A number of years ago I did a lot of testing with frozen blocks of bits of decks and hulls and was convinced that I could determine moisture content of frozen core ..... I was wrong ! What worked in the kitchen did not work in the field. In the kitchen at ambient temperature the meter was warm enough to create a thin film of moisture on the frozen bit and register on the meter. Outside in below freezing temperatures this did not occur.
When water freezes it expands by about 8%, The crystalization that causes the expansion results in a lot of non-conductive space within the water thereby greatly reducing the effectiveness of the meter. When meters are unreliable we can use the percussive sounding method, unfortunately this too goes out the window because frozen core whether balsa or plywood sounds solid. I need the work as I don't have much in the winter months and will survey a frozen boat (balsa cored hulls excepted) as long as you are willing to live with the limitations. My best advice .... wait til' spring.

As they send out a very weak signal, these meters do not read reliably much more than 1/4" deep and not more than 1/8" deep on some laminate schedules although some manufacturers claim up to 1" ( not in my experience). Many unnecessary epoxy bottom jobs are initiated by the improper use of moisture meters and many more of these jobs fail for the same reason.

A high reading on the meter in conjunction with a dull sounding report from my little brass hammer is an almost certain indication of a problem. If you find a little brass hammer in your bilge could I please have it back ?

Remembering that the meter is actually measuring the conductivity (or capacitance) of the material, we must also consider that the fiber/resin ratio, whether chopped strand, roving or mat and different types of resins will all have an affect on conductivity. Also remember that bottom, topsides, decks and superstructure of the same boat will often have different laminate schedules and construction techniques all of which again affect conductivity and therefore meter readings.

The average 30' uncored hull can absorb a maximum of about 3% water as contrary to popular beliefs, polyester resin is hygroscopic. For a 30' uncored boat this is somewhere between 20 and 30lbs. and it can take months to dry out ... if ever. The only way to accurately measure moisture content is to cut a piece from the hull, weigh it, bake it for a couple of weeks or burn it, weigh it again and measure the difference.

These meters can give an indication of relative moisture content or at least point out anomalies across a given area but this is just the beginning. Once anomalies have been identified we must confirm the reasons by examining the inside surface of that area for causes other than moisture or perhaps removing an area of bottom paint and re-testing.
It should be remembered that proving a 6" X 6" area wet or dry does exactly that and only that !

Assuming the same substrate (the layup is likely different in several areas of the bottom), these meters can show different levels of moisture across a given area. This can be useful if the bottom has been stripped of gelcoat and we are trying to determine if it is dry enough to to accept an epoxy barrier coat. If the meter reads XXXX when the hull is first stripped and XX two months later then moisture content has been reduced. If after another two months the meter still reads XX it does not mean the hull is dry, just as dry as it's going to get. Whatever the reading, it is relative and does not actually show 5% or 30% water content or whatever other number the meter generates. Let me repeat ....These meters CANNOT give a moisture percentage in FRP !

Moisture content is NOT a predictor of blisters and don't let anyone tell you different. While blisters without moisture are highly unlikely, thousands of boats out there have had high moisture levels for decades without a blister in sight.

I use the meter in my left hand and a hammer in my right. I use it as a backup and second guess and yes I make mention of the readings in my survey reports but only because so many people ask for them. I don't personally believe they are of much value on the bottom of a boat unless the previously mentioned "Code of Practice" is followed and even then it's only going to tell you what we already know.....boat bottoms are wet !

There are several places now advertising these meters to the general boating public suggesting they are cheaper than hiring a surveyor. Don't waste your money, even an untrained ear can detect soft balsa core under FRP in most cases. Invest about $9.00 in a small brass hammer instead.

I am often surprised by the ignorance of many marine surveyors regarding these issues. Even the hammer is not infallible, if the laminate is thicker than usual or wet core has separated from a thicker than usual FRP laminate or even how the boat is blocked or supported may affect percussive soundings. What we are really dealing with is an educated guess. Sometimes there is just no way around taking a core sample but the meter should never be used as the sole arbiter.

Cored bottoms are different animals all together and one day I will post my opinions on cored hulls vs. moisture meters Why on earth do so many builders think putting balsa below the waterline is a good idea ?