I'd never hear of this guy before and found the information on his site to very very much in tune with my thinking on just about everything , I guess this makes him brilliant
. He obviously has a very good understanding of moisture meters but I disagree with his advice to buy one. For the novice I still think a little brass hammer is sufficient to find the really bad stuff.
Most articles on moisture meter are so full of baloney ( including the Electrophysics website) it makes me gag. Here is the artcile from my own site
Moisture Meters
FACT: Moisture meters do not measure moisture.
FACT: Fiberglass is not waterproof.
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.
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 methods necessary. These include: 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 paint to be scraped as necessary for an accurate determination.
Generally buyers are also not willing 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 100% sure of the condition.
Some examples of meter confusion.........
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 the laminate the meter may show "dry" as the moisture is too deep for the weak signal of the meter to penetrate the laminate.
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 sure I buy this!). 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.
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.
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 not 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.
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 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 hulls are different animals all together and one day I will post my opinions on cored hulls vs. moisture meters.