Electrophysics makes a digital version that I have used for years and the numbers do not "jump" around. Over the years I have had the opportuniy to see what the numbers actually mean as I have been present when the suspect area was opened up for inspection. A number of times I ahve seen erroneous readings with no discernible cause. below are a couple of related articles I wrote.
Moisture Meter Mythology
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. ) which specifies the methods necessary.
"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 ?
( a highly simplified explanation )
If I knew what an osmosis test was I could be doing a lot more business as I get asked for it all the time. Dock talkers often use "blisters" and "osmosis" as interchangeable terms however, there are different types, causes and degrees of blisters and using the word "osmosis" just confuses the issue. I am often asked if I can perform an osmosis test to which my response is " What's an osmosis test ?". This response is met with a blank look as the people asking the question really don't know what they are asking. Many people also think that "osmosis" can be predicted by using a moisture meter unfortunately these meters do not measure moisture and the only way to determine actual moisture content is to cut out a chunk of your hull and send it to a lab.
This is one of the reasons I dislike the word "osmosis" as applied to boats, it has become a generic term to describe all types of blisters and moisture content in fiberglass hulls cored and uncored. Strictly speaking for osmosis to occur you need fluid on both sides of a membrane. If you have this situation on a boat, you better start bailing. What you are really encountering is simply water soaking into a porous material. Polyester, epoxy and vinylester resins are not waterproof, they are hygroscopic ie. They can absorb and retain water! (less so with epoxy and vinylester). I have heard many convoluted definitions of osmosis in attempts to justify the use of the word but what we really have is simply a process of absorption.
OSMOSIS : The tendency of fluid substances, if separated by a porous membrane to filter through it and become equally diffused.
So lets forget about the semantics and get to the issue................
There are many of causes of blistering, To list a few ....... trapped moisture during moulding, under cured resin, over cured resin, aerated resin, incorrect timing of subsequent layers, absorbent fillers, voids, trapped moisture in core materials, stale catalyst, emulsion bound mat, dusty mould, hygroscopic dust, cold mould, inadequately mixed resin, uncontrolled temperature and humidity levels during moulding process, uncontrolled temperature and humidity of raw materials in shipping, orthopthalic (cheap) resins. OK .... enough ! If I really thought about it I am sure I could come up with more but let's just say this, it is an extremely complex issue and "osmosis" just does not cover it. all you need to know is about blisters.
If you insist on calling it osmosis then it follows that all fiberglass boats have it !
NO VOIDS = NO BLISTERS (maybe) : There are dozens of reasons for voids in a laminate (some chemical) and they can range from tiny champagne size bubbles to several square feet although most are less than 1/2" in diameter. The average laminate may be 8-15 layers of various types of glass fabric made up of millions of miles of microscopically thin glass strands wetted out with resin. It is unreasonable to assume that all voids will be filled whether the wet-out is accomplished by five guys with rollers or one of the admittedly better (but not perfect) vacuum bag processes. All fiberglass layups have voids, some more than others. The higher the void content, bigger the voids and the more likely and earlier you will see blisters.
Polyester resin is hygroscopic, it can absorb water. A typical 30' , uncored sailboat hull can absorb about 30lbs. of water or roughly 3% maximum weight of the laminate. The glass fibers do not absorb any water and the resin is chemically incapable of absorbing more than 3% so theoretically 3% water content is saturation point of the material (voids excluded). As all polyester reinforced glass fiber and gelcoats are water permeable to some degree, all fiberglass boats left in water long enough will absorb water and probably develop blisters. This is rarely a structural issue (at least in our lifetime) although it can drive the sailboat racers nuts !
If the hull is cored with balsa or the laminate is all chopped strand (read - very cheap boat) then you may have a more serious and expensive issue to deal with as the balsa rots when wet and in the case of chopped strand fiber, the millions of exposed fiber ends wick water like so many straws.
The more common gelcoats are simply pigmented polyester resin of varying levels of quality and these pigments combined with the aeration caused by spraying the product into the mold can make it more permeable than the resin used in the laminate and therefore most blisters appear in the gelcoat. These blisters are usually small (1/8 - 1/4" dia.) and round in shape. While this does have an effect of the sale value of the vessel it is rarely a cause for concern. Vinylester gelcoats are becoming more common and are much more resistant to blistering.
Under the gelcoat is usually a "skinout" mat of chopped strand glass fibers that does not contribute much to the strength of the hull but is used primarily to hide the basket weave pattern of the heavier woven fiber which (hopefully) makes up most of the laminate. When water passes through the gelcoat it may wick up the chopped strand fibers of the "skinout" mat. These blisters are usually small, elongated and again not a major concern except to the value of the boat as they can scare buyers.
Water that has passed through the gelcoat and skinout mat into the structural laminate may combine with soluble elements that may occur in the voids. These elements could be uncatalysed resins, silane, glycol or salts (not the table variety) or any of the other chemical soup of ingredients that results from the resin curing (or uncured) process. When water combines with these molecules a new, usually larger molecule forms (hydrolysis), thus preventing escape of the fluid since the molecule is now bigger than the microscopic hole it came in through.
HYDROLYSIS : A chemical process by which the oxygen or hydrogen in water combines with an element or some element of a compound to produce a new compound.
As these newer, larger molecules multiply deep in the laminate they can get big enough to start to pushing apart the various layers of the laminate as the resin dissolves. This can be a serious issue, it is however relatively rare.
Put on a pair of safety goggles and puncture a few blisters. If they are dry or release a clear fluid, you likely do not have the serious type of blister. If an acidic vinegar like fluid appears this could be the more serious "Hydrolysis" type blister. Be careful as some of these blisters contain fluids under tremendous pressure. If the blister is very large and cannot be punctured with an ice-pick, it is likely very deep in the laminate, in this case drilling a hole for closer inspection may be warranted.
THE CURE ? Sorry..... regardless of the chemical companies hype there is no cure. You can only delay the inevitable but a very high percentage of blistered boats will still outlast you. I have surveyed one local boat three times over the last 10 years and each time it was getting another $10k "epoxy bottom job". I refused to survey it a fourth time because a well respected shop was about to do it again using the same improper techniques as the previous three jobs done by others. Whoever buys this 42' motor yacht will be doing it again soon (if there is any laminate left).
The more dry a boat is the better chance you have of getting a barrier coat to stick. Unfortunately once a boat has been in the water for a few years the water is deep in the skin at a molecular level and I believe its impossible to get out. Many years ago I peeled my own boat in September, put it in a heated shop with heat lamps on it until the following May then applied 8 coats of interprotect ...... five years later the blisters were back !
Small blisters in the gelcoat may be repaired by sanding, drying and applying an epoxy or vinylester bottom coat. This may help the resale value of your boat. Blisters in the skinout mat can be repaired by the same method but with much more aggressive sanding and perhaps some patching. The larger hydrolysis blisters require complete removal of the gelcoat and probably the skinout mat and perhaps a layer of the laminate (perhaps in local areas or over the entire hull in which case new cloth may be needed). The hull must then be dried to the point where the epoxy or vinylester bottom coat will adhere and washed frequently with fresh water during the drying process to wash off as much of the hydrolytic fluid as possible. This fluid and water may weep from the hull for weeks, months or even years so washing is crucial to providing a clean surface to ensure the adhesion of the new barrier coating.
This can be a bit of a gamble. Many bottom coats fail because the hull was not dried or washed properly. I have seen boats put under heat lamps for six months before bottom coating and new blisters appeared within a few years. The moisture is so deep in the laminate at a molecular level that it is not easily evacuated. A new system of applying heat under vacuum holds promise for drying hulls but for the most part it's still a gamble.
Although blisters will have a negative effect of the the resale value I have only ever seen two cases where I thought they posed a structural threat to the hull.
Give this careful consideration before plunking down $5,000.00 - $10,000.00 - $20,000.00 or more for a bottom job and always ask for a written guarantee (unlikely). Consult an Accredited Marine Surveyor® before spending you're hard earned money.
The hysterical laughter you hear as you drive a way in your"new" boat ..... is the seller.