Moisture Meter Facts and Myths
Our continuing discussions on various aspects of the marine survey and some of the logistics involved in getting it done provides us with a perfect lead in for this a discussion on moisture meters. It’s been a while since we discussed these little guys and since moisture readings can be one of the more confusing of the items noted on a survey report let’s see if we can dispel some of the myths bandied about surrounding these infernal little devises. I say infernal with a bit of humor thrown in because in all reality as a surveyor I have to admit that I have come to rely on the data that they present, but only when in the hands of an experienced operator.
First of all let’s take a look at where the moisture meter originated and how it happened to wind up in the hands of marine surveyors and fiberglass repair professionals. The moisture meter was originally developed by the lumber industry to assist in the scaling and grading of wood. Early versions were all of the analogue type and each had 2 sharp pins extending from the end of the meter’s housing which the operator pressed into the wood and the reading was displayed by the needle on the meter. About 20 or so years ago the meters lost the sensor pins which were replaced by a sensor pad on the body of the meter which was simply held against the subject matter and the display produced a reading. This improvement allowed the meter to be used on various materials such as drywall, stucco and yes fiberglass. It wasn’t long before any marine surveyor serious about their craft had one of these guys in their tool kit. By the way the pin type meters are still in use by the lumber industry. Since no formal training or instruction was provided on how to interpret the information produced by the meter (remember they were mainly for use on lumber) many different approaches to the meters usage and data interpretation surfaced and became commonplace. Luckily since then most experienced surveyors have become proficient in their use but there remain many myths and confusions that still surround the use of these devises.
The first comment that I would like to make is that incorrect usage of the moisture meter has probably been responsible for more unnecessary repair and bottom jobs to fiberglass boats that all other diagnostic methods combined.
Secondly the moisture meter does not actually measure the moisture content of the substance. In actuality it measures the electrical conductivity of the material in question.
The theory is that since water is a conductor of electricity, the more moisture contained in a substance the higher the reading will be. This reading is then displayed on the meter, usually as a percentage.
This brings me to my next comment. Moisture readings should NEVER, NEVER be interpreted as, or reported as a percentage. My answer is always “as a percentage of what?” Here’s why. All fiberglass resins all have some level of moisture in their core makeup, some more and some less. Unless the surveyor was present on the day that the boat’s hull was popped out of the mold and if he was able to take moisture readings at that time, put the information on file for 15 years or so or until the next time the hull’s moisture levels were measured there is no way that percentages have any bearing on the resulting information. I still come across survey reports that report moisture levels as a percentage. Add to that the fact that a boat hull’s make up is actually a combination of resins, gel coat (which is actually a thickened, pigmented resin) spun glass re-enforcement (of various configurations) and possibly a core material (balsawood, closed cell foam or plywood). Then add a few coats of anti-fouling paint and we have enough to seriously confuse any self-respecting moisture meter. This where the experience factor of the operator comes into play. The moisture meter is in reality a very simple devise to use. You place the sensor pad on the substance in question and read the display. That’s the easy part. Trying to understand and interpret what the meter is telling us is where the difficulty sets in.
The next question that arises is how deep into the substance can the meter actually read? Moisture meter manufacturers make claims and I don’t want to say that they are false but it appears that any meters depth of accuracy, again will depend largely on the substance being measured.
Does anybody notice a pattern developing here? The point that I’m trying to make is that there are usually more variables in the use of a moisture meter than there are absolutes and interpretation of the readings is the real key. I have taken moisture readings on hundreds of boats over the years have to admit that I still do not have it all figured out but I have learned enough over the years to know when to question the readings and investigate further by other methods. I see moisture meters being offered for sale to boat owners at retail outlets and when I’m questioned as to whether they are a good investment for any boat owner my answer is usually no. In most cases what happens is that inexperienced meter operators are easily confused by any or all of the issues listed here and they wind up calling someone such as myself to re-examine and make recommendations.
Taking moisture readings in ambient air temperatures below freezing is also a bad idea and since I have mentioned this previously many times in this column I won’t go into it here.
One thing that the moisture meter does do very well is to prove the positive, this is if a substance is dry the meter will show it easily. Its when the readings are elevated that the confusion sets in.
So I think that you can see that while the moisture meter is in reality, is a simple devise to use there’s a lot more to it and accurate interpretation of the data is the real key.
Reprinted From “The Seaworthy Surveyor” Ontario Sailor Magazine
Original Article By David Sandford AMS®