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1. The meter measures the radio frequency electrical conductivity of the material it is in contact with (and below the surface to a variable depth). The meter does not directly measure water.
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According to the manufacturer, pinless meters (ie CT-33) actually measure the capacitance, that is the ability of the material to store an electrical charge. In effect the back of the meter has an electric plate, which acts as one plate of a capacitor. The water, which is conductive, acts as the other plate, and the fiberglass, which does not, is a dielectric between the two plates. This is why moisture (dew) on the surface of the fiberglass dry will give spurious (high) readings.
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Primarily what I look for is large variations around chainplates, deck mounted Genoa track (often a culprit), and other penetrations.
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Right on!! The theory of how they work is less important than where and how you test, and interpret the results.
I bring a deck / hull diagram with me, and make sure that my meter is calibrated immediately before I use it (it can vary from day to day with humidity) and try to establish a baseline on the particular boat that I am checking. The baseline reading is at a point that I feel confident has not been compromised by moisture. I then at moisture prone areas, and only look for large variations in the readings. I have found that most decks that I have checked have a baseline, on the CT-33 meter, between 5 and 10. When I see the needle pass 25% I note that area as being "moist" on my diagram. I then search around the deck and continue to note the 25% moisture cline on my diagram as I scan around.
HTH!
Ed