Join Date: Dec 2002
Thanked 0 Times in 0 Posts
Rep Power: 12
Buying Boat in Winter
Moisture Meters do not directly measure moisture content (MC) or dampness - but show changes in electrical resistance, capacitance, or radio wave reflection/absorption in the tested substrate, depending on the type of electrical moisture meter in use.
As general on site diagnostic tools electrical Moisture Meters are suitable, provided they are used correctly with the necessary skill and expertise, together with a full understanding of their limitations. They are clean and non-destructive and will not effectively damage the property, a factor most important in a pre-purchase survey.
Unfortunately, the majority of surveyors do not appear to understand how an electrical moisture meter should be used, and the manufacturers of the meters do little to educate the user. With most surveyors the meter is simply stuck into (onto) the surface of the hull, the indicator goes into the red and, hey presto, rising dampness! That is certainly not the way to use electrical moisture meter, and using it like this will almost certainly lead to a high number of wrong diagnoses! Similarly, it is not a valid exercise to directly compare readings from one material with that of another in respect of moisture content (MC).
Electric Moisture Meters measure an electrical property of a sample, and convert that measurement to a corresponding moisture content (MC) value, usually read directly on the meter. Depending on the type and manufacturer of the meter, the electrical property measured may be resistance, conductance, dielectric constant, or power-loss factor.
The majority record electrical resistance between two applied electrodes; a few measure capacitance, and more recently some measure the reflection of radio frequency emissions from the meter: moisture, together with one or two other materials, affects these properties. Thus, such meters can reflect the presence (or absence) of moisture and/or other factors which may be present.
The correct method for using an electrical moisture meter is to take a vertical & horizontal SERIES of readings in order to obtain a PATTERN of readings. It is the pattern (distribution) of readings which gives us guidance on the diagnosis. This is where skill and expertise of the surveyor are required in the interpretation of these patterns. The diagnosis is based on the overall distribution of meter readings, and not so much the actual reading itself.
All moisture readings must be corrected for the ambient temperature and relative humidity. An increase in temperature of the test sample will increase the rate of conductivity across a "wheat-stone" bridge, or increase the capacity of a laminate to store an electric charge. The electrical resistance at any given moisture content, increases as the temperature decreases. Portable moisture meters need to be calibrated and interpreted in a consistent manner, to enable comparable results. Meters are usually calibrated for 70-80 degrees F. If the hull is above or below this temperature by 20 degrees F or more, corrections must be made. Correction tables are often available from the meter manufacturer.
Since the more important finding is any potential differing DISTRIBUTION (pattern) of readings, rather than an absolute MC number - a Moisture Reading can still be useful & informative in freezing winter conditions.
FWIW & IMHO