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What causes moisture meters to have false readings?
Monday, November 16, 2020
There are three types of moisture evaluation devices and each uses a different technology. Pin-type meters measure electrical condutivity. Contactless meters check density with radio waves, and infrared cameras show differences in temperature across the surface of the material. Each type has its own unique advantages and shortcomings.
Electrical conductivity increases in tandem with the moisture level in a material. But any salts that have settled in the material, such as from urine or other chemicals, also increase conductivity. Plus, these meters are primarily designed for measuring moisture in wood. The moisture readings are stated as a percentage of water in wood (% H20 WME - Percent Wood Moisture Equivalent). But a reading of 12% moisture in wood is dry, whereas 12% in brick is saturated.
So, when working with materials other than wood, it is often better to use a comparison of readings between a known dry area and one that is suspected of being wet, rather than relying solely on a single percentage reading, even with an adjusted range based on the material tested. And checking your conclusion with one or both of the other moisture meter types is always a good idea.
Another limitation of pin-type meters is the damage caused by pin-penetration of a surface to get a good reading. It is not necessary, or recommended, to push the pins all the way into a material. But, in most cases, you have to slightly puncture the surface to get a reading, and that may not be acceptable for some home owners. Also, some surfaces are too hard to be penetrated.
These are good for a quick scan of an area within arm’s reach, and easily reset for different types of construction materials. But the readings jump up when over any hidden metals in the tested material, such as conduit, wiring, steel connectors, or foil backing on insulation. Again, cross-checking with another type meter is good due diligence.
INFRARED CAMERA (THERMOGRAPHY)
Best tool for a quick scan of a large area, although not as useful outdoors in daylight due to sun-induced temperture variations over surfaces. Because any moisture exposed to air will immediately begin to evaporate, and the evaporation cools the surface of the material, thermography uses this temperature difference to locate wet areas. But lower surface temperature in a location can also be caused by, for example, lack of insulation or a leaking air conditioning duct behind the material. An experienced thermographer can often tell the difference by the shape/pattern of the cool areas, although it is still possible to be fooled. So double-checking with another meter is good for verification.
Calibration can also be an issue for any moisture meter, although being out-of-calibration is more likely for older contactless and infrared, rather than pin-type, meters.
And then there is one final, low-tech tool. Your fingers are quite sensitive to wetness. “Fingertips are a great moisture-sensing tool. And the best part is that you recalibrate them every time you wash your hands,” according to Joe Lstiburek, a nationally known engineer and construction material researcher for Building Science Corporation. So, if an area looks suspicious, touch it to feel for wetness. Like the other types of moisture meters, fingers have their own limitation: moisture that does not reach the surface cannot be detected this way.
Also see our blog post What tools do you use for a home inspection?
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