Moisture Meters Demystified
Conductance (resistance) pin type meters used primarily for wood, utilize electrodes (pins), of varying lengths from 3/8-inch to 3 inches.
These pins are pressed or driven into the wood and measure the resistance to the flow of direct current or low frequency alternating current, so what does this mean in terms we can understand? When the pins are driven into the wood there is a current running between the two pins, this resistance or conductance is measured and converted to a moisture content reading (MC), water is a conductor of electricity, more moisture equals higher conductance.
With the Conductance wood meters, there are two types of pins, insulated and non-insulated. Insulated pins have a non-conducting coating except at the tips of the pins; this is more common with the one to three inch pins. These longer pins are generally on a slide hammer or something similar, so that they can be driven into the wood material, and attached to the meter via an external connection (Photo 2). Un-insulated pins do not have a coating along the length of the pins therefore; the reading will register at the point where the presence of moisture is the highest between the pins.
Pins that are affixed to a meter are usually non-insulated and are 3/8-inch to 1/2-inch (Photo 1, two outside meters), and can only read to the depth that the pins are inserted. So why the difference between insulated versus non-insulated?
Insulated pins, reading only at the tip of the pins, have the ability to measure gradient moisture content, as the pins are driven into the wood surface, readings can be taken at different depths to determine what the moisture content is at that particular depth.
When using pin type meters insert pins running parallel with the grain unless the manufacturer of the meter recommends otherwise (Photo 2).
Conductance meters read between the pins only and are sensitive to the temperature and species of the wood so make sure to refer to manufacturers manual or the manufacturer of the meter to determine the temperature and species adjustments.
Dielectric wood meter, (pictured in Photo 1 in the middle). This type of meter is a non-intrusive meter which means, it does not penetrate the surface of the material being tested. These types of meters are also referred to as capacitance meters. Readings are obtained by firmly placing the meter, which has a platen on the underside, in full contact with the substrate. This type of meter takes an average reading of everything within the sensing area and depth penetration. Results are displayed immediately and several readings can be taken in a very short time, as it is a non-intrusive test.
Depth of reading can vary with manufacturers, this particular meter that was used reads at a depth of 3/4-inch (Photo 3).
For engineered wood products (Photo 6), contact the manufacturer to get the recommended setting, as these types of tests are more of a relative measurement rather than a semi-quantitative measurement, this is due to the glue and resins that are present in engineered products.
Concrete meters (Photo 6) - Years ago resistance type tests were done by nailing two concrete nails into the concrete surface and then placing two wire terminals connected to a meter, many of these types of meters are still being used today but in a different form. Spring loaded moisture meters are considered Power loss meters and react to resistance of the material.
Dielectric meters for concrete use the relative density or specific gravity of the concrete to determine a value and use a platen (Plate) placed firmly on the concrete surface, to register a reading (Photo 7, right side).
Concrete moisture meters DO NOT give a quantitative value such as the calcium chloride test. The numbers displayed on the meters are relative, meaning that it is displaying a number only, although most manufacturers recommend a not to exceed value with their meters. One can get a relative reading by taking several readings in an area to get a general idea to determine the variance in the readings. Each manufacturer uses their own set of numbers displayed on their meters, so a reading from one meter will not coincide with the numbers from another manufacturers meter. A concrete moisture meter will not give a proper reading over wood and the same applies to wood meters over concrete, each type of meter is designed for a specific use and function.
Are the meters used in situations where they are exceeding the recommended limits of their use?
Here are some tips when using meters
• Make sure you understand the specifics of each manufacturer's meter as they have their differences.
• Readings on the wood meters generally read from 6 percent to 27 percent for the Capacitance meters, and from 5 percent to saturation point, 30 percent on the dielectric meters.
• Make sure there is no surface moisture present prior to testing as it can skew the results.
• Pins, on pin meters MUST be in new like condition to measure accurately.
• When using conductance meters with pins that are driven into the wood material, if it is an installed product, make sure you request permission to use this type of meter as it is intrusive testing and will leave permanent holes where the pins are driven.
• Follow the manufactures guidelines for temperature operating range.
• Wood Meters- Check with the manufacturer to see if meters are to be used parallel with the grain of the wood or if orientation of the meter is not affected by grain direction.
• Know the size of area the meter is reading.
• Make sure you are getting full contact with the substrate being tested.
• Document, document, document; this is critical if there are any issues in the future.
Remember, if you are providing a quality product and professional installation, and fail to conduct a moisture test, it does not matter how good the product or the installation, if it fails due to moisture that was not checked and documented prior to installation either by you but preferably, an independent third party, the entire installation would be considered a failure from the beginning. Without moisture testing documentation, the liability can still fall on the installer, even though the installation procedures may not have been a contributing factor to the actual cause of the failure.