The Lies Meters Tell
January 4, 2007
Restoration professionals be warned: All moisture meters give false information. This can lead the user to false positive or negative results when looking for water in building materials. Readings can also be easily misinterpreted. Fortunately, with a little education and practice, these problems can be minimized so that meters can provide effective readings.
Moisture SensorThe moisture sensor (Image 1) is the most basic of moisture detection instruments. It is designed to detect the presence of moisture in carpet and pad, indicating “what is wet.” A moisture sensor works by using an electrical circuit to test the conductivity of a material through two short, non-insulated pins that are inserted into the material.
Although the amount of moisture does influence the frequency and/or intensity of the instrument’s response, no numerical value is given by the device, preventing the technician from evaluating progress. This instrument is therefore a sensor and not a meter.
It is a misuse of the moisture sensor to measure moisture in other materials present in a water-damaged structure. Because the instrument has been designed only for high levels of moisture in carpet and pad, it is possible that the meter will give an unaffected response even if materials remain wet. Moisture sensors can also supply false indications of moisture migration if high levels of urine are present in the carpet, padding or subfloor.
Invasive Moisture Meter (Penetrating)The invasive moisture meter (Image 2) operates on a principle that is very similar to the moisture sensor described above. Electrical circuits test the conductivity of the suspect material, and higher degrees of moisture result in a response from the instrument that indicates the target is “wetter.” The primary difference between invasive moisture meters and moisture sensors is the ability to apply a value to the moisture readings.
As with all meters, different brands and models of invasive moisture meters have unique scales; two meters inserted into identical materials with the same moisture content can produce two entirely different readings. Use the same meter each monitoring visit to nip this problem in the bud.
Invasive moisture meters offer a variety of accessories and attachments designed for various building materials and construction methods. These attachments, especially hammer probes, suffer from abuse and can easily give false readings due to broken electrical connections.
It is important to check pin insulation frequently. As the pins are driven into structural material such as plywood and hardwood, the insulation will begin to wear. To test the integrity of the insulation, simply run fingers up and down both metal pins, on each side of the pins. If the meter displays elevated numbers, the insulation has been compromised and the pin(s) must be replaced. Failure to replace the pin(s) will lead to inaccurate moisture content readings in the material being monitored.
Many restorers communicate all readings as a “percent moisture content.” Actually, all readings taken are of a relative nature. They are compared to a dry standard in the unaffected area. In these cases, report the readings as “points” or simply as the number without any explanation, except its relationship to the drying goal.
Non-Invasive Moisture Meter (Non-penetrating)Non-invasive moisture meters (Image 3) use radio frequency signals and conductive pads to measure either impedance or capacitance though a sample of the suspect material. Higher levels of moisture will result in higher transfer of the signal through the material, and therefore will result in higher readings displayed by the instrument.
Non-invasive moisture meters give false positive readings most often because of metal hidden within or behind the material being monitored. Common building components that influence a non-invasive meter’s accuracy include wallboard corner bead, metal studs, HVAC ducting, foil linings, etc.
Most non-invasive meters beep when testing concrete, even when the concrete is unaffected. Use an unaffected reading to make proper comparisons with concrete readings.
False negative readings are also possible with non-invasive moisture meters. These false readings result from unseen air gaps behind and beneath target materials. Most non-invasive moisture meters are only capable of carrying a signal through solid materials. The presence of air will limit the signal. Common building surfaces that may present air gaps behind and or beneath them include:
- Wall paneling over gypsum wallboard
- Veneer finish panels on cabinetry
- Swollen baseboard
Applying moderate pressure with one hand while placing the moisture meter on the surface with the other hand will aid in eliminating air gaps behind these materials.
Thermo-HygrometerA thermo-hygrometer (Image 4) measures temperature (thermo) and relative humidity (hygro). Some thermo-hygrometers are capable of calculating GPP.
Thermo-hygrometers take a significant amount of time to acclimate. When the proper amount of time is not taken, false readings are provided by the meter. The temperature side of the meter is the slower to acclimate, therefore be sure that the temperature has adjusted fully before writing down the reading.
How low will it go? When taking readings on dehumidifiers especially, it is important to recognize that some thermo-hygrometers have a lower relative humidity limit, such as 25 percent RH. Just because the RH side of the meter stopped going down doesn’t always mean the reading is correct.
Of the instruments discussed here, the thermo-hygrometer is one of the most important to calibrate on a regular basis. Relative humidity probes are prone to loss of calibration and should be tested frequently. Additionally, it is necessary to monitor the meter’s accuracy more frequently if the instrument is exposed to abnormally high or low temperature or humidity. One common way thermo-hygrometers are exposed to unacceptable temperatures is when technicians leave them on vehicle dashboards. Freezing temperatures and heat from direct sunlight can adversely affect the meter’s set calibration.