Ascertaining the moisture content of wood flooring is critical to the success of flooring installation, as well as in the case of post-installation water damage. Moisture testing is carried out by using both pin-type and non-destructive (pinless) meters. So, how accurate are they?
The ultimate and most accurate test for moisture content of a material such as wood is the gravimetric or oven-based testing method, when a sample of the wood is taken and weighed before having its moisture removed through oven drying, and then weighed again. The percentage difference in weight is the percentage moisture content. Quality non-destructive and pin-type moisture meters can replicate this measurement with either qualitative or quantitative readings, depending on their use and degree of accuracy and precision. The moisture content in a wood sample is calculated using a moisture meter through the measurement of the changes in the wood sample’s electrical properties and comparing them with predefined data from gravimetric testing methods.
Pin-type meters are resistance meters that measure the electrical resistance levels in wood. The meter translates the resistance values into a moisture content percent measurement based on the corresponding empirical data results of gravimetric testing.
Non-destructive meters use capacitance to measure the moisture content of the wood sample. A low frequency electronic signal is transmitted into the material via the electrodes in the base of the instrument. The meter determines the strength of the current and converts this to a moisture content value, again using gravimetric testing as a baseline.
Pin-type meters test a very precise location in a sample of wood, between the two pins. Quality pin-type meters therefore are typically more precise than non-destructive meters, which test a much larger sample size. The non-destructive meter sample size is the area or “footprint” of the electrodes on the back of the meter, which can sometimes measure at different depths.
There are advantages and disadvantages to both methods, as well as adjustments that can be made to both that enhance their accuracy.
Pin-type meters tend to be less accurate when testing woods with moisture content values of less than approximately 7 percent MC. This is due to the fact that when the wood is dry and the percent MC drops below 7 percent, the electrical resistance of the wood increases exponentially and the readings can become less and less meaningful.
Quality non-destructive meters are better at reading the moisture content at these lower levels. In addition, they, obviously, have the advantage of not being invasive, so can be used to cover a large area in a short period of time and, of course, without leaving pin holes.
Non-destructive meter readings often have been considered qualitative. This can be because, in the field, they may be used to read through coatings, varnishes, and paints without damaging the floor surface, and due to the sample size being larger and deeper than the pin-type sample, there is a potential for the readings to be influenced by a substrate beneath the wood flooring. The user then has needed to compare those readings with quantitative readings using a pin-type meter.
To improve this non-destructive method of in-the-field testing of flooring products that already are installed, there have been remarkable developments in the shallow depth feature. The shallow depth feature greatly reduces and even eliminates the influence of the substrate on the non-destructive moisture content measurements. This feature now is available in several moisture meters currently on the market.
Measuring Moisture in Wood
The moisture content in a wood sample is calculated using a moisture meter through the measurement of the changes in the wood sample’s electrical properties and comparing them with predefined data from gravimetric testing methods.
Another very notable update to the non-destructive shallow depth feature is the ability to adjust the readings to take into account the SG (Specific Gravity) of the wood. The benefit of this SG adjustment feature is two-fold. First, when testing in non-destructive mode, the SG adjustment can be adapted to the density of the wood species under test, making the non-destructive test even more accurate. Second, if the SG of the wood is not known, it is easy to match up the non-destructive meter reading to a high-quality pin meter reading, by simply adjusting the SG until the non-destructive readings match the actual pin readings. This allows the user to scan the floor non-destructively using the same density setting and achieving the same results as they would with a pin-type meter.
There also have been technological advancements in pin-type meters, some of which now include preset species menus built into the meter to select from, as well as built-in infrared surface thermometers that allow for the meter to make the necessary corrections to the moisture content readings depending on the temperature of the wood.
Traditionally, prior to the development of the very helpful shallow depth and SG and temperature adjustment features, the meters would give readings based on a wood standard (for example the U.S. forestry Douglas Fir standard) calibrated at a certain temperature and with a certain SG. The user then would need to refer to different charts, such as temperature correction charts, wood species correction charts, or SG adjustment tables.
For additional information about moisture meters, check out the NWFA’s Installation Guidelines at nwfa.org/technical-guidelines/.
The use of wood standard settings, without any adjustments made, may be all the adjustment that is needed. There also may be some added value brought to the accuracy by adjusting for species and temperature when, for example, drying water-damaged wood so as to avoid overdying, or when working in more extreme conditions.
Having that option is key, and so with certain meters, now the user can choose to test with either a traditional wood standard without adjustment, or to turn on the advanced settings and avail of the built-in adjustment features as needed, thus allowing the user to see both the wood and the trees.