By Grete Heimerdinger
It may sound easy to take moisture measurements: just stick the needles in the wood, or place the meter on the board. However, there are some essential variables installers and inspectors need to be aware of, to correctly measure the moisture of a wood floor.
Checking your moisture meter:
You can check the primary functions of your meter by performing the following simple tests:
Pin or pinless:
First, hold the meter in the air and take a reading with nothing touching the pins or the measuring plates. The meter should show the lowest possible measurable value. Depending on the make of the meter, the meter may indicate “LOW” or “MIN.” It is important to be aware of this low limit because your meter cannot indicate a lower moisture content.
Wood can have a very low (many times non-measurable) moisture level in high desert areas such as Colorado, or in the cold winter months in places like Minnesota. The theory behind it is that there is so little water in the wood that the meter has reached the limit to indicate a lower moisture level. The dry wood blocks all measurements.
Some pinless meters can measure lower moisture contents than some pin meters.
Next, make sure your meter is actually reading. Touch the pins with two fingers or put the measuring plates on your forearm, push the “READ” button and you should be able to get a value above 12 percent.
It is also recommended to use a calibration check block provided by the meter manufacturer. You cannot check a pin and a pinless meter with the same check block. Based on the different measuring technology, two different check blocks are required. Meters from different manufacturers also use different check blocks.
It is highly recommended to perform the two tests described in addition to using the calibration check blocks.
Measuring with your pin moisture meter:
Pin meters measure the electrical resistance between 2 (or 2 plus) pins. If there is more water in the test sample, the resistance gets lower, and the meter indicates a higher moisture value. Less water and the electrical resistance gets higher; the meter indicates a lower moisture value.
Measurements are obtained from the space between the pins. If pins are Teflon-coated, the meter measures moisture where the tips of the pins are placed. This is the only way to obtain core measurements, measurements from a subfloor, or moisture gradients within the entire system without removing the floor planks.
Three factors affect the indicated moisture value by a pin meter:
1. Differences between different boards of the same wood species. Each piece of wood, even within the same wood species, is different in color, structure, and electrical characteristics. Moisture readings from sample boards of the same species at the same moisture content show small deviations. Below fiber saturation point at about 25 percent, these deviations are small and in general stay within 1-2 percent.
2. Differences between different wood species. More significant variations are found between different wood species. To stay within the variations of 1-2 percent, moisture readings have to be corrected for the wood species you are measuring at the moment.
Calibration corrections are done internally by the moisture meters.
The user selects either a wood group
(species with similar correction algorithms are in the same wood group) or the individual wood species. Wood group codes and species settings are specified by the manufacturer.
Note, the correction algorithms for pin meters are not based on specific gravity; they have to be found by oven tests and mathematical calculations.
3. Differences between different wood temperatures. Measuring moisture of hot or cold lumber requires further corrections. The electrical resistance changes with the wood temperature. Moisture meters are calibrated for a wood temperature of 70˚ F. If the wood is between 55-85˚ F, the difference in readings is small. Above 85˚ F or below 55˚ F, the measured value should be corrected for the actual wood temperature. Some pin meters have built-in temperature corrections while others come with a correction table. Caution: It is hard to establish the correct wood temperature at the measuring depth where the tip of the pin has been driven to.
If the ambient temperature has been stable for the last few hours, you can take the ambient temperature. If the ambient temperature has changed, then use the average during the last few hours.
Measuring with your pinless moisture meter:
Most pinless meters use electromagnetic wave technology. The indicated moisture content includes all materials in a three-dimensional field underneath the measuring plate(s). (See photo, the entire area between surface and core is included in the measuring field.) If there is more water in the test sample, the indicated moisture content is higher. Less water gives a lower moisture content until the meter cannot measure anymore.
If moisture varies within the three-dimensional measuring field, the average moisture is indicated. If different materials, for example, wood and concrete, are within the three-dimensional measuring field, the indicated value is not a true moisture percentage.
If there is not enough material underneath the measuring plates, the indicated moisture value is not accurate either. It could be because the test sample is too narrow to cover the entire measuring field, or maybe because the test sample is not thick enough and there is not enough material underneath the measuring plates to give an accurate reading. In other words, wood planks that are only 1/2″ thick cannot be accurately measured when the meter is reading 3/4″ deep. The subfloor will influence the results. Some meter manufacturers offer dual-depth meters to accommodate for these variations in material thickness.
Two factors affect the moisture value indicated by a pinless meter:
1. Differences between different boards of the same wood species. Pinless meters change the indicated moisture value based on the material inside the measuring field. Each piece of wood, even within the same species, differs in structure and density, which causes small variations between measurements within the same wood species at the same moisture level. These variations mostly stay within 1-2 percent if the moisture content is below the fiber saturation point at 25 percent. However, if the density within the same wood species varies too much, those limits will be exceeded.
2. Differences between different wood species. Corrections for different wood species are necessary because of density changes between different wood species. There is a large difference in density between a lightweight cedar board at specific gravity (SG) of 0.3 and a tropical hardwood with an SG of 1.1. To obtain accurate moisture percentages, the meter needs to be adjusted for different wood species.
Many meters now offer built-in correction settings as an upgrade from printed correction tables. Wood species with the same density are put into the same wood groups. Most meters use the average density of the wood species as a code number. For instance, the wood group No. 45 is for wood species with a density of 0.45.
The temperature of the wood does not have a great impact on the indicated moisture values when using a pinless meter.
It is very obvious that moisture meter measurements with any meter, pin or pinless, need to be corrected for the wood species measured. When documenting your readings of moisture measurements, it is crucial to always note the make of meter, the wood species settings, and where applicable, the wood temperature correction settings.
For accurate record keeping, simply take a photo of the indicated moisture value and the chosen wood species (and temperature) settings. If possible, show the place where the photo was taken. If you ever have to go back for a claim, measure the same spot and compare measurements.
What can be done if the meter has been set to the wrong wood species code number? It happens from time to time that an entire series of measurements is taken with a meter on the wrong wood species setting.
Whether you’re using a pinless or a pin meter, the best solution would be to call the manufacturer and give the actual measured wood species, the moisture value, and the wrong setting number. They should be able to give you the corrections.
If this was a pinless meter, you might be able to find the correction yourself. Take a stack of papers and measure the moisture content using the wrong setting. Add more papers or moisten some sheets until you get the same moisture content as out on the job. Then change the setting to the correct wood species setting and take a reading. The correct value is displayed. Now, you can determine the difference in moisture percentage between the correct setting and the wrong setting. Correct the other values by the same difference within the same moisture range. You can also call the manufacturer to ask how to correct the values.
The worst case scenario is if moisture measurements have been taken and it has been forgotten to note the chosen wood species settings, and if applicable, wood temperature settings. Those measurements are useless, sorry.
With the right moisture meter, following the manufacturer’s instructions, anyone can get reliable moisture measurements, which represent the moisture distribution of the wood floor at the time the moisture readings were obtained.
Grete Heimerdinger is a Technical Specialist for the Lignomat moisture meter division. She can be reached at firstname.lastname@example.org.