Temperature and Wood Moisture Measurement
It is necessary to use moisture meters for wood to measure moisture content (MC) due to continual changes that occur in the relative humidity (RH) of a wood’s surrounding environment. Wood absorbs moisture from its ambient environment when the RH rises, and when RH decreases, wood releases moisture. However, the correlation between RH and the wood’s MC does not apply equally to the relationship between temperature and the wood’s MC if using a pin-style moisture meter. This is why a pin-style wood moisture meter reading is only as good as its ability to account for wood temperature.
Lumber producers utilize control systems in their kiln drying process to lower the MC in freshly-cut ("green") timber. Optimally, they dry the lumber to its optimum Equilibrium Moisture Content (EMC) target in order to balance the wood’s MC with its ambient environment. As any weather watcher can attest, the RH of our atmosphere is ever-changing. The air naturally holds varying proportions of moisture, and RH is the catalyst for those fluctuations.
Wood usually does not shrink or swell in use except if its MC changes in some way. The MC of wood changes when the RH changes. Unfortunately, if the wood was at the wrong MC when initially put into use, it will adjust to achieve its EMC with its environment and therefore may shrink or swell initially quite a bit during this process.
When you consider changes in MC, think changes in RH.
Temperature alone does not cause any significant size changes in wood. Heating does cause moisture changes to occur faster. When the air is heated, its RH drops unless moisture is added in some way.
For example, if it is 30 degrees Fahrenheit outside with 100% RH (snowing), the EMC would be 28%.
If outside air is brought into a home and heated, the following EMC values will be seen:
Heated to 40 F will result in 68% RH and 13% EMC.
Heated to 50 F will result in 47% RH and 9% EMC.
Heated to 60 F will result in 34% RH and 7% EMC.
Heated to 70 F will result in 24% RH and 5% EMC.
In North America, most heated homes and offices will run at approximately a 6% EMC in the wintertime and 9% EMC in the summertime.
Temperature does dry out the moisture in air (naturally). Without the addition of more humidity, the EMC of the wood will eventually change. To keep the wood’s EMC in balance during winter months, home owners can add moisture to the air by installing a humidifier in the furnace, as an example.
Therefore, some changes in EMC can correlate to a degree with rising temperatures, but most changes are due to RH fluctuations brought about by the drying effect of the heat itself. When measuring wood moisture levels, ambient temperature is a very minor source of influence compared to that of RH.
But what about the temperature of the wood itself?
Moisture Meter Readings
A pin-style wood moisture meter must be corrected for temperatures above or below 70 degrees F (Fahrenheit). This is to ensure that accurate MC readings are still obtained even when the wood temperature varies.
Wagner Meters produces a line of pinless hand-held moisture meters for wood that do not require adjustments for most wood temperatures. Even frozen wood with a MC below 15 percent can be measured with minimal temperature concerns (it is recommend that a person call Wagner Meters before measuring frozen woods with possible higher MCs).
The Wagner Meters MMC220 Extended Range Moisture Meter works for common hardwoods, softwoods and rare exotic species. Wood flooring and woodworking specialists may also utilize the Wagner Meters MMC210 Digital Proline Moisture Meter. Wagner Meters give you the confidence to trust the accuracy of your MC readings regardless of wood temperature.
Remember the basic fundamentals to successfully manage your wood’s MC.