Moisture Meters in the Wireless Age
A Brief History of Moisture Meters
Old-school Moisture Meter Technology
Wood moisture meters were initially developed to reduce the cost of inspections and avoid losing valuable lumber to the oven-dry method. While the oven-dry method of determining the moisture content of wood is extremely accurate, it’s time-consuming and wastes good lumber because it renders it unusable later. Here’s how it works…
The wood is weighed and then placed in a special drying oven. As it’s drying it’s taken out periodically and weighed. (As it dries, its weight will decrease.) When its weight stops changing, it’s weighed again, and that weight is compared to the weight it had before being placed in the oven. The difference between those two weights is the wood’s moisture content.
Forest Products Laboratory developed a pin-style moisture-testing device in the 1920s that came to be known as the ‘Blinker’ machine because of its two blinking bulbs. It worked like this…
When the instrument was set to a certain standard moisture content, the first bulb would flash according to that rate. If the wood being tested was wetter than this standard rate, a second bulb would blink faster than the first bulb. If the wood was drier, the second bulb would blink slower. It also came with headphones that allowed the user to listen to clicks instead of watching flashing bulbs.
The ‘Blinker’ could be used to determine both wood’s relative moisture content (above or below the selected moisture content) and to determine its exact moisture content by dialing the second bulb so that its timing approximated that of the first bulb. It measured wood moisture content from 8-24% and was accurate to within about 1% moisture content.
According to a July 1944 report by the Forest Products Laboratory titled “Electrical Moisture Meters for Wood,” there were two types of wood moisture meters available at the time:
- The resistance meter (pin meter) evaluated the moisture content of wood by measuring the wood’s resistance.
- The capacity meter (an early version of the pinless meter) measured the electrical capacity of the wood. The capacity varies in direct proportion to the amount of moisture in the wood.
This 1944 report also contains a list of moisture meter makers and dealers in the US at that time.
Moisture Meter Technology Today
Moisture meter technology has come a long way since the clumsy ‘Blinker’ device with its mercury-filled tube. Today, pinless moisture meter technology enables meters to quickly scan large areas of wood in no time flat without damaging the wood. Some of the features found on top-of-the-line moisture meters today include:
- The ability to use internal memory to store data from readings as the meter scans the wood. This data can then be uploaded later to other devices and programs.
- Bluetooth® connectivity. Today’s moisture meters can connect to smartphones and integrate with apps.
- Apps that allow users to create maps of floors to mark where tests have been taken and generate reports based on the data.
- The ability to calculate the Equilibrium Moisture Content (EMC) of wood. See the discussion below for more information about the EMC.
- No need to send the meter back to the manufacturer for recalibration. Moisture meters today can be easily recalibrated in the field via an external calibration device.
- Pinless meters can now take readings at two depths: ¼ inch and ¾ inch.
- Finally, they also have the ability to ‘’overlook’’ slight amounts of surface moisture on the wood so that it doesn’t affect the reading.
As you can see, moisture meters have certainly evolved. With today’s new technology, moisture meters can do things they weren’t even dreaming about back in the 1920s, 30s, and 40s. Of course, this doesn’t mean that every moisture meter on the market today possesses all these features. In fact, most inexpensive moisture meters probably don’t have any of these features. However, quality, high-end wood moisture meters will usually have most, if not all, of these.
Why You Should Use a Moisture Meter
All wood is hygroscopic. This is a fancy way of saying that wood is something like a sponge in that it both absorbs moisture from the environment and releases moisture back into the environment. This process continues until the wood eventually comes into balance with its ambient environment. That is until it’s no longer absorbing or releasing moisture. This is referred to as the equilibrium moisture content of wood (EMC).
Now, as wood absorbs and releases moisture its physical size actually changes. As wood absorbs moisture, it expands, and as it releases moisture, it shrinks. Therefore, anyone who works with wood needs to make sure that it has reached its EMC before using it in a project. If you don’t, the wood will continue shrinking or expanding after the project is complete. The result will be deformities like buckled floors, problems with cabinet doors or drawers, etc.
To sum up, anyone who works with wood, whether they’re a serious hobbyist or a professional woodworker, needs to own and know how to use a quality moisture meter. In fact, if you’re a woodworking professional, your reputation could even depend on it.
Larry Loffer is a senior technician at Wagner Meters, where he has over 30 years of experience in wood moisture measurement. With a degree in Computer Systems, Larry is involved in both hardware and software development of wood moisture measurement solutions.