Moisture Meter

The moisture meter seems to becoming a common component in the woodworker's toolkit. These can be a somewhat mysterious black box with magic inside. Their operation is somewhat complex and their principles are even more complex.

In this article I hope to uncover some of the mysteries of this device and describe my experience with my moisture meter giving hints as to its best use.

There are two basic types of meter, the pin type meter and the inductive meter. The pin type has several advantages, they are typically cheaper and you can drill holes into the wood to get a better moisture profile. The operating principle is pretty simple. The moisture in the wood conducts electricity so the meter figures out how much water is in the wood based on how much current it passes. The up side is they are cheap and simple to operate. The down side is that you have to get inside the wood to find out how much moisture it has. Not much good at the lumber yard where the owner would take a very dim view of cutting a few mm off the end of every piece of wood in a stack. Also a pain at home where you want to check the state of a flitch by having to cut the end off and re-seal.

However the inductive type are easier to use and you do not have to cut or drill the wood to get a good reading of the moisture content but you must have a flat surface to place the meter against. It works by using an RF (radio frequency) signal and testing how much it is affected by the wood. The better the meter the more sophisticated the technique used in the meter. I bought this one for about $58 off ebay. You can get meters that cost $350 or more  but the truth is that you can do all of what you need with one of these. The dearer meter may have a deeper penetration and better build quality but that is about it. This meter has a penetration of about 30-40mm. (For those electronic engineers I know the operation is more complex but this is a layman's overview.)

You need a flat surface on the wood being tested and typically the long grain will provide a more accurate reading then end grain since the cell cross section may distort the reading but not by much. The surface can be rough sawn but a jagged surface will produce inaccurate readings. Also make sure you keep your hand away from the reading area. This will give a high reading. Since the reading is affected by the density of the wood you need to adjust the meter for the wood's density or SG (specific gravity) using the Density selection. Most meters come with a table providing the setting (typically 0-9) versus Density (SG usually) and wood species for the more common woods.

Let me do some school revision for you regarding SG. The SG is a number which relates the density of the material to pure water which has an SG of 1. In short if the material has an SG of less than 1 it floats and is lighter than the equivalent quantity of water. If the material has an SG of higher than 1 then it will sink and is therefore more dense than water. Almost all woods will have an SG of less than 1 and will therefore float but some rare woods such as ebony will actually sink and Ebony has an SG of about 1.1. There are many tables online that provide SG values for various woods but it is easy enough to calculate with a ruler, scales and piece of wood. Again this is covered elsewhere using various techniques but the one I prefer is to square a sample of wood on all six faces, calculate the volume, divide it into the volume of one litre (10cm cubed) and divide by the weight in kg. This should give you a number between about 0.3 and 1.2 and this will be the SG.

I had to do this for a piece of mallee for which I could not find a value online. I think it came out to about .4, I should have made a note.

If you are planning to use local timber or even local hardware store timber then an inductive moisture meter is a welcome investment and can save a lot of grief.