Wood will settle (for a while) at its equilibrium, that is when it is equilibrium with its surroundings. When wood is green it contains a lot of water, some of this is sort of loose and is easy to dry. There is a huge amount of water bound up in the individual cells of the timber, water that is necessary for the cells to function as cells.this is the stuff that is difficult to persuade to leave. It is a slow process that can be speeded up to some extent by by controlling the environment the wood is kept in. This is the basis of the kiln, and is a great deal more involved than heating it and dehumidifying. Well it is if you want some useful timber at the end of it. In a centrally heated Home, you want to be fairly low, 10% ish or better. In a less controlled atmosphere 15% could be considered dry. Bare in mind boats are often made of wood and they float on water! Whatever figure you decide is right you will need to be aware that the wood will always strive to reach an equilibrium with its surroundings and this may mean that it will take water into its structure after you have taken pains to achieve a lower level. All changes in moisture level may produce changes in dimensions. Good joinery techniques and choices made very early on in a project in terms of timber choice and milling will help to mitigate this, as will choice of Finnish that can help to isolate the timber to some extent from its surroundings. If you walk round a timber yard with a meter, you need to be aware of a number of issues. The yard are often almost outside or actually outside, and therefore the humidity in the air is likely to be high in this country at least. Temperature is involved in the moisture reading and the meters are calibrated usually for a particular species and at a temperature usually about 20 deg C. When I used to have to work for a living I used a moisture meter many times a day every day. I still have my meter, and I remember it cost me about £700 when I bought it about 15 + years ago so I do wonder how good the cheap ones are. Mine was not the dearest on the market at the time, although I guess the technology could have got cheaper over time. A meter is useful to take relative measurements, as in has it got drier over the last week/month, but don’t rely on it to provide you with a definitive moisture content. There is only one accepted method to determine absolute moister content expressed as a percentage, and it is a destructive method that involves weighing a sample, totally drying it and doing the maths. There is also a similar method using a pressure device, a sample of wood or dust and calcium carbide, that reacts with the water in the sample to produce acetylene gas, the pressure of which is displayed on a meter fitted to the top of the device. This method is often used with brick or concrete dust from buildings. However, a moisture meter has its uses when used as a reference to show trends. Unless you cut your timber and measure in the middle of the very fresh cut you will likely get a different reading from the outside edges of the timber. If you mill your own timber, do it in good time, oversize, and allow it to stabilise ideally in the conditions that it will eventually live in, to allow it to move. You may be surprised how much it can move when you have released the stresses in it by cutting it, and that’s before the moisture element comes into play. If I’ve been teaching my grandmother to suck eggs so to speak, then I apologise unreservedly. Hey if it was easy, we wouldn’t want to do it (hammer)
