Woodworker's Journal 2008-32-6, страница 38

The three moisture meters above perform the same task using slightly different methods. The Lignomat meters use pins of different lengths to test the moisture content. The Merlin meter uses radio waves and has no pins.

essential, they are nonetheless very desirable, for they provide a real indication of whether the wood will perform as expected.

Depending upon whether it is softwood or hardwood, there can be a considerable range of MC in a newly-felled tree. As well, the sapwood will contain more moisture than the heartwootf. The average MC is about 75%. but it can exceed 200% in some species! The desirable MC for furniture-making is 8% to 10%, so clearly a lot of moisture must be removed.

Making Wood

Usable

By Ian Kirby

Converting logs to boards suitable for furnituremaking.

In our September/October issue, Ian described how wood is formed in a living tree. He also gave us a vocabulary to name the structural components of wood. The sum of those components is the whole of a piece of wood. But for wood to become lumber, it must be cut from a log. Then it must be processed to be suitable for use. In this article, Ian explains how and why that processing occurs. Why does one board twist or cup, and another remains flat? Read on.

— The Editors

A , ,

L 1 truck loaded with newly felled logs destined for a sawmill to be converted into boards ready for drying could just as aptly be called a water tanker as a logging truck. That's because practically half the weight of each log could be due to water. We call this its moisture content (MC). Since the properties of timber depend a great deal on the amount of moisture it contains, knowing what that amount is allows us to dry and use it to the best advantage.

Determining Moisture Content

MC is a variable and its weight is expressed as a percentage of a constant, which is the oven-dry weight of the wood.

Weight of water x 100 _ _Q/ Dry weight of wood

If you have a scale or balance accurate to about half a gram, the oven-dry method of determining MC is simple. Take a sample from a board, weigh it wet, dry it, weigh it dry, and do the math.

Initial weight of sample - dry weight of sample x 100 Dry weight of sample

This was the simplest method of determining MC prior to the advent of moisture meters. All that is required now to get fast results is to follow the manufacturer's instructions. Although I don't consider meters absolutely

If you remove moisture from wood, you reduce its weight. This fact is at the bottom of the original and simplest way to calculate moisture content (prior to moisture meters). Weigh it wet, dry it, weigh it dry, then do the math.

Woodworker's Journal December 2008

37