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Per Hallgren -> RE: Per Hallgren Wood Hygrometer (Jan. 15 2008 2:13:49)
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I can offer the hygrometer for sale to those who don't want to make one himself, but only in very small quantities since it would be my son who would make them, well, most of it at least. I have too much to do myself to drop my guitarmaking to produce hygrometers. E-mail me for details.
However it is quite easy to make a wooden hygrometer so why don't you? You need some spruce as wide and long as you want the wooden stick to be long. 15 cm x 15 cm is fine. Glue two pieces together with the grain crossed 90 degrees using polyurethane glue. Thin one of the layers almost to the glueline but be sure to leave ca 0.3-0.5 mm. Thin the other layer to ca 2-3 mm. Cut strips ca 5 mm wide so that the thin side spruce is long grain along the length of the stick and thus the thick side is cross grain. Finish sand gently without breaking the stick. Glue one end of the stick on a surface but be sure that the rest of it is free to bend. Refer to the pictures below.
Now that you have understood the principle behind it you can use other dimensions on the spruce. You don't need to use two similar sized pieces as long as the cross grained side is the thick one and the long grained is the thin one. If the stick is made only ca 5 mm wide it will react faster than if it is made wider. What is important is the the wood shall have reached equilibrium moisture content for the relative humidity that you want the stick to be straight in, i.e. that the wood is not gives or takes moisture from the air in the room where it is and will be the reference. This will take some time, hours, days or months depending on dimensions of the wood and how dry it is from the beginning.
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