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Hygrometer, old style.


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Works on a principle similar to the bi metallic strip, except this works on the different rates of moisture absorption/release of different woods, both long grain and end grain. I used epoxy to glue the indicator together, largely because water based glues have a strong effect on how straight the indicator is when it comes out from the clamps. I glue up at 50% RH, so the 6 'O' clock position is therefore 50% RH. 

I've had this in the workshop for around 5 years and it still tracks my digital hygrometer pretty well, just at a slower pace. I sent one to Minnesota and he sent me a picture of it in action. They must be experiencing fairly high humidity because the indicator had swung a marker and a half to the right.




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I tried with the poplar plywood I had on hand but the resulting strip is too soft and plastic, I do not think it works.

I'll try to do things properly using spruce from violin top blanks.

I found this interesting video on youtube on the making, although aesthetically speaking your hygrometer is decidedly more refined and inspiring .....


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You don't really need it to react fast. I use mine in the workshop as a traffic light. Under or over a certain mark and I won't glue anything that is cross grain. If you made the indicator narrower it might have a greater overall swing. It may even react quicker but I'm guessing. I suppose you could spend a long time varying the wood types, cut type and the various dimensions. A lifetimes work for sure but not quite the ten lifetimes of messing with a bit of tree sap and oil. All for what? Just to daub it on a bit of wood! What do the ants and the bees think of us?

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Very cool stuff.


A quick search indicates the best wood to use would be Oak, Hickory, Dogwood, or Beech.  Birch and Sugar Maple are close 2nd choices.  On the other end of the spectrum (smaller reaction to changes in humidity) are Teak, Redwood, Cedar, mahogany, Pine, Ash, Butternut.  Spruce is only a little better.  This is based on readily available shrinkage rates from green to kiln dry.


Also interesting is that tangential (slab cut) rates of change are typically double that of radial (quarter cut).


So i'm going to find some slab cut Oak and see what I can make!





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  • 3 months later...

An update. 

I made a hygrometer based on Oak glued cross-wise to Maple.  This means the tangential shrinkage is being compared to the axial movement.  I used thin pieces for each and laminated them together with epoxy to keep the 50% point near the center (it was about 50% humidity when i did the epoxying).  The oak is 1.3mm and the maple about 2.0mm.

In late summer the humidity in my workshop was about 45-55%.  Now it's drier - with a cold front moving this week in it dropped to about 20% today.  So I've seen a fairly wide range of humidities on on the hygrometer.

The hygrometer is 76cm long (~30 inches), and moves about 21cm from 46% (its center) to 21% (today's measurement).  I used aniline dye to color the hygrometer but not seal the wood.


The hygrometer represents the % of humidity in the workshops better than a digital hygrometer which reacts more or less instantaneously to changes in ambient humidity.  


Next: The sensitivity to humidity to the thickness is very high.  By reducing the thickness by10 (10mm to 1 mm) the movement of the hygrometer should increase by about 3x.  A good result should be obtainable with two Maple veneers, I'll try this when I get some time.







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