maybe a not-so-whimsical humidity indicator.

[Bob Hammond]

An awareness and control of relative humidity is important while building. I glued up this bit of whimsy perfectly straight when I thought that conditions were ideal at 47% RH and 72F. Since then I've kept an eye on it and observed the curvature change over the winter and summer. I think it's a practical instrument. It responds to changes in RH over the course of hours, and is visually interesting. It'd be great for a shop, to explain to clients about the effects of ambient humidity upon instruments.

The 9"L indicator was made from crosscut 3/32 x 1" elm and a strip of 0.020 x 1" mahogany veneer, bonded together with hot hide glue, in a cross-grain orientation. Along with the cross-grain orientation, I think that the combination of thicknesses and species of woods, plus the water-absorptive properties of hide glue, accentuate the changes in curvature.

So now I'll calibrate it against an electronic instrument, and maybe by a psychometric dry/wet bulb method demonstrated by Mario Proulx. Briefly described, the method draws a large volume of air from the shop over a temperature sensor, which gives a better estimate of the overall humidity in the shop rather than a location that may have still air that does not represent the environment.

1. Place a dry thermometer near the intake of a shopvac or dust collector. Wait a minute or three, and then note the temperature.
2. Wrap the thermometer bulb (sensor) with a wet paper towel, and again hold it near the intake of the shopvac. While the bulb is still wet, wait until the temperature stabilizes, and note that temperature.
3. Calculate the change in temperature, and then consult a published dry/wet RH table to determine the relative humidity.

[Markus Schmid]

Bob, I use the same type of hygrometer (this one) since 2011 and it works a treat. I also "calibrated" it using this home made sling psychrometer, and it turned out that the "bi-wood" hygrometer gets a bit less sensitive over the years. But more important than precise RH% numbers is getting aware of RH changes during one build, and here the "bi-wood" thing really shines because it reacts really quick and small changes become so obvious (open the window on a rainy spring day and you can watch from across the room how the indicator moves).

Have fun with yours!

[Patrick Hanna]

Nothing too whimsical, if you ask me. I have used a similar device for more than 20 years now. Sure.....it's not ultra precise. However, it gives me a VERY good indication of the relative humidity in my shop. You know....relative to last night or last month or whatever. I don't expect it to be pure-D accurate. I expect it to give me a relative picture of the humidity in my shop, and it does.

[Pat Foster]

I made something like that a while ago. What I liked about it was that it indicated the state of the wood, which to me was more important than the state of the air (which of course influences the wood). I mean, we really want to know what the moisture content of the wood is, right? I was surprised how quickly it responded to RH changes. I also found that it became less sensitive over the years, which I figured was because the wood was becoming seasoned. That meant it needed replacement. I read somewhere that Carleen Hutchins used something similar. I eventually got a mechanical Abbeon, which I'm happy with.


Pat

[Bob Hammond]

I wouldn't be surprised if it loses sensitivity as the wood ages. But it's easy to make a new one at anytime.

I've forgotten where I got the idea for the crossgrain layout, and I wonder if it's necessary at all. I do think that they hygroscopic nature of the hide glue on one side increases sensitivity. I wonder if that nasty Franklin Liquid hide glue would be more hygroscopic. I'd made another object with it last year, and it seemed to take a very very long time to set, if it ever really does. That will be easy to test.

[Murray MacLeod]

I do think that the hygroscopic nature of the hide glue on one side increases sensitivity.

I would have thought that the opposite would be true, and that the less hygroscopic the glue, the more efficient the hygrometer would be. Polyurethane glue would be my choice here.

[Bob Hammond]

The more moisture that glue absorbs, the more it will expand. Does polyurethane glue absorb or release moisture after it is catalyzed?

[David King]

If you are wanting a large difference in expansion rates then using two different woods makes less sense than one wood and something totally impermeable and unstretchable like brass.

[Bob Hammond]

note that the veneer is quite thin - it keeps the crosscut strip from splitting and then breaking off like a crispy potato chip