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View Full Version : taking advantage of nylons hygroscopic properties



darryl
10-17-2012, 01:14 AM
So nylon absorbs water and changes size- I'm thinking this could be the basis for a humidity indicator, and a small shop project at the same time. I don't recall what it's called, but there's a mechanism which uses two parallel elements close together. At one end they are bonded, and at the other they work two pivot points which are close together. When one element expands, an indicator attached to the pivot points gives an expanded reading. My high school metalworking textbook shows some indicators that use this principle. Guess I'll have to find that book again-

I wonder how stable a strip of nylon would be in this application, and how easily you could cancel out temperature effects-

Doozer
10-17-2012, 08:08 AM
It is called a Humidastat.
Used for room humidifiers.
Buy one at Home Depo or Lowes.

-Doozer

Lew Hartswick
10-17-2012, 08:41 AM
It is called a Humidastat.
Used for room humidifiers.
Buy one at Home Depo or Lowes.

-Doozer
Yep. Originally they used hair (I guess human) but for at least the last 50 yrs
or so they have been using a plastic material. (don't think it was Nylon).
It is not in the "bending" mode but in the "extension" mode. At least the one
in the de-humidifier I had back then. :-)
...lew...

ed_h
10-17-2012, 11:27 PM
I once worked for a company that made water sprinkler timers and accessories (for irrigation). We wanted a simple, cheap moisture detector to help control watering cycles. One of the approaches we tried was gluing together strips of two materials that expanded differently in the presence of moisture. This had the effect of multiplying the differential expansion. Nylon was one of the materials we were working with. We tried it with brass, stainless, and with other plastics. It didn't pan out in the end, partly because we never found the right adhesive.

We ended up using small pieces of sycamore wood , but not in a glued composite. In the right orientation, the wood changes dimension quite a bit with changes in moisture.

Black_Moons
10-18-2012, 11:03 AM
Something tells me a good old screw might work better in nylon then glue.

ed_h
10-18-2012, 01:15 PM
Something tells me a good old screw might work better in nylon then glue.

Moons--

Gluing the strips together gives something akin to the "bimetallic" strips used in some thermostats--Two metal strips with different thermal expansion coeficients are bonded face to face along their lengths. A change in temperature causes the assembly to bend such that the side with the more expansive metal is convex.

Bonding along the entire length is important. If the strips were just fastened at a couple of points, like with screws, the more expansive one could just buckle between the fasteners.

Black_Moons
10-19-2012, 02:18 AM
Moons--

Gluing the strips together gives something akin to the "bimetallic" strips used in some thermostats--Two metal strips with different thermal expansion coeficients are bonded face to face along their lengths. A change in temperature causes the assembly to bend such that the side with the more expansive metal is convex.

Bonding along the entire length is important. If the strips were just fastened at a couple of points, like with screws, the more expansive one could just buckle between the fasteners.

Yea, but he was planing on using them bonded at only one end, using a much greater change in size then bimetal strips, and using the realative motion of the two unbonded ends.

ed_h
10-19-2012, 10:34 AM
Yea, but he was planing on using them bonded at only one end, using a much greater change in size then bimetal strips, and using the realative motion of the two unbonded ends.

Moons--

I reread the OP, and you're right--he was thinking a lever type arrangeent to multiply the small movement of the nylon piece. I need to read things more closely.

That being said, I wonder if a little more investigation into a "bimetal" approach might be worthwhile. In a way, the lever action is built in to the assembly, and some quick calculations I just did suggest that some pretty high effective lever ratios would be easy to achieve. But then, there is still the problem of the adhesive.