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I am aware that some "Super Absorbent Polymers" can swell up enormously when they absorb water. However, I am looking for a material which swells up (maybe about 30% to 100%) when in a moist environment, and which will contract again when moved to a less moist environment.

I.E. by measuring the expansion of the material, one could determine the moisture content of the environment the material was in. It would act like a moisture measurement device.

Are there any such materials? Perhaps there are some "Fairly Absorbent Polymers"

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  • $\begingroup$ Superabsorbers are perfect for that. They swell by an order of magnitude or more in (distilled) water, but in moist air, you surely stay below a factor two. $\endgroup$
    – Karl
    Jul 2, 2019 at 20:40
  • $\begingroup$ @Karl - Thanks. And is the effect reversible? Moved back to les moist air, will they reduce in size again? $\endgroup$ Jul 3, 2019 at 7:29
  • $\begingroup$ Quite reversible, if you keep the swollen material at low stress. $\endgroup$
    – Karl
    Jul 3, 2019 at 18:40

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A classic hygrometer consists of bundle of a dozen or so human hairs (blonde are supposedly best, because they're typically thinnest, i.e. react fastest to humidity changes), stretched between a metal spring with a pointing device and a fixed point.

A uniaxial effect is much easier to record precisely, and hair is thin (=fast) and far more mechanically reliable than a piece of swollen hydrogel.

https://de.wikipedia.org/wiki/Haarhygrometer#/media/Datei:Haarhygrometer.PNG

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  • $\begingroup$ Nice answer +1! That's the Hair tension hygrometer one of the possible classical hygrometer, right? $\endgroup$
    – G M
    Jul 5, 2019 at 11:57
  • $\begingroup$ @GM I think that's the classic version. What others are there? $\endgroup$
    – Karl
    Jul 5, 2019 at 21:19
  • $\begingroup$ I know there is also the metal-paper coil type which is one of the classical cheap hygrometer hygrometer.net/types-hygrometers-multiple-uses $\endgroup$
    – G M
    Jul 6, 2019 at 15:15
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Many hygroscopic materials have this behaviour but sometimes they might have some "memory" of the previous swellings cycles, or the swelling might vary depending on the arrangments of the polymers chains, making them often not so reliable for such a device. You might want to do some testing on Nylon which is hygroscopic (more detail in the link). Also, Polycarbonate and Polyethylene Teraphthalate are hygroscopic and swells due to RH fluctuation. I personally worked on natural polymers used as artworks materials these are some results I collected on parchment and paper samples. enter image description here

Monitoring microclimate-induced deformations on hygroscopic materials using conoscopic holography sensors , Paper 11058-37, SPIE2019, Munich to be published

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