Imagine I'm breathing through a wide straw/tube in my mouth. At the far end of the straw is a resistance mechanism. I want it to be that as I exhale there is no resistance, but as soon as I start to inhale I'm met with strong resistance so that little or no air can pass. But over the course of a few seconds, I want that resistance to drop to zero (either to drop smoothly or to "pop" to little or no resistance after a couple seconds). Furthermore, I want the timing of that resistance fall off or "pop" to be independent of how hard I am inhaling -- either a strong or a light inhale will produce the same result: strong resistance dropping (or popping) to little or no resistance over a couple seconds.

I'm trying to think of a way to do this purely mechanically.

Three mechanical timing mechanisms come to mind:

  • the balance wheel of a mechanical watch (which causes ticking at a constant rate regardless of how tightly the watch's mainspring is wound)
  • an hourglass (the concept here is that an inhale of any strength would flip the hourglass, and the number of grains fallen would represent the decreasing air resistance)
  • a hydraulic door-closing mechanism (where the door closes at roughly the same speed regardless of the pressure applied)

All of those are just "inspirations" in the sense that they seem like they can be used as the basis for a mechanical timing mechanism, but I don't know how to turn any of them into a concrete design for this purpose. Any suggestions welcome.

  • $\begingroup$ It sounds exactly like a pumping mechanism. So what is the motivation for not using it as reference? $\endgroup$
    – Jonathan
    Commented Jul 3, 2017 at 11:11
  • 1
    $\begingroup$ Have you checked the Tesla Valve? It can be an interesting application for reverse flow restriction, if you add a timing to turn it around somehow. $\endgroup$ Commented Aug 31, 2017 at 13:13
  • $\begingroup$ @GürkanÇetin, thanks, yes a friend did point me to the Tesla valve, which is a cool one-way restrictor without moving parts. But it still leaves me with the problem of a purely mechanical timing mechanism. $\endgroup$
    – M Katz
    Commented Sep 1, 2017 at 19:18

1 Answer 1


Use a flap valve - allows air out and if you design the "catch" correctly then won't allow air to pass until a certain load or depression is achieved.

  • $\begingroup$ By "certain load or depression" you mean a certain amount of air pressure, right? So if the inhale pressure is not enough the valve will never open? $\endgroup$
    – M Katz
    Commented Jul 3, 2017 at 6:22
  • $\begingroup$ Actually the pressure difference between either side... $\endgroup$
    – Solar Mike
    Commented Jul 3, 2017 at 6:36
  • $\begingroup$ I can't quite tell if this is actually an answer to my quest. Do you have a link that shows an example? $\endgroup$
    – M Katz
    Commented Jul 3, 2017 at 7:58
  • $\begingroup$ First link after a search, giving examples but you will have to design yours from there : flexseal.co.uk/products/flap-valves $\endgroup$
    – Solar Mike
    Commented Jul 3, 2017 at 15:28
  • $\begingroup$ This is not a solution as far as I can see, since if the inhale is not very strong, then the pressure difference won't be sufficient to open the valve - which needs to open at a set time even in this scenario $\endgroup$ Commented Feb 27, 2018 at 17:26

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