I am designing a device that requires a gas (air) valve that takes the same volume of gas from two different input ports IP1 and IP2.

The output port OP will have lower pressure than both of the inputs, however, the pressures at the inputs are not not guaranteed to be constant and the same.

What is the simplest design that can guarantee that roughly the same volume of gas is been taken from both inputs despite the pressure fluctuations?

Additional criteria, the valve must not use any electronics or external power. It has to be a purely mechanical design.

Follow up questions:

  • Can this be designed without using moving parts?
  • Can we modify the design such that the gas volume taken from IP1 and IP2 is a different fixed ration e.g. 1:3
  • How would you design it different if you were to guarantee that gas taken from IP1 is at least 1/3 of the total volume in the output?

3 Answers 3


Such a thing exists for hydraulic system. It is not simple or cheap, but passive.

You need two positive displacement machines for the airflows, like sliding vane or rotary lobe pumps. At least the first also exist as pneumatic actutators, I thinkt this is how you source the parts. Then you link the shafts so that airflows are in the correct ratio. So if both are the same size and you want a ration of 1:2 you would need a linkage so the the rpm are in this same ratio. If the ratio is 1:1 ist's simply the same shaft.

The idea is that the pressure loss across the whole system drives both machines, with the linkage they are forced to turn in the correct ratio. This does no regulate pressure! There will be some pressure loss, depending on friction in the system etc.


This might be solved by some kind of compliant mechanism. The pressure difference could be used to control flows through orifices.

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Strictly speaking, no mechanism is required, but you would need right combination of compliant (flexible) and stiff materials. The hinge effect can be achieved this way.

Design for a predefined gas mixing should also be possible. Main disadvantages of this approach are computational complexity and manufacturing issues. You would probably need a lot of prototyping or extensive use of numerical simulations (FEA and CFD with two-way fluid structure interaction).

  • $\begingroup$ This is more or less what I have in mind! Upvoted. But I marked the other answer because that answer describes the more general design. $\endgroup$
    – Karl H
    Mar 31, 2023 at 18:57
  • $\begingroup$ very nice concept. Was this published somewhere by chance? would think it should flow right-to-left, fwiw (i.e., the pressure-sensing side of it ought to be downstream of the variable restrictions, to equalize the output pressures, not the input pressures; after equalization, a pair of orifices would establish the ratio). $\endgroup$
    – Pete W
    Jan 15 at 13:21
  • 1
    $\begingroup$ @PeteW I don't know if something like this was published somewhere (I did this figure myself, the idea came from compliant mechanisms in general). It is possible that there is a simpler version already in use somewhere. I am not sure, if it could work in reverse, because as it is now, you can have different pressures in both chambers (which are required to bend the core), provided that the adjustable passages on the right provide also different pressure drops, so the mixture pressure can equalise. Maybe with some modifications... $\endgroup$ Jan 15 at 17:00

Any orifice has a coefficient of discharge, varying pressure will affect the delivered amount.

If both inputs vary at the same time fine but if they vary independantly then you will need active control.

  • $\begingroup$ That's not necessarily true. A very easy yet complex solution would be to have two independent regulators to independently regulate the pressure of both IP1 and IP2 down to an intermediate pressure, and then have the same sized orifice from both intermediate pressure chamber to the output port. However, I am looking for a much simpler design. E.g. a "flutter valve" that have one moving element, that moves between IP1 and IP2, always towards the lower pressure side to create a smaller opening. $\endgroup$
    – Karl H
    Dec 16, 2022 at 23:05
  • $\begingroup$ A flutter valve has a moving part, which you excluded… $\endgroup$
    – Solar Mike
    Dec 17, 2022 at 5:32

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