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I've been asked to create a system that will output a constant flow of liquid (water) even when the input pressure is variable.

I can't use gravity filtration because the system is for a straw filter (diameter of 1-2cm). It's a system that should take in the water when sucked by a person, send it through a filter with a constant flow rate and be available for drinking. The final straw would look something like this

So far I haven't been able to think of any design that's possible for a straw. I'm starting to wonder if it even is possible considering the dimensions involved.

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closed as off-topic by Emilio Pisanty, Jon Custer, ZeroTheHero, Qmechanic Aug 29 '18 at 18:55

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  • $\begingroup$ why is a constant flow rate necessary in this application? $\endgroup$ – niels nielsen Aug 29 '18 at 17:56
  • $\begingroup$ I actually don't think it is, but the PhD student I'm helping wants a system like that. I'm thinking a filter that works at the maximum expected flow rate would be easier to build and fit into the design. $\endgroup$ – user43334 Aug 29 '18 at 23:13
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This could be potentially done with a variant of a constant flow valve.

There is a pressure differential associated with a flow. This pressure differential could be used to control a valve in a negative feedback fashion, i.e., when the flow and the pressure differential increase, the plunger is pushed to restrict the flow.

Below is a diagram that could possibly give you an idea how to proceed.

enter image description here

When the flow increases, the difference between $P_1$ and $P_2$ increases, which pushes the disc to the right and restricts the flow.

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a simple and cheap constant-flow regulator is used in drip irrigation systems; it is a rubber plug with a hole running through its center. it is mounted in the sprinkler water line so the sprinkled water must flow through the hole. high pressure deforms the plug so as to close off the hole, low pressure allows the hole to get bigger. in this manner the plug drops the supply pressure (40 to 80 PSI) down to the necessary constant low value needed to feed the drippers over a fairly broad range of inlet supply pressures.

In this case, the plug is fairly stiff but if a softer grade of rubber were used it might work for your application.

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