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I need a source of air pressure for an air bearing (3D printed cylinder that expands air upwards like an air hockey table). I would like to be able to control the flow of air. I don't have a lab air supply or advanced equipment, this will basically be a homemade build.

An off the shelf electric pump might be a good supply source. What do I need to regulate the flow?

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    $\begingroup$ If you want to regulate the pressure, a $10 pressure controller is sufficient. If you want to regular the flow itself (maintain constant flow no matter what happens downstream), you will need a mass flow controller, which are 10 to 100 times more expensive. $\endgroup$ – Tiger Guy Feb 3 '20 at 20:19
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As was mentioned in the comments, you can get off-shelf air compressors, which are quite good at regulating pressure. Now if you want to regulate the flow rate, that becomes a bit more tricky.

Your first (cheap ~ £10) option is to go for something like this (see https://www.smc.eu/en-eu/products/as-standard-w-built-in-one-touch-fitting~134135~nav for details):

enter image description here

This will allow to set your flow rate to a constant value for a given pressure, but you'll have to measure the flow rate in some other way. Also, if the pressure changes, your flow rate will also change, even if the setting on the speed controller hasn't changed.

Your second and more expensive option, is to go for a mass flow controller, such as this one (see https://www.bronkhorst.co.uk/en/products/gas_flow_meters_and_controllers/elflow_select/ for more details):

enter image description here

Now this will allow you to measure and control the flow rate to whatever value you desire and maintain that flow rate regardless of inlet pressure variations. This is controlled by a software running on your PC and communicating with the instrument via a serial cable (you'll need a serial to USB converter). Now this will set you off ~£1,200 so not cheap, but it's pretty fool-proof.

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A quick search for "how does a gas regulator work" resulted in a clear explanation from the wiki world.

single stage regulator

Verbatim explanation:

Single stage regulator

High pressure gas from the supply enters into the regulator through the inlet valve. The gas then enters the body of the regulator, which is controlled by the needle valve. The pressure rises, which pushes the diaphragm, closing the inlet valve to which it is attached, and preventing any more gas from entering the regulator.

The outlet pressure and the inlet pressure hold the diaphragm/poppet assembly in the closed position against the force of the large spring. If the supply pressure falls, it is as if the large spring compression is increased allowing more gas and higher pressure to build in the outlet chamber until an equilibrium pressure is reached. Thus, if the supply pressure falls, the outlet pressure will increase, provided the outlet pressure remains below the falling supply pressure. This is the cause of end-of-tank dump where the supply is provided by a pressurized gas tank.[citation needed] With a single stage regulator, when the supply tank gets low, the lower inlet pressure causes the outlet pressure to climb. If the spring compression is not adjusted to compensate, the poppet can remain open and allow the tank to rapidly dump its remaining contents. In other words, the lower the supply pressure, the lower the pressure differential the regulator can achieve for a given spring setting.

The outlet side is fitted with a pressure gauge. As gas is drawn from the outlet side, the pressure inside the regulator body falls. The diaphragm is pushed back by the spring and the valve opens, letting more gas in from the supply until equilibrium is reached between the outlet pressure and the spring. The outlet pressure therefore depends on the spring force, which can be adjusted by means of an adjustment handle or screw.

two stage regulator

Double stage regulator

Two stage regulators are two single stage regulators in one that operate to reduce the pressure progressively in two stages instead of one. The first stage, which is preset, reduces the pressure of the supply gas to an intermediate stage; gas at that pressure passes into the second stage. The gas now emerges at a pressure (working pressure) set by the pressure adjusting control knob attached to the diaphragm. Two stage regulators have two safety valves, so that if there is any excess pressure there will be no explosion. A major objection to the single stage regulator is the need for frequent adjustment. If the supply pressure falls, the outlet pressure may change, necessitating adjustment. In the two stage regulator, there is improved compensation for any drop in the supply pressure.

I've disassembled the regulator for a helium balloon inflator tank and found it to be virtually identical to the image. For balloon inflation, precise regulation is not important and the regulator was single stage.

It was comprised of a number of o-rings, seating the metal diaphragm to the machined parts and of course, the needle valve adjustment mechanism.

The failure of the regulator requiring repair was erosion of the o-rings and easily repaired.

One could construct a DIY version using a thin sheet of spring steel, but the critical aspect of the build would be to ensure the enclosure will hold the expected pressure. One would not expect a 3D printed pressure chamber to manage any appreciable pressure, although an explosion of PLA or ABS would be less severe than that of steel or other metals.

Off the shelf items for the needle valve assembly might be a carburetor kit for a mower engine or similar powerplant. Experimentation is warranted for the DIY side of things.

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    $\begingroup$ That would regulate the pressure, not the flow. $\endgroup$ – am304 Feb 3 '20 at 15:25
  • $\begingroup$ @am304 control the pressure and the flow $\endgroup$ – Solar Mike Feb 3 '20 at 18:58

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