# Meter-in or Meter-out

We are wanting to control the speed at which a pneumatic cylinder extends while pushing a load and retracts when unloaded. When looking for flow control valves we have found metered-in and metered-out, but not sure which we need. Any tips?

you want meter out. You want to control the flow of air coming out of your tanks.

Meter out on double acting cylinders, but not for the reason @ARaulinaitis said.

Meter In fittings (which you donâ€™t want) let air slowly into the cylinder. The cylinder will be stationary until enough pressure has built up to break any static friction and actually move it, at which point it can move very rapidly, until the air has expanded and dropped in pressure enough that it stops. Thereâ€™s still some damping because exhausting high pressure air from the other side isnâ€™t instant, but itâ€™s not going to be a super controlled motion.

Meter out fittings will slowly let air out of the exhausting side of the cylinder and quickly pressurize the other side. The cylinder will move as soon as the differential pressure between the sides is high enough to break static friction. Then the incoming air will expand, but pressure wonâ€™t drop as much because itâ€™s already high pressure. Pressure on the exhausting side will increase, and eventually a balance will be hit thatâ€™s ultimately set by how fast air can be pushed out of the meter out fitting at a given pressure. Because the speed is set by this balance, the actuation speed is generally less affected by changes in load. Meter out is pretty much the only way of slowing an unloaded cylinder. The only trick is that you need to make sure the exhausting side always starts at the same pressure if you want consistency. So donâ€™t dump air out of the exhaust side and then pressurize the other side, or youâ€™ll get a rather rapid jump followed by a slower movement.

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Youâ€™ll want the meter-out valve coming out of the cylinder for the reasons @Kevin_Sevcik listed.

Thanks for the info, do I need a meter-out fitting on both ports, or just on the exhaust?

If you want to control speed in both directions, both ports of the cylinder.

I think most flow control valves actually have a check-valve that opens to a higher flow path on input, so you canâ€™t put them in to meter in.

But it will matter which end of the cylinder you put it on if youâ€™re only doing one flow control valve on a particular cylinder.

but like flmmel says, just put two on it and you canâ€™t go wrong. If you have space.

Techinically there is no such thing as a meter-out or meter-in fitting. There are fittings that you can adjust the flow from the PTC side to the threaded side (which are typically sold as meter-in) and those that can adjust the flow from the threaded side to the PTC side (which are typically sold as meter-out).

Put a â€śmeter-inâ€ť on the solenoid and it becomes a â€śmeter-outâ€ť.

A -in on the extension port or a -out on the retract port will both control the speed at which the cylinder extends. However Kevin is right, using my example having the flow control on the retract port will give more consistent control and be easier to make fine adjustments of the extension speed of the cylinder.

This shouldnâ€™t be an issue since most teams will use a double solenoid to control a double acting cylinder as the switch from pressure to exhaust on one port is almost simultaneous as the other port switches from exhaust to pressure. Either way the timing of those actions is not something that can be adjusted by the team.

Now if you used two single acting solenoids to control one double acting cylinder then yes could create a situation where there could be a significant delay between switching one port to exh and putting pressure to the other port.

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I donâ€™t know why youâ€™d say this. Image of the only cylinder-mount one AM

sells. Itâ€™s Meter out. Automation direct sells them too. They are all sold as â€śMeter Inâ€ť or â€śMeter outâ€ť.

In one direction, they meter. In the other direction is a check valve with a high flow path.

I say it because it is the truth.

I did say that many people sell them as -in and -out and the assumption that they make that the threaded side is connected to the cylinder.

If you read the bottom line of the product description it states that it allows flow to be controlled in one direction, while the opposite flow is not restricted.

No where that I can find on the product description does it say that it is â€ścylinder mountâ€ť or that it can only be connected directly to the cylinder.

Fact is you can put the flow control fitting on the solenoid and depending on the application that may be the better location for it. Maybe there isnâ€™t access to the cylinder once it is installed and there is good access to the solenoid. Maybe there isnâ€™t room for it on the cylinder or it is in a vulnerable location if placed on the cylinder.

Of course you can always go this way. https://www.mscdirect.com/product/details/80083769 and since the connection type on both ends are the same they do not try to force an -in or -out tag on it. The beauty of this particular flow control device is that it has a proper pneumatic diagram on its side depicting the internal construction so that the user understands what is going on. Some instead have a large arrow in the direction of unrestricted flow and a small arrow in the direction the flow is adjustable. Many mfgs put that on all of their flow control devices regardless of the connections

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I was thinking of 3-way center exhaust valves, where theyâ€™d start with one side unpressurized. Which are admittedly rare in FRC

applications, but still.
And now Iâ€™m thinking about pneumatics and pondering if I could make a center-blocked valve legal. Itâ€™s possible with check valves plumbed to working pressure, but thereâ€™s a non-zero chance someone would rule itâ€™s â€śconnecting the output of two solenoidsâ€ť even though the check valves are to prevent pressure from reaching the cylinder.

center-blocked valve will leave air pressure, energy, in the robot after air system drained. Pretty sure this not legal.

If all you do is plumb the output of a center blocked solenoid to a cylinder, this is correct. However, thereâ€™s a way to plumb it with check valves so the pressure will exhaust when the rest of the system loses pressure:

This works fine from a pneumatics standpoint. From an inspection standpoint I think it should be legal as well, since check valves arenâ€™t solenoid valves, so youâ€™re not plumbing the output of multiple solenoid valves together. But you may end up with a less experienced inspector that doesnâ€™t understand this distinction.

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looks like that works to me. Kudos.