I have a question concerning pressure changes. We are trying to make small pressure changes to a static pressure of 5psi by varying the volume in a cylinder. We have had no problem making these pressure changes, but an interesting phenomenon is occurring. At the leading and trailing edges of the pressure pulses there are very fast pulses that are approximately 2-3x the amplitude of the pressure pulse. I do not know what is causing these extra pulses, or even what to call them, but they are dependent on how fast the velocity changes. Does anyone know what I would call these extra pulses, or does anyone know how to attenuate, or eliminate these extra pulses?

1. I believe what is occuring is comparable to what is happening in water by sudden flow changes, examplified by suddenly closing a valve: pressure peaks / shock waves when the kinetic energy is transferred to pressure energy.

2. By water you try to avoid this by slower flow, using wider pipelines and slower movements and longer closing (opening) times, sometimes also water expansion tanks, pressurized with constant overpresuure air to expand /damp the shockwaves.

3. How to cope with your particular problem would be to try to balance your total system, depending on how exact and how fast you have to have your application to react.

Some suggestions:

a. Slow down all movements to maximum allowed time.
b. Check if any components (vessels, pipelines, all) or restrictions should be larger or smaller to constrict or even out the flow.
c. Look at check valves. Select spring loaded or soft closing types and 'correct' sizes (perhaps smaller?).-
d. Closing time and sizes for solenoid valves, throtteling of any ports in and out?
e. Set in needle valves some places to experiment with slowing /constricting the flow?

Yes.  It takes a finite amount of time to move air into or out of the 400 ml volume, and a much smaller finite amount of time to fill/empty the tubing and components connected to it.  FWIW, the same thing happens with any fluid...if you have instruments that can respond fast enough to see the pressure transients.

"  I have a couple other questions, when is air considered compressible vs noncompressible?  "

Mmm.  Air is compressible.  All (real) fluids are compressible.  Incompressibility is a fictitious condition used to simplify analysis.  You can sometimes ignore the compressibility, sometimes not.  Aeronautics and fluid flow generally says to ignore compressibility when you are below Mach 0.3, or pressure ratios that will drive those speeds.  

But, acoustics treats air as "linearly compressible", if you will, at zero speed (pressure waves travelling thru still air).  That's not quite correct, but you get my meaning.

In your case, you are deliberately compressing the air to create a pressure change, so you should model the air as being compressible (and that multiply recursive statement oughta make the English teachers shudder).  If you tried your experiment with water, you know what would happen - your stepper motor would stall, because the water is much less compressible than air, and the motor wouldn't have enough torque to move the piston.

"Depending on how fast I move the pneumatic cylinder the extra pulse would grow substantially, why is this?"

You are compressing the cylinder side volume, causing its pressure to rise.  The pressure decays thru an outlet (to the 400ml bottle) that can pass only a fixed rate of air per sqrt(diff pressure).  Smash down the small volume faster, and the pressure goes up.

A little spreadsheet model of the volumes of the cylinder and bottle, tracking the mass of air in each, and connected by the flow thru the tube (just model it as an orifice) will teach you a great deal.  And, once you do this, you will have taken the first steps on the path to Computational Fluid Dynamics (CFD), using Finite Difference Methods (well, really finite volume methods, but hey).  

Next thing you know, you'll be coloring in pretty pictures and drawing streamlines...and that way lies madness.  

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