Atmospheric Pressure -- Actuator Calculations

Does anyone account for Atmospheric Pressure when specifying pneumatic actuators? Maybe it's dependent on how the plant air compressor measures it's pressure. If it's relative to atmospheric or if it's measured absolute.

Anyone care to chime in?

I would say that basically nothing in "industry" is based on absolute pressure. Typically if XXXpsi is stated, it will be implied that that is PSI_G.

Unless you are doing anything around gas flow/thermodynamics/states of matter/etc then absolute pressures probably do not factor in to your worries.

I do my calculations relative to absolute 0. Even air at half an atmosphere can be compresses to be many times a atmosphere. Usually the pressure will go below atmospheric when decelerating overhauling loads quickly. This will happen in both pneumatic and hydraulic systems.

Thank you for the replies.

Am I safe to assume that atmospheric pressure will change the way pneumatic actuators move in our plant? I'm just thinking if we had an application where the actuator needs to be carefully specified, how much would role would it play? I know there's millions of other factors that could also change the outcome.



Thanks for the help!

Sticky cylinders from impure air will be the biggest problem if the cylinders are smooth to begin with.
If the actuators are only going from end to end it isn't a big deal but if you are trying to do precision control then keeping the cylinders moving with constant friction is a must.


We have pretty a pretty good pneumatic cylinder but it still has rough spots.
http://deltamotion.com/peter/Videos/PneuMove2.mp4

It depends on what you are doing. If you are working with actuator forces or other calculations that are based on pressure differences you can ignore atmospheric pressure and work with gauge pressure. If you are doing thermodynamic calculations and are working with ratios then you want absolute pressure, which is gauge plus atmospheric.

Most compressor gauges are using psig. With shop air at 80 to 100 psig the effect of atmospheric pressure is usually negligible.

as Peter said,

sticking air cylinders, valves, water in the compressed air, and even certain types of compressor oil will play a big factor in the application.

ALL air cylinders have different levels of friction in the seals and require different amounts of air to get them moving. the only way around this issue is to pay a fee to the factory to test each cylinder and send you closely matched cylinders. but when one goes bad, you're back to square 1.

if the system is critical, put a frl and then a 5 micron coealescent filter in front of the quick dump exhaust feeding the system.

the 5 micron filter will need to be monitored frequently to remove the trapped material.

james