Fluid Power Society's Pneumatics Specialist certification question 1

[Vitorio]

Hello,
I am studying to become a pneumatics specialist, but have come across something in the Fluid Power Society's study guide that I don't understand. If someone could explain it to me, I would be very appreciative.
The question and my solution are attached.
My misunderstanding comes from the units of the answer. I can calculate the correct numerical value (as the answer key says the correct answer is c, 48psig), but my units would be in psia, not psig. Can someone explain why psig is used here and not psia? From all my studying where we have to size a cylinder, we use psia, not psig, so I'm confused why this question uses psig. I understand the difference b/w absolute and gauge pressure, but not why gauge pressure is used here instead of absolute.
Thanks in advance.
Vitorio

 

[kcj]

Just quick reviewing, the reason is simple: your answer is correct, answer C is in psig. Since force is differential across the piston, gauge is correct. For expansion, gas laws, or other types of calcs, psia is used.

The FPS study guide used to be riddled with known errors. I am CFPE, but took the pneumatics test a few years back for some other reasons. The study guide 1/4 inch thick came with a photocopied set of about 30 known errors and corrections. Hopefully it has gotten better in later printings. It was really bad.

On the + side, it caused you to exam your work thoroughly and be convinced you had the right answer before asking here. So that is some confidence building.

kcj

 

[Vitorio]

Ahhh, thanks kcj. I think you're right: I was confusing using psia in gas law eq'ns with psig used here, although I'm still not clear on what you mean by "since the force is differential across the piston, gauge is correct". Could you elaborate a little more on that? Does it have something to do with the atm. pressure acting on both sides of the piston, and so does not affect the net pressure, or something? I'm still a little confused b/c I thought the pressure residing withing the cylinder would be relative to absolute zero, not atm, but I'm not sure.
Regarding the FPS study guide, it's the 2002 version and did not come with any errata so let's hope they've ridden the manual from all the errors it had in the past.
Thanks again for your response.
Vitorio

 

[kcj]

the error ridden one I have is 2000, so yours may be better, but be on your guard and trust your own knowledge. You may be right, the 'book' may actually be wrong. Also applies to many 'experts' in real life......experts, media, internet info, etc.

The math: You are correct in understanding, atmospheric is on both sides, which is why it cancels out.

I was taught if in doubt go back to the very basic physics, without the assumptions and deletions and work through the units, and see what cancels out or is assumed negligible.

In this case, the pressure on the closed side of piston is P gauge, which is actually Pc + 14.7.

On the other side, the rod side, the pressure is Pr, or actually Pr + 14.7. If you apply them both, the 14.7 on the annular net area rod side cancels out and using P gauge is correct.

There is still the 14.7 acting on the closed side, against the rod steel area. That would seem to calculate a larger force than if calculating with Pgauge only. However, the outside world still has 14.7 pressing on the area of the steel rod, so the 14.7 x steel rod area also cancels out.

Long way through, but it explains the basic concepts. Utlimately, areas and gauge pressures give the same results because the 14.7 is acting on both sides.

 

[kcj]

this case, the pressure on the closed side of piston is P gauge, which is actually Pc + 14.7.



my error, should say P absolute, which is Pc + 14.7



I can't find an edit button. Is there one somewhere?
kcj

 

[Vitorio]

Thank you, kcj, for your explanation. I understand now fully. I appreciate you taking the time to explain it in some detail. Thanks for your efforts.
Vitorio