Fluid Power Society's Pneumatics Specialist certification question
Fluid Power Society's Pneumatics Specialist certification question
(OP)
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
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





RE: Fluid Power Society's Pneumatics Specialist certification question
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
RE: Fluid Power Society's Pneumatics Specialist certification question
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
RE: Fluid Power Society's Pneumatics Specialist certification question
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.
RE: Fluid Power Society's Pneumatics Specialist certification question
my error, should say P absolute, which is Pc + 14.7
I can't find an edit button. Is there one somewhere?
kcj
RE: Fluid Power Society's Pneumatics Specialist certification question
Vitorio