09-25-2006, 05:04 AM
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#12 (permalink)
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SRTforums Member
Join Date: Apr 2004
Member Number: 9691
Location: Central Ct.
Posts: 3,267
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Quote: Originally Posted by Corners
Quote: Originally Posted by 16g4g63
I'm currently in college for engineering and am taking a fluid dynamics class. According to the professor, as I understand it, vac pressure can not be greater than the atmospheric pressure. I was wondering how my boost gage can read 21 psi vac at idle when the atmospheric pressure is 14.7 psi.
Well, first off, you need to read your guage again. The stock guage reads inHg (inches of mercury) for a vacuum, and psig (psi guage) for positive pressure. I'd suggest you get your fluids book out and review the whole gauge vs. absolute pressure.
I'm a licensed ME and I specialize in thermal fluids design, and I can tell your professor is right IF you assume atmospheric pressure in psia (psi abolsute). If you're using psig, atmospheric pressure is 0. Just make sure you know the difference between psia, psig, and psid (that's absolute, guage, and differential), and you're good to go. 
FINALLY...........someone who KNOWS what thier talking about 
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