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Natural Science Forum / Physics / General Physics / July 2008A question about pressure
Hello, I have a couple of questions regarding pressure and its definition. I know that pressure is defined as force on some surface. 1Pa = 1N/1m^2. I know that this is very small pressure and zhat in life a much higher units are used. Many people use bar and atm as units because "it's more natural". How ever, I have found reading one article that negative pressure is mentioned, for example -0.5 bar. Since negative pressure doesn't exist (at least I cannot understand anything different), I suppose that this means 0.5 bar below some referent point. Is that referent point normal atmospheric pressure 101 325 Pa = 1.01325 bar or some other value? Thank you very much [snip] > How ever, I have found reading one article that > negative pressure is mentioned, for example -0.5 bar. Since negative [quoted text clipped - 3 lines] > Is that referent point normal atmospheric pressure 101 325 Pa = > 1.01325 bar or some other value? Little hard to tell because you are not holding "one article" high enough for me to read from here. But it is probably pressure relative to atmospheric. Another term you may see is gauge pressure. It is common to measure the pressure difference between a test region and the atmosphere. Socks > Hello, > [quoted text clipped - 10 lines] > 1.01325 bar or some other value? > Thank you very much If anyone comes up with a negative pressure, they simply did not have the 0 pressure correct to begin with. That is known as operator/engineer error of calibration. :)  SignatureJames M Driscoll Jr Spaceman > Hello, > [quoted text clipped - 10 lines] > 1.01325 bar or some other value? > Thank you very much I'm guessing that's a reference to gauge pressure, not absolute pressure. And yes, gauge pressure is referenced against atmospheric. PD > > Hello, > [quoted text clipped - 15 lines] > > PD Thank you for your replies. Well, article is saying about pressure in boilers and furnaces. It is engineering practice to manage with negative values. I also concluded this must be relative to atmospheric pressure of 1.013 bar, because if pressure in furnace (that burning coal) is higher than normal at. pressure it would force fire out. Is this "negative pressure" is called vacuum or difference must be greater in order to call it vacuum? Thanks > > > Hello, > [quoted text clipped - 24 lines] > greater in order to call it vacuum? > Thanks That's a matter of taste. Some people refer to "vacuum" to refer to the absolute sense of the word (zero absolute pressure), with various adjectives in front to indicate partiality or degree. Interestingly, "low vacuum" typically means a slight pressure reduction below atmospheric and "high vacuum" means very close to absolute zero pressure. When you suck on a soda straw, you generate a low vacuum in your mouth, but it is atmospheric pressure that pushes the liquid up into your mouth. PD >> > Hello, >> [quoted text clipped - 24 lines] >greater in order to call it vacuum? >Thanks Rather than speculate about terminology, why don't you say what your problem or observations are and then maybe we can help. Boilers make steam and work with gage pressure that is high. A furnace heats hot air, and either in the ducts, or in the chimney, they depend on very low pressure differences to draw the heated air up. This is customarily measured in "inches of water", with an inclined device (to magnify the deflection of the column) to measure the very low pressure differences. The sensitivity for water density which is 0.44 psi/foot or about 27 inch/psi, which could be increased by 10 times by setting the tube 6 degrees from the horizontal. A chimney can suffer a pressure reversal, sending the smoke back down, from gusts for example. John Polasek > > Hello, > [quoted text clipped - 15 lines] > > PD When engineers (also plumbers, HVAC techs, etc.) are using gauge pressure, they use negative values for pressure to indicate partial vacuum. Tom Davidson Richmond, VA >Hello, > [quoted text clipped - 10 lines] >1.01325 bar or some other value? >Thank you very much You could find negative pressure with a tank and a U-tube siphon taking water out. The column of water in the downtube will suck the tank water, constituting a negative pressure for all practical purposes. And, yes it is relative to atomosphereic. I use such a system to drain a flat roof. Wehn leaves etc. plug the exit I send water up to fill the siphon, remove hose, and there follows a torrent of mud and water filling a 2" PVs that is very impressive. (My new scheme: put plastic cap over the end of the pipe, and as it drip fills, the cap pops off by itself, and again, a geyser! John Polasek |
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