 We're now going to make a few comments about pressures and specifically we're going to take a look at absolute gauge and vacuum pressures beginning with absolute and sometimes you'll see this with a little a to denote the fact that it is an absolute pressure don't confuse that with Pascal's that's meaning pressure absolute but what this is so this is a pressure measurement with respect to a zero pressure reference so a zero pressure reference would be zero Pascal's or zero pressure an example of this atmospheric pressure is one oh one three two five Pascal's and that is an absolute pressure measurement with respect to zero Pascal's so P atmospheric is an example of that being an absolute pressure measurement the next form that we're going to look at is a gauge pressure and gauge pressure is referenced to the atmospheric pressure and it is often given the symbol P little G so PG is always higher than atmospheric pressure and it's referenced to atmospheric pressure and then finally we can have pressures that are below atmospheric and reference to the atmospheric pressure and those are referred to as being vacuum pressures and typically if you see it it might have a little V there denoting that it's a vacuum pressure and just like gauge it as well as reference to atmospheric pressure so let's take a look at a plot of these or a diagram showing the different pressures so what I'm going to plot on the vertical is going to be the pressure itself and first of all let's assume that our atmospheric pressure is here and we'll call that absolute so down here would be zero Pascal's and typically atmospheric is 14.7 psi or one oh one three two five Pascal's and we'll denote that as being P A T M so that's atmospheric pressure if we have a gauge pressure that would be above the atmospheric pressure so let's say we have a gauge pressure someplace up here and let's say the magnitude of that is PG equals 400 Pascal's so that would be a pressure then in terms of absolute pressures we'll continue this line over if we were to draw an absolute pressure for that we would have one oh one three two five plus four hundred which is one oh one seven hundred and twenty five Pascal's so that would be an absolute what that gauge pressure would be and let's say we have a pressure that is lower than atmospheric that would be our vacuum pressure and let's say we have a vacuum pressure that is minus 51,325 Pascal's and I'll call that P little V that would be vacuum and the pressure there would be one oh one three two five minus 51,325 that would take you to a pressure absolute pressure of 50,000 Pascal's so those are different pressures don't get confused with them whenever you see gauge it's reference to atmosphere and whenever you see PV that will be referenced to atmosphere as well but it'll be below it and if you see P atmosphere that usually is one oh one three two five or it could be fourteen point seven PSIA sometimes you'll see the a there and that denotes an absolute pressure as well so those are some things with pressure that we you'll need to remember as we work through problems and you'll see that constantly gauge or absolute vacuum we don't use as much but typically we quite often use gauge and absolute pressure