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Old 10-05-2012, 10:01 AM   #79
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Join Date: Feb 2008
Location: Mountain Top
Posts: 6,404
My Ride: 2005 M3 Coupe
Originally Posted by SeanC View Post
Terra, I know the algebra, but thanks for laying it out for those who may not. Technically it is a called special case of the ideal gas law when n and V are constant. It is not called a "simplified" version. If you insist on writing it for 2 different conditions, you need to have the knowledge of 3 of the 4 unknowns in the form that you write it (1 equation 4 unknowns). That means you first have to use the PV=nRT twice to get to those 3 unknowns (actually you'd know all 4 by then). How's this for simplified?
I meant it's a simplification in that it's an algebraic simplification by definition. If you assume volume and molecules of gas are constant, you can simplify the equations by canceling the terms out. I realize that it is a special case and doesn't apply in every situation. No one is saying that the ideal gas law always simplifies to that form.

Originally Posted by WDE46 View Post
I'm glad we have come to the same conclusion. I can think of no engineering justification for the 2 bar cap . I also don't think a 1 bar cap will help with the longevity of the ET. I think it WILL help prevent a headgasket failure in the event of an overheat, but in day to day operation, the system never runs over 1.4 bar and that is generous. I will continue to try to justify a 2 bar cap, but I don't think I ever will.
I think it's time one of us measures the pressure and see if the theory matches the reality. The only thing I can think of is that if you reach atmospheric pressure while the heatercore is running, and then subsequently shut off the heat, the coolant level will rise a bit and decrease the volume for air. I don't know if that would be enough to bring the pressure to 2 bar (let alone 3) in any circumstance though.

I should probably mention - I'm just a senior in college, not majoring in engineering or chemistry, so my knowledge will have gaps compared to both of yours. But nonetheless, this has been fun

Last edited by TerraPhantm; 10-05-2012 at 10:10 AM.
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