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Old 10-04-2012, 11:40 PM   #64
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Originally Posted by lcoleman View Post
Thinking off the top of my head here...pv=nrt would probably only work for a static system. The entire cooling system has a pressure differential (consider the points directly before and directly after the water pump) which causes flow. Also, the volume of the system is fixed, and the volume of a given amount of air changes depending on pressure, no?

Regardless of the science behind it, I agree with TxZHP04 in post #13; all this modification will do is introduce a leak as the "mean system pressure" in the ET exceeds the limits of the lower cap. Changing the cap will not change the operating pressure of the system, as it simply bleeds excessive pressure (which only happens if the T gets too high, right?). Think more like underdrive pulley if you want to reduce the pressure...and have a head gasket handy.

Long story short: waste of time.
PV=nRT (or PV=NkT for those who prefer that form) is applicable to gasses. So as the temperature increases, the pressure exerted by the gas in the ET will increase. So while the flowing water is not applicable to the ideal gas law (as it is not a gas), it still has relevance. Remember Bernoulli's equation 1/2(rho)v^2 + (rho)gh + P = constant... The "P" value will increase as a result of the pressure exerted by the gasses.

Originally Posted by SeanC View Post
The equation you wrote is a special case of the ideal gas law. Not its simplified version. Ideal gas law does NOT have a simplified version.

And it's not steam, but air. Steam won't be produced until you boil your coolant. Do you use steam in your expansion tank to fill it up 100% or just close the cap?

Please refrain from posting stuff that goes over your head
Sean, what WDE46 wrote is indeed a simplification. Since we are assuming volume and n are constant, we can write the equations for the 2 scenarios as follows --

P1*V = n * R * T1
P2*V = n * R * T2.

Rearranging, we get:
P1 / T1 = nR/V
P2 /T2 = nR/V


Obviously this only applies when n and V are constant, but I think it's pretty obvious that WDE46 understood that.

Last edited by TerraPhantm; 10-04-2012 at 11:41 PM.
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