Date: Mon, 12 Jun 2000 13:32:52 -0400
Reply-To: "John W. Parkins" <jwp6@PSU.EDU>
Sender: Vanagon Mailing List <vanagon@gerry.vanagon.com>
From: "John W. Parkins" <jwp6@PSU.EDU>
Organization: Acoustics Consultant
Subject: Detection of air bubbles in coolent system
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I had some thoughts on coolent system air bubbles, and thought I would
share them.
First off, I changed the heads on my 87 Westy, and wanted to be able to
check for leaks in a convenient way. I put together a pressurizing system
that was suggested by a listserver member. My design was slightly
different. I found a brass T fitting at my local hardware store. They
also sell a bicycle pump adapter that will screw onto the T. The store
also sold a pressure gauge that would screw right onto the T. You then
clamp a thick hose onto the T that will also fit on the coolent tank (not
the expansion tank). Take off the expansion tank hose and clamp on your
pressurization system hose. Use a bicycle pump to pump up the coolent
system to 14psi. You can then remove the pump. If the cooling system
doesn't hold pressure, look for the leaks. This rig costs around $10-$15,
and is well worth it. (You can't use this system to measure the pressure
generated in the cooling system when the engine is running though.)
Now, I found that I had no leaks in the system, but the temperature was
reading high by about a needle's width. When I pumped up the coolent
system, I found that the level went down at least three inches. Now here
is my thought. Since the fluid is basically incompressible, the fluid must
have gone into air pockets. Now the pressure times the volume of the air
in the coolent system is a constant (if you allow the system to remain at
constant temperature), or P1*V1=K (V1 is the initial volume of the air
pockets, and P1 is the pressure in the air pockets). Now if you pump up
the system, you get a new pressure and volume and P2*V2=K. If you pump up
the system to 14.7psi, you have doubled the pressure in the system (since
atmospheric pressure is 14.7psi). Thus P2=2*P1. Now, the change in volume
of the air bubbles is V1-V3, where V3 is the volume of air pumped into the
coolent tank (which can be estimated). Now, P1*V1=P2*V2 and V2=V1-V3. So,
V1=2*(V1-V3). Solving for V1, V1=2*V3.
In other words, if you pressurize your cooling system by pumping air into
the coolent tank ( to achieve 14.7psi), and the cooling tank level goes
down, there is air in your system. The amount of air in your system is
equal to twice the volume of air you pumped into the cooling tank.
Any comments on this? I think it is a good way to determine how much air is
in your cooling system. I have a significant amount of air in my system,
and I think this is the reason for the higher temperature reading. Cheers.
John
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