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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