Vacuum Gauges Paint a Picture for You
This will give you the anticipated vacuum you should expect when you know the given line length, diameter and GPH, plus the height from the bottom of the tank to the middle or shaft of the fuel unit. Let's look at an example of a manufactured home 14' x 70' and lets say the fuel line is 50' of 3/8ths line from the tank to the fuel unit and that the lift is about 4'. Here is how you can determine the vacuum reading you should be seeing. If you haven't already, you're going to want to make a vacuum gauge, a part of your tool arsenal. As my friends at Bacharach would say, "If you don’t test you don’t know."
L x Q = ? + H = V
L = Total length of fuel line.
Q = Fuel line diameter x GPH.
H = Total lift from the bottom of the tank to the middle of the fuel unit.
V = Vacuum hg. inches mercury.
(Q) (.0086 x .60 GPH = 0.00516) (H) (.75 x 4 = 3)
(L) 50’ x (Q) 0.00516 = 0.258 + (H) 3 = (V) 3.00258
Three inches of vacuum is well within the limits of the fuel unit, however if the fuel unit constantly needs to be bled and the vacuum is 1" this would tell you that you have an air or oil leak. A vacuum higher than 3" would indicate there is a restriction in the system caused by a kinked or plugged line, plugged fuel filter or fuel gelling, etc. See how easy that was? :)
You'll find carrying a small calculator and this information with you on the job will prove to be very useful.
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Your friend in the industry,
Alan R. Mercurio OilTechTalk.com