> Umm, this might confuse a couple of principles. Yes, lifting
> a column of liquid by atmospheric pressure limits the column
> to 34 feet (of water, or the equivalent), and the figures
> above reflect that for the lower density of gasoline. But,
> the fuel system is not pumping against atmosphere. It's a
> closed system (at least when the float valve is closed), and
> with the gauge directly connected to the fuel line, it's
> definitely a closed system from pump outlet to gauge. And,
> by definition, the gauge reads PSIG, i.e., pressure above
> atmospheric, regardless of ambient air pressure.
True, but regardless of whether there is atmosphere on top, the pressure
goes down as you get higher along a column. In effect, the weight of the
fuel (or whatever) inside the column is sitting on top of the pressure at
the bottom, so pressure goes down as you go up.
> In a closed system, pressure equalizes at all points in the
> system, so,
> 2.7 psi at the pump would be 2.7 psi at the gauge.
I disagree. If, for example, you set up the system I suggested with a 6'
length of pipe, 2 psi at the bottom and no fuel coming out the top; then you
plug the top with a pressure gauge, the pressure does not magically increase
just because there is a plug there. The gauge still reads 0 psig.
Normally, this effect is too small to notice, because we work with much
higher pressure in hydraulic systems. 3 or 4 psi in a system that works at
hundreds of psi (like a clutch) doesn't make enough difference to notice.
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