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Re: [TR] What is the correct way to measure fuel pressure

To: "'Michael Porter'" <>, "'Sujit Roy'" <>, "'Triumphs'" <>
Subject: Re: [TR] What is the correct way to measure fuel pressure
From: "Randall" <>
Date: Mon, 21 Oct 2019 21:02:15 -0700
References: <> <2E75E051C17D4893B10A381E3652D5BE@RYPC> <>
Thread-index: AdWIh/r61Uy0awCSQZWrURkbDM9T5gAA/UEQ SSwn2GiHQa9xwZmcPNwf3Dd+VpANURNQ++qvbkI/O7RAhNuNPm6eHE8hItrwww3tpM5CoEZ8EgTX5bImqm85qjwtTARpiOm76qZnxCwSJUjWvZh85zadZEqY
> 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.

-- Randall

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