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

[michael muller]

Bernoulli's equation, for irrotational flow (i.e. incompressible and non-turbulent), states:
P/rho + 1/2*v^2 + g*z = constant
The higher a fluid goes (bigger z), the pressure (P) must decrease.  So physically lifting a pressure probe decreases the pressure at the probe.  In many (most) cases this is not too significant, i.e. at high pressures or for light fluids (P/rho is much larger than g*z).  But for heavy fluids (e.g. gasoline) at low pressures (2-3 psig), it can make a difference.  Obviously Sujit is looking for a big difference in pressure - 0 psi vs. 2.7, in his original post, so holding the probe a few inches higher or lower does not explain his issue - it is more likely the probe is defective.  
But the basic idea of holding a tube 6' above the fuel line is a valid and simple way to test.
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Message: 8
Date: Mon, 21 Oct 2019 21:02:15 -0700
From: "Randall" <TR3driver at ca.rr.com>
To: "'Michael Porter'" <mdporter at dfn.com>, "'Sujit Roy'"
    <triumphstag at gmail.com>, "'Triumphs'" <triumphs at autox.team.net>
Subject: Re: [TR] What is the correct way to measure fuel pressure
Message-ID: <704EE09E9A4E4074BEB301067361C5FA at RYPC>
Content-Type: text/plain;    charset="iso-8859-1"


> 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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