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

 To: triumphs@autox.team.net Re: [TR] What is the correct way to measure fuel pressure michael muller Tue, 22 Oct 2019 21:47:42 +0000 (UTC) mharc@autox.team.net triumphs@autox.team.net WOW64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/77.0.3865.120 Safari/537.36
 ```--===============0258712820653987557== boundary="----=_Part_3630760_330838495.1571780862748" ------=_Part_3630760_330838495.1571780862748 Content-Transfer-Encoding: quoted-printable Bernoulli's equation, for irrotational flow (i.e. incompressible and non-tu= rbulent), states: P/rho + 1/2*v^2 + g*z =3D constant The higher a fluid goes (bigger z), the pressure (P) must decrease.=C2=A0 S= o physically lifting a pressure probe decreases the pressure at the probe.= =C2=A0 In many (most) cases this is not too significant, i.e. at high press= ures or for light fluids (P/rho is much larger than g*z).=C2=A0 But for hea= vy fluids (e.g. gasoline) at low pressures (2-3 psig), it can make a differ= ence.=C2=A0 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 highe= r or lower does not explain his issue - it is more likely the probe is defe= ctive.=C2=A0=C2=A0 But the basic idea of holding a tube 6' above the fuel line is a valid and = simple way to test. ------------------------------ Message: 8 Date: Mon, 21 Oct 2019 21:02:15 -0700 From: "Randall" To: "'Michael Porter'" , "'Sujit Roy'" =C2=A0=C2=A0=C2=A0 , "'Triumphs'" Subject: Re: [TR] What is the correct way to measure fuel pressure > Umm, this might confuse a couple of principles.? Yes, lifting=20 > a column of liquid by atmospheric pressure limits the column=20 > to 34 feet (of water, or the equivalent), and the figures=20 > above reflect that for the lower density of gasoline.? But,=20 > the fuel system is not pumping against atmosphere.? It's a=20 > closed system (at least when the float valve is closed), and=20 > with the gauge directly connected to the fuel line, it's=20 > definitely a closed system from pump outlet to gauge.? And,=20 > by definition, the gauge reads PSIG, i.e., pressure above=20 > 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.=C2=A0 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=20 > system, so, > 2.7 psi at the pump would be 2.7 psi at the gauge. I disagree.=C2=A0 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 yo= u plug the top with a pressure gauge, the pressure does not magically increas= e just because there is a plug there.=C2=A0 The gauge still reads 0 psig. Normally, this effect is too small to notice, because we work with much higher pressure in hydraulic systems.=C2=A0 3 or 4 psi in a system that wor= ks at hundreds of psi (like a clutch) doesn't make enough difference to notice. -- Randall =20 ------=_Part_3630760_330838495.1571780862748 Content-Transfer-Encoding: quoted-printable
Bernoulli's equation, for= irrotational flow (i.e. incompressible and non-turbulent), states:

P/rho + 1/2*v^2 + g*z =3D constant

The h= igher a fluid goes (bigger z), the pressure (P) must decrease.  So phy= sically lifting a pressure probe decreases the pressure at the probe. = In many (most) cases this is not too significant, i.e. at high pressures o= r for light fluids (P/rho is much larger than g*z).  But for heavy flu= ids (e.g. gasoline) at low pressures (2-3 psig), it can make a difference.&= nbsp; Obviously Sujit is looking for a big difference in pressure - 0 psi v= s. 2.7, in his original post, so holding the probe a few inches higher or l= ower does not explain his issue - it is more likely the probe is defective.=

But the basic idea of holding a tube 6' abov= e the fuel line is a valid and simple way to test.

------------------------------
=
Message: 8
Date: Mon, = 21 Oct 2019 21:02:15 -0700
From: "Randall" <TR3= driver@ca.rr.com>
To: "'Michael Porter'" &= lt;m= dporter@dfn.com>, "'Sujit Roy'"
&nbs= p;  <triumphstag@gmail.com>, "'Triumphs'" <triumphs@a= utox.team.net>
Subject: Re: [TR] What is t= he correct way to measure fuel pressure
Message-I= D: <704EE09E9A4E4074BEB301067361C5FA@RYPC>
Content-Type: text/plain;    charset=3D= "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 val= ve 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.
<= /div>

True, but regardless of wh= ether there is atmosphere on top, the pressure
go= es down as you get higher along a column.  In effect, the weight of th= e
fuel (or whatever) inside the column is sitting= on top of the pressure at
the bottom, so pressur= e goes down as you go up.

> In a closed system, pressure equalizes at all points in the
> system, so,
> 2.7 ps= i at the pump would be 2.7 psi at the gauge.

=
I disagree.  If, for example, you set up the sy= stem 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 i= ncrease
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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 Current Thread [TR] What is the correct way to measure fuel pressure, Sujit Roy Re: [TR] What is the correct way to measure fuel pressure, Chad Re: [TR] What is the correct way to measure fuel pressure, Sujit Roy Re: [TR] What is the correct way to measure fuel pressure, Sujit Roy Re: [TR] What is the correct way to measure fuel pressure, Randall Re: [TR] What is the correct way to measure fuel pressure, Michael Porter Re: [TR] What is the correct way to measure fuel pressure, Randall Re: [TR] What is the correct way to measure fuel pressure, michael muller <=