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

From: Sujit Roy <[email-masked]>
Date: Mon, 21 Oct 2019 11:42:02 -0700

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I borrowed a Harbor Freight fuel pressure gauge from a fellow Triumph
owner. I disconnected the supply to the ZS carbs and connected the pressure
gauge directly to the feed.
The gauge read less than 1 PSIG. I have a Stag and it is using an SU
electric fuel pump.The stag pump should be around 3 psi.
Am I measuring the pump correctly? Should I Tee in the gauge inline with
the carbs instead?

Sujit

-- 
Sujit Roy
Cupertino, California

https://triumphstagblog.wordpress.com/

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<div dir=3D"ltr">I borrowed a Harbor Freight fuel=C2=A0pressure gauge from =
a fellow Triumph owner. I disconnected the supply to the ZS carbs and conne=
cted the pressure gauge directly to the feed.<div>The gauge read less than =
1 PSIG. I have a Stag and it is using an SU electric fuel=C2=A0pump.The sta=
g pump should=C2=A0be around 3 psi.=C2=A0</div><div>Am I measuring the pump=
 correctly? Should I Tee in the gauge inline with the carbs instead?</div><=
div><br></div><div>Sujit<br><div><br></div><div><br clear=3D"all"><div><br>=
</div>-- <br><div dir=3D"ltr" class=3D"gmail_signature" data-smartmail=3D"g=
mail_signature"><div dir=3D"ltr"><div>Sujit Roy<br>Cupertino, California</d=
iv><div><br><a href=3D"https://triumphstagblog.wordpress.com/" target=3D"_b=
lank">https://triumphstagblog.wordpress.com/</a><br><br></div></div></div><=
/div></div></div>

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** [email-masked] **

Archive: http://www.team.net/pipermail/triumphs  http://www.team.net/archive

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From: michael muller <[email-masked]>
Date: Tue, 22 Oct 2019 21:47:42 +0000 (UTC)

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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" <[email-masked]>
To: "'Michael Porter'" <[email-masked]>, "'Sujit Roy'"
=C2=A0=C2=A0=C2=A0 <[email-masked]>, "'Triumphs'" <[email-masked]=
am.net>
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

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<html><head></head><body><div class=3D"ydp3c1e3c10yahoo-style-wrap" style=
=3D"font-family: Helvetica Neue, Helvetica, Arial, sans-serif; font-size: 1=
3px;"><div><div dir=3D"ltr" data-setdir=3D"false">Bernoulli's equation, for=
 irrotational flow (i.e. incompressible and non-turbulent), states:</div><d=
iv dir=3D"ltr" data-setdir=3D"false"><br></div><div dir=3D"ltr" data-setdir=
=3D"false">P/rho + 1/2*v^2 + g*z =3D constant</div></div><div dir=3D"ltr" d=
ata-setdir=3D"false"><br></div><div dir=3D"ltr" data-setdir=3D"false">The h=
igher a fluid goes (bigger z), the pressure (P) must decrease.&nbsp; So phy=
sically lifting a pressure probe decreases the pressure at the probe.&nbsp;=
 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).&nbsp; 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.=
&nbsp;&nbsp;</div><div dir=3D"ltr" data-setdir=3D"false"><br></div><div dir=
=3D"ltr" data-setdir=3D"false">But the basic idea of holding a tube 6' abov=
e the fuel line is a valid and simple way to test.</div><div dir=3D"ltr" da=
ta-setdir=3D"false"><br></div></div><div id=3D"ydpfe32bfb1yahoo_quoted_1885=
058362" class=3D"ydpfe32bfb1yahoo_quoted"><div style=3D"font-family:'Helvet=
ica Neue', Helvetica, Arial, sans-serif;font-size:13px;color:#26282a;"><div=
><div dir=3D"ltr">------------------------------<br></div><div dir=3D"ltr">=
<br></div><div dir=3D"ltr">Message: 8<br></div><div dir=3D"ltr">Date: Mon, =
21 Oct 2019 21:02:15 -0700<br></div><div dir=3D"ltr">From: "Randall" &lt;<a=
 href=3D"mailto:[email-masked]" rel=3D"nofollow" target=3D"_blank">TR3=
[email-masked]</a>&gt;<br></div><div dir=3D"ltr">To: "'Michael Porter'" &=
lt;<a href=3D"mailto:[email-masked]" rel=3D"nofollow" target=3D"_blank">m=
[email-masked]</a>&gt;, "'Sujit Roy'"<br></div><div dir=3D"ltr">&nbsp;&nbs=
p;&nbsp; &lt;<a href=3D"mailto:[email-masked]" rel=3D"nofollow" targ=
et=3D"_blank">[email-masked]</a>&gt;, "'Triumphs'" &lt;<a href=3D"ma=
ilto:[email-masked]" rel=3D"nofollow" target=3D"_blank">triumphs@a=
utox.team.net</a>&gt;<br></div><div dir=3D"ltr">Subject: Re: [TR] What is t=
he correct way to measure fuel pressure<br></div><div dir=3D"ltr">Message-I=
D: &lt;<a href=3D"mailto:704EE09E9A4E4074BEB301067361C5FA@RYPC" rel=3D"nofo=
llow" target=3D"_blank">704EE09E9A4E4074BEB301067361C5FA@RYPC</a>&gt;<br></=
div><div dir=3D"ltr">Content-Type: text/plain;&nbsp;&nbsp;&nbsp; charset=3D=
"iso-8859-1"<br></div><div dir=3D"ltr"><br></div><div dir=3D"ltr"><br></div=
><div dir=3D"ltr">&gt; Umm, this might confuse a couple of principles.? Yes=
, lifting <br></div><div dir=3D"ltr">&gt; a column of liquid by atmospheric=
 pressure limits the column <br></div><div dir=3D"ltr">&gt; to 34 feet (of =
water, or the equivalent), and the figures <br></div><div dir=3D"ltr">&gt; =
above reflect that for the lower density of gasoline.? But, <br></div><div =
dir=3D"ltr">&gt; the fuel system is not pumping against atmosphere.? It's a=
 <br></div><div dir=3D"ltr">&gt; closed system (at least when the float val=
ve is closed), and <br></div><div dir=3D"ltr">&gt; with the gauge directly =
connected to the fuel line, it's <br></div><div dir=3D"ltr">&gt; definitely=
 a closed system from pump outlet to gauge.? And, <br></div><div dir=3D"ltr=
">&gt; by definition, the gauge reads PSIG, i.e., pressure above <br></div>=
<div dir=3D"ltr">&gt; atmospheric, regardless of ambient air pressure.<br><=
/div><div dir=3D"ltr"><br></div><div dir=3D"ltr">True, but regardless of wh=
ether there is atmosphere on top, the pressure<br></div><div dir=3D"ltr">go=
es down as you get higher along a column.&nbsp; In effect, the weight of th=
e<br></div><div dir=3D"ltr">fuel (or whatever) inside the column is sitting=
 on top of the pressure at<br></div><div dir=3D"ltr">the bottom, so pressur=
e goes down as you go up.<br></div><div dir=3D"ltr"><br></div><div dir=3D"l=
tr">&gt; In a closed system, pressure equalizes at all points in the <br></=
div><div dir=3D"ltr">&gt; system, so,<br></div><div dir=3D"ltr">&gt; 2.7 ps=
i at the pump would be 2.7 psi at the gauge.<br></div><div dir=3D"ltr"><br>=
</div><div dir=3D"ltr">I disagree.&nbsp; If, for example, you set up the sy=
stem I suggested with a 6'<br></div><div dir=3D"ltr">length of pipe, 2 psi =
at the bottom and no fuel coming out the top; then you<br></div><div dir=3D=
"ltr">plug the top with a pressure gauge, the pressure does not magically i=
ncrease<br></div><div dir=3D"ltr">just because there is a plug there.&nbsp;=
 The gauge still reads 0 psig.<br></div><div dir=3D"ltr"><br></div><div dir=
=3D"ltr">Normally, this effect is too small to notice, because we work with=
 much<br></div><div dir=3D"ltr">higher pressure in hydraulic systems.&nbsp;=
 3 or 4 psi in a system that works at<br></div><div dir=3D"ltr">hundreds of=
 psi (like a clutch) doesn't make enough difference to notice.<br></div><di=
v dir=3D"ltr"><br></div><div dir=3D"ltr">-- Randall<br></div><div dir=3D"lt=
r"><br></div></div>
            </div>
        </div></body></html>
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