<html><head></head><body><div class="ydp3c1e3c10yahoo-style-wrap" style="font-family: Helvetica Neue, Helvetica, Arial, sans-serif; font-size: 13px;"><div><div dir="ltr" data-setdir="false">Bernoulli's equation, for irrotational flow (i.e. incompressible and non-turbulent), states:</div><div dir="ltr" data-setdir="false"><br></div><div dir="ltr" data-setdir="false">P/rho + 1/2*v^2 + g*z = constant</div></div><div dir="ltr" data-setdir="false"><br></div><div dir="ltr" data-setdir="false">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. </div><div dir="ltr" data-setdir="false"><br></div><div dir="ltr" data-setdir="false">But the basic idea of holding a tube 6' above the fuel line is a valid and simple way to test.</div><div dir="ltr" data-setdir="false"><br></div></div><div id="ydpfe32bfb1yahoo_quoted_1885058362" class="ydpfe32bfb1yahoo_quoted"><div style="font-family:'Helvetica Neue', Helvetica, Arial, sans-serif;font-size:13px;color:#26282a;"><div><div dir="ltr">------------------------------<br></div><div dir="ltr"><br></div><div dir="ltr">Message: 8<br></div><div dir="ltr">Date: Mon, 21 Oct 2019 21:02:15 -0700<br></div><div dir="ltr">From: "Randall" <<a href="mailto:TR3driver@ca.rr.com" rel="nofollow" target="_blank">TR3driver@ca.rr.com</a>><br></div><div dir="ltr">To: "'Michael Porter'" <<a href="mailto:mdporter@dfn.com" rel="nofollow" target="_blank">mdporter@dfn.com</a>>, "'Sujit Roy'"<br></div><div dir="ltr"> <<a href="mailto:triumphstag@gmail.com" rel="nofollow" target="_blank">triumphstag@gmail.com</a>>, "'Triumphs'" <<a href="mailto:triumphs@autox.team.net" rel="nofollow" target="_blank">triumphs@autox.team.net</a>><br></div><div dir="ltr">Subject: Re: [TR] What is the correct way to measure fuel pressure<br></div><div dir="ltr">Message-ID: <<a href="mailto:704EE09E9A4E4074BEB301067361C5FA@RYPC" rel="nofollow" target="_blank">704EE09E9A4E4074BEB301067361C5FA@RYPC</a>><br></div><div dir="ltr">Content-Type: text/plain; charset="iso-8859-1"<br></div><div dir="ltr"><br></div><div dir="ltr"><br></div><div dir="ltr">> Umm, this might confuse a couple of principles.? Yes, lifting <br></div><div dir="ltr">> a column of liquid by atmospheric pressure limits the column <br></div><div dir="ltr">> to 34 feet (of water, or the equivalent), and the figures <br></div><div dir="ltr">> above reflect that for the lower density of gasoline.? But, <br></div><div dir="ltr">> the fuel system is not pumping against atmosphere.? It's a <br></div><div dir="ltr">> closed system (at least when the float valve is closed), and <br></div><div dir="ltr">> with the gauge directly connected to the fuel line, it's <br></div><div dir="ltr">> definitely a closed system from pump outlet to gauge.? And, <br></div><div dir="ltr">> by definition, the gauge reads PSIG, i.e., pressure above <br></div><div dir="ltr">> atmospheric, regardless of ambient air pressure.<br></div><div dir="ltr"><br></div><div dir="ltr">True, but regardless of whether there is atmosphere on top, the pressure<br></div><div dir="ltr">goes down as you get higher along a column. In effect, the weight of the<br></div><div dir="ltr">fuel (or whatever) inside the column is sitting on top of the pressure at<br></div><div dir="ltr">the bottom, so pressure goes down as you go up.<br></div><div dir="ltr"><br></div><div dir="ltr">> In a closed system, pressure equalizes at all points in the <br></div><div dir="ltr">> system, so,<br></div><div dir="ltr">> 2.7 psi at the pump would be 2.7 psi at the gauge.<br></div><div dir="ltr"><br></div><div dir="ltr">I disagree. If, for example, you set up the system I suggested with a 6'<br></div><div dir="ltr">length of pipe, 2 psi at the bottom and no fuel coming out the top; then you<br></div><div dir="ltr">plug the top with a pressure gauge, the pressure does not magically increase<br></div><div dir="ltr">just because there is a plug there. The gauge still reads 0 psig.<br></div><div dir="ltr"><br></div><div dir="ltr">Normally, this effect is too small to notice, because we work with much<br></div><div dir="ltr">higher pressure in hydraulic systems. 3 or 4 psi in a system that works at<br></div><div dir="ltr">hundreds of psi (like a clutch) doesn't make enough difference to notice.<br></div><div dir="ltr"><br></div><div dir="ltr">-- Randall<br></div><div dir="ltr"><br></div></div>
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