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Thread: Fuel Pressure sensor versus fuel flow sensor

  1. #1
    Senior Member
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    Default Fuel Pressure sensor versus fuel flow sensor

    Just curious to know what others are using/doing......

    In my situation, I have the Rotax 912 ULS (carbs) with the the fuel return line. It seems to me that this return line will screw up both pressure and flow readings?

    I like the idea of knowing the fuel pressure but from reading comments on here, it seems that the fuel pressure fluctuations are common with the Rotax and more so now with the return line.....


    So do most people use pressure or flow, or nothing?


    I have the MGL fuel flow sender that came with my used Oddessey and it looks "cheesy" so I didn't install it. I like the idea of all AN fittings and the thought of having to "hose clamp" in this plastic turbine sensor bothers me.

    I have a back up Facet pump and my thought would be to kick it on if the engine acts weird and find a place to land. Just kind of wondering, since this would be my goto anyway, and probably the "goto" if you did get a low fuel pressure warning from a sensor, is it even worth bothering with either?

    Curious to know what others have done....Thanks

  2. #2
    Senior Member jiott's Avatar
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    Default Re: Fuel Pressure sensor versus fuel flow sensor

    I fly the 912uls, carbureted with a return line, and have both a fuel pressure sensor and a fuel flow (red cube) sensor with no problems.
    First of all, if your return line has the Rotax specified 0.014" orifice restrictor it will definitely not affect fuel pressure to any measurable degree because that size restrictor limits fuel return flow to about 0.5 gal/hour, which is a very small flow rate compared to what the engine is consuming. If your return flow uses a larger orifice and it reduces the fuel pressure, this means that the combined fuel flow is way to close to the fuel pump maximum capacity and you are asking for trouble if the pump flow capacity reduces due to aging and wear.
    Secondly, with the small 0.014" orifice the very small 0.5 gal/hr bypass flow rate is easily calibrated out of the much larger engine consumption flow rate by tweaking the "K" factor in the red cube setup procedure. By doing this you don't need two fuel sensors, and after a little adjustment of the "K" factor your fuel flow accuracy will be fine. I have been flying this way for about 9 years, 1200 hours. I know there are folks who strongly disagree with me on this, so make up your own mind; I ran this by the factory tech guys at EPI who make the red cube and they agreed that what I was doing was acceptable.
    Jim Ott
    Portland, OR
    Kitfox SS7 flying
    Rotax 912ULS

  3. #3
    Senior Member Delta Whisky's Avatar
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    Default Re: Fuel Pressure sensor versus fuel flow sensor

    As Jim said (or sort of said) - to each his own. I don't disagree with him - I just think of the hundreds if not thousands of high wing aircraft flying with nothing except a fuel line in and figure if it was good enough for them, it is good enough for me. (No fuel pressure or flow measuring.) I do check fuel burn rate at each fill up and verify fuel flow via a timed test at each condition inspection.

  4. #4
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    Default Re: Fuel Pressure sensor versus fuel flow sensor

    Thanks all.

    Appreciate the comments.

  5. #5

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    Default Re: Fuel Pressure sensor versus fuel flow sensor

    @Delta Curious where you do that test? Do you pull a hose at the carb, mech fuel pump, gascolator, somewhere else?
    Brian Cass
    KF Model IV Speedster
    Rotax 912UL

  6. #6
    Senior Member Delta Whisky's Avatar
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    Default Re: Fuel Pressure sensor versus fuel flow sensor

    I remove the fuel hose on the "in" side of the fuel pump. It it is a little tricky to catch the fuel coming out at the level of the pump but it can be done. Also, (because this is a year-to-year comparative test) to compare apples-to-apples the fuel level in the tanks has to be pretty much the same during each test.

  7. #7

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    Default Re: Fuel Pressure sensor versus fuel flow sensor

    Thanks! That makes perfect sense.
    Brian Cass
    KF Model IV Speedster
    Rotax 912UL

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