# Level switches in the fuel system for alarms - Yes, No, Why?

Discussion in 'Firewall Forward / Props / Fuel system' started by wsimpso1, Dec 14, 2018.

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1. Dec 15, 2018

### BJC

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lots of good discussion here. Two comments:

With thin, low dihedral, wings and multiple tanks, the venting system design is challenging and very important.

When my EFIS is powered up, a discrepancy (I don’t recall the triggering amount / percentage) between the total measured fuel in four tanks and the totalizer-computed fuel on board triggers an alarm to check fuel.

BJC

2. Dec 15, 2018

### Dan Thomas

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Large airplanes with big tanks that run a long way out in the wings will use multiple level sensors. The signals are summed for the gauges. If I were Bill I'd install three resistive sensors along the length of the tank, connect them in series, and feed that to a calibratable digital gauge.

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3. Dec 15, 2018

### rv6ejguy

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I took most of my flight training in PA-28-151s which is where my distrust of float types fuel gauges came from. Later flying a Grumman Tiger and my RV, same thing. They give you a rough guide of fuel remaining only and dance around pretty good in anything but glass smooth air. Certainly not like my BMWs which are accurate to less than a half liter and give you very accurate range remaining through the ECU.

Some time in PA-31Ps and King Airs and these had pretty accurate gauges which you could rely on.

Yes, it seems some pretty poor stuff made it through certification on light aircraft. Notice the regs don't specify accuracy of said gauges...

I only cared that my RV gauges read reasonably accurate at the 1/4 mark and they do when I tested them. I know I have 4.5 - 5 US gallons remaining there and I never feed fuel from them below that point. When flying though, I rarely look at them, just using time since TO to switch tanks and calculate rough fuel remaining.

4. Dec 16, 2018

### Dan Thomas

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The dancing fuel gauge is because the sensor is measuring fuel level in only one spot in the tank, and it isn't the middle. The middle is the most stable spot, but sensors are almost always at the lower end of the tank. Multiple sensors, wired in series, will cancel the sloshing out. If the fuel surges to one end of the tank and raises that float, the other end drops its float and the total resistance stays the same. Airplanes like the King Air will have multiple sensors. The Lockheed Electras I worked on had five per tank, and there were two tanks per wing. The whole wing, between the front and rear spars, from about five feet out from fuselage centerline, was tanks.

Cars put their sensors in the middle of the tank, and the tank isn't usually long and shallow and skinny like some aircraft tanks. I'd bet they also use damping circuitry to stabilize the gauge needle. Some old cars (60 and 70 years ago) used a thermally-driven gauge; its temperature was controlled by the tank sensor's resistance, and a bimetal strip moved the needle. Very stable. My '51 International had a thermal float sensor, too: the float moved a lever that drove a tiny cam that moved a metal reed toward a bimetal strip; the strip would heat and bend away from the reed and break the circuit. It thereby send an on-off signal to the gauge. When the tank was full the cam forced the reed harder against the bimetal strip and it had to heat longer before it would bend far enough to open the switch, and the gauge's strip got current longer, got hotter and the needle went up farther. Empty was the opposite.

5. Dec 16, 2018

### rv6ejguy

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BMWs have shallow, flat tanks as they are under the back seat which is why I used that example. Whatever they do, they work very well compared to light aircraft. Could be lots of digital averaging between the sensing unit and gauge via the ECU I'd guess.

6. Dec 16, 2018

### Toobuilder

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We should keep in mind that this is Bill's airplane and he says he has already installed capacitance senders in the wing tanks. That element of the design is locked down. The discussion concerns the header tank.

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7. Dec 16, 2018

### wsimpso1

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It is large enough to use as part of your enroute fuel. Sounds like another Princeton gage is appropriate.

8. Dec 16, 2018

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