Fuel Level float accuracy

RV-7 with the float style gauges wired into an AFS4500.

The fuel level does not seem accurate until down to about 12 gallons. It will show full and then all of a sudden drop to 15. I have a fuel totalizer that is cross checks with and I get an audible to check fuel level.

I am currently installing hotel whiskey tanks and have the main tanks out. Is there something I can do to the floats/calibration to fix this problem?

The totalizer appears to be very accurate.

Floats will not measure fuel above about 15 gallons as you've noticed, due to the dihedral of the wings. Since the floats are in the inboard part of the wing they will stay at the top of the tank (giving you the "full" reading) until the fuel level burns down to about 15 gallons.

Kyle,
I have an RV7A with the GRT EIS 4000. It measures fuel two ways: (1) by fuel flow, and (2) by fuel tank float position. My RV?s float system is really accurate between 2 to 17.5 gallons with the first 3 to 4 gallons burned in each tank and the last two gallons measured very accurately by the fuel totalizer (fuel flow computation). The ?fuel flow? measurement is the most accurate for determining total fuel burned and is automatically subtracted from the initial fuel setting by the EIS and displayed as ?Total Fuel?.

I had to go through an extensive calibration process on each wing tank recording raw EIS SF and auxSF readings for every one gallon change. I did this process as I both drained and then filled my tanks to ensure I had reliable and comparable measurements. I then use GRT?s formula to compute the required EIS settings. Having gone through the process several times until I got it right, I can tell you RV fuel tank float measuring systems work quite well when properly calibrated in an RV7A. I use the float system primarily to insure aircraft lateral balance but it agrees real well with my Total Fuel reading within the 2 to 17.5 gallon range per tank.

Both measurement systems together (since they work independently) keep me informed of how well I?m managing fuel.

"Float" and "Accuracy" are not normally words that collide in the same sentence......

IMHO, by far the best way to know how much fuel you have on board is to know what you started with and how fast you are burning it - especially with EFIS. I have floats and VANS steam gauges as a cross-check and to guard against a fuel leak/refuelling error/EFIS failure. On my RV-10, they don't really make much sense until I get below 15usg in a tank. Below 10usg they become really quite accurate - which means that I would have at least 20usg if/when I became aware of a problem.

I have the floats and a red cube. I love this set up because it offers a bit of redundancy/cross check of fuel level. The red cube is very accurate... usually within a gallon on fill ups. While flying along on a cross county, the floats give me, (once below the 15 gallon mark), a second source of a fuel level check to verify the accuracy of the red cube. The red cube gives me the courage to press on a little if needed as long as the flows/fuel remaining are verified by the floats. I use to switch tanks every 5 gallons. I've changed that to a much simpler system now. I do an 8-8-8-8 which is roughly 3 1/2 to 4 hrs with about 5 left per tank. By now, I need to make a head call anyway.