Fuel calibration

[wombat]

Since I bought my plane last year I've known that the fuel gauges were off by quite a bit.  For example when the gauges read 10 gallons, I can fly for about 75 more minutes, burning 20 GPH.    Also, they never measure more than 37.5 gallons per side, and the actual total is 55 per side.

I've got an EI MVP-50, which has fuel gauges that can be calibrated.

So I took some time today (3 hours) and did it.   I learned that the fuel pump at S35 will only accept any given card twice in one day.  Fortunately I found a friendly local who was willing to let me pump gas on his card and I could just pay him in cash.   And the weather was amazing, about 70F and sunny with a light breeze.

 

What I did was I had a 5 gallon gas can, and I would fill it from the pump, dump it in, then refill it.   Then I'd wait a while for the gauges to read right, and for fuel to flow between the main/aux tanks, then record what the sight gauge said, what the engine monitor said, and the depth with a dipstick.   Then I'd dump in the next 5 gallons, wait, and do the same thing.

Some top-line notes:

• The monroy long-range tanks in my plane are supposed to hold 55 gallons.    I got 54.6 gallons in, after letting it burble for 10 minutes or so.   After flying back home on the other tank, I'm sure I can get in at least another half gallon.    But this is the first time I'd tried to get it ALL the way full and measure exactly how much I put in.
• The tops of the fuel tank fillers are about 4.75" different in height.   So for the last 4.75" of fuel in the long range tanks, you need to keep the caps on the mains.   The flapper will keep fuel from gushing out, but it does leak past it and will eventually overflow if you don't put the actual cap on and seal it.
• The bottoms of the fuel tanks at the inboard edge of the filler are about 8" different in height.   So I'll have 8" of fuel in the mains before the dipstick in the aux tank even gets wet.

Here are my notes from empty and at each 5 gallon interval.

 

At empty, the engine monitor reads 176 for its raw value.

 

• @5 gallons
  • Engine monitor reads 425
  • The dipstick is barely wet
  • I forgot to look at the sight gauge
  • The aux tank is dry
• @ 10 gallons
  • Engine monitor reads 691
  • The dipstick reads 3"
  • Wing sight gauge accurate
  • The Aux tank is dry
• @15 gallons
  • Engine monitor reads 870
  • The dipstick reads 5.5"
  • Wing sight gauge accurate
  • The Aux tank is dry
• @20 gallons
  • Engine monitor reads 984
  • The dipstick reads 7"
  • Wing sight gauge accurate
  • The Aux tank is dry
• @25 gallons
  • Engine monitor reads 1070
  • The dipstick reads 8.25"
  • Wing sight gauge reads 22.5
  • The Aux tank is just barely wet under the filler
• @30 gallons
  • Engine monitor reads 1488
  • The dipstick reads 9.5"
  • Wing sight gauge reads 25
  • 2" on the dipstick aux tank
• @35 gallons
  • Engine monitor reads 1640
  • The dipstick reads 10.5"
  • Wing sight gauge reads 28
  • The Aux tank reads 3.5"
• @40 gallons
  • Engine monitor reads 1785
  • Sight gauge over 30 with the back end pointing at 13.5
  • I forgot to get dipstick measurements
• @45 gallons
  • Engine monitor reads 1930
  • The dipstick reads 14", or just above the flapper
  • Sight gauge reads opposite 15, or bottom left of the letter "L"
  • The Aux tank reads 6.25"
• @50 gallons
  • It took a minute or two let air burble out in order to fit 50 gallons in.
  • I forgot to get a engine monitor reading.
  • The main will overflow if the lid isn't locked
  • The sight gauge points at the bottom left of the letter "L"
  • The aux tank was completely full for a minute or two.  I didn't use the dipstick again before adding more fuel, so I have no reading.
• @54.6 gallons
  • It took multiple small additions and about 10 minutes to get the fuel in
  • Engine monitor reads 2160
  • My wife was calling me asking where I was.  I'd already missed my previous meeting by more than 2 hours. 

After I flew back home (20 minutes from engine start to shutdown, including 5 minute cool-down) which I did exclusively on the other tank, I had maybe 1" to 2" of room in the aux tank to add more fuel.

[Pinecone]

I wrote about doing this for mine plane in Mooney Flyer. Present Position (themooneyflyer.com)

Got 5 5 gallon jugs.  I dumped the first one in, figuring under 5 gallons would not be measurable (I did not calibrate my electric gauges).

Then I dumped half of one into the empty so did increments of 2.5 gallons.

My 252 with Monroy are supposed to hold 52 per side.  I managed to get 55 into the right tank from sputtering.

[Fly Boomer]

10 hours ago, wombat said:

Also, they never measure more than 37.5 gallons per side, and the actual total is 55 per side.

If you are talking about the Rocket, I don’t believe the aux tanks have fuel senders, so starting from full, you have to burn off quite a bit before the senders can start to come down.  I think usable in the mains is 37.8, and 14.5 in the aux (each side).

[wombat]

18 minutes ago, Fly Boomer said:

  I think usable in the mains is 37.8, and 14.5 in the aux (each side).

I don't actually remember if the original number on my engine monitor was 37.5 or 37.8   That's less than 1 minute of fuel at cruise power which is a finer distinction than I need to make with how I plan flights. 

Since there is no way to separate the tanks, starting somewhere between 20 and 25 gallons, fuel starts filling the aux.     

While the sight gauge on the L tank stops increasing somewhere between 40 and 45 gallons, the engine monitor shows different readings above 45 gallons.

At cruise power (20 GPH) I am burning 0.1 gallons every 18 seconds.

[Pinecone]

That is why I did a calibration table for the sight gauges and sticking both fuel caps.  

[wombat]

1 hour ago, Pinecone said:

That is why I did a calibration table for the sight gauges and sticking both fuel caps.  

That's basically what I was doing too.

Here are the graphs of the measurements.   The engine monitor values are on the right axis, all the others are on the left, but be aware that the blue (Sight gauge) measurement is in gallons and the dipsticks are in inches.

[PT20J]

If you have a G3X and are absolutely anal about fuel quantity measurements,  the G3X allows storing two calibration curves; one in flight attitude and a second at ground attitude. This is primarily for tail wheel airplanes, but Mooneys sit about 5 degrees nose up on the ground, so there is a small difference. 

[Yetti]

I recently found the fuel gauge table in the Dynon Sky view is editable.   Using my wood paint stir calibrated stick and the current voltage and the stick I am making the measurements pretty darn accurate.