ArVeeNiner

ArVeeNiner

Well Known Member
I calibrated my capacitance fuel gauges before my first flight a few weeks ago. I also set up my fuel computer at the last fill up. Today, I wanted to see if can hang my hat on either one of these readings and it turns out that I can't.

I disconnected at the carb and ran each tank dry in a calibrated bucket to verify the readings. Here is what I found:

****
Fill up: 13.3 hours
Time at check: 19.0
Elapsed Time: 5.7 hours

Fuel computer reads: 2.0 gallons remaining

Left tank reads: 4.0 gallons
Actual left tank: 3.07 gallons
After 1 gallon drained, fuel quantity dropped from 4.0 to 1.8 gallons.
After 2 gallons drained, fuel quantity stayed at 1.8 gallons.
After 3 gallons drained, fuel quantity dropped to 0.0 gallons.
After an additional 16 ounces drained, fuel quantity stayed at 0.0 gallons.

Right tank reads: 4.0 gallons
Actual right tank: 2.27 gallons
After 1 gallon drained, fuel quantity dropped from 4.0 to 1.8 gallons.
After 2 gallons drained, fuel quantity dropped from 1.8 gallons to 1.2 gallons.
After an additional 40 ounces drained, fuel quantity stayed at 1.2 gallons.

Total actual fuel both tanks: 5.34 gallons.
****

My calibration seemed fine but I did get the warning about the readings not changing when they really did. Here are the numbers I got:

Left Tank

971
1011
1050
1092
1094
1135
1176
1218
1301
1428

Right Tank

994
1023
1056
1098
1133
1164
1191
1248
1307
1434

When graphed, the curves track pretty well except the left tank number are a bit low in a couple of areas.

So, the fuel computer does not equal the fuel gauges which do not equal the actual gallons in the tank.

I will recalibrate the tanks this weekend (even though I'm pretty sure I did it right) but I'm not sure what to do about the fuel flow computer. Any ideas would be appreciated. I'd like some piece of mind.
 
The fuel computer relies only on the fuel flow transducer and has nothing in common with the fuel gauges and their calibration. You must set the K factor for the fuel flow transducer and the number that Dynon gives you is only a starting point. Over several fill-ups you can see which way and how much to move the number. If you do it correctly, you will end up with a fuel computer that is very accurate and repeatable. Use the formula in the book to figure out how much to move the number till you get it close.

Sounds like you need to re-calibrate the tanks. You must follow the directions exactly to get em to work correctly. While I have never seen a 100% accurate fuel gauge in an aircraft, the capacitive ones work pretty well once setup correctly. They are not perfect and since I have a taildragger, they don't work very well on the ground since you calibrate em for in flight and the Dynon does not have dual calibration.

Here is my results from a few years ago now:


Brantel said:
Here are my results after calibrating a second time.

The left looks OK but the right bothers me. How can two tanks built exactly the same way be this different? There is no reason I can think of why that right tank has several points the same right in the middle of its range????

3518j2f.jpg


rjgm8i.jpg
Click to expand...
 
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Thanks Brantel.

I found the fuel flow tweek instructions in the manual. I didn't think to look there because I thought putting in the proper K-factor from the tag was good enough. Silly me was thinking some guy in a white lab coat carefully calibrated each and every unit to exact specifications. Wrong! I guess if that really happened, the cost would be much higher. It is probably more like iPhone in one hand, texting while yapping about the latest World of Warcraft spell he has invoked (sorry, I'm not a player. Do they even have spells?) as they did the "cal". :D

By the way, my graph is much closer than yours. I wish I knew how to post it. There is a slight variation at two points not not as extreme as yours. It's strange that two of your points seem like mirror images.

Anyway, I'll start dialing in my fuel flow this weekend.
 
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They actually do calibrate each flow sender, but that's at a specific flow rate with a specific install. If you run 18" of perfectly straight tubing to and from the flow sender, mount it perfectly horizontal and vertical, and fly at 30 GPH all the time, it will be really close. So will a stopwatch ;)

The install does make a few % difference in how the sender reacts, so that's why you almost always need to tweak from the published number. Once you do that it can be really close. Last time I filled up some tanks it said we burned 29.0 gallons and the fuel pump put in 29.042.

Also, be careful calibrating fuel flow with anything but normal flying. Pouring fuel into a bucket runs way higher fuel flow rates than the real plane and that has an effect. You really want to just fly the plane, keep a log of how much fuel you put in, and then when you have put in about 100 gallons, tweak the K factor. Don't chase it over one or two fill-ups.
 
dynonsupport said:
They actually do calibrate each flow sender, but that's at a specific flow rate with a specific install. If you run 18" of perfectly straight tubing to and from the flow sender, mount it perfectly horizontal and vertical, and fly at 30 GPH all the time, it will be really close. So will a stopwatch ;)

The install does make a few % difference in how the sender reacts, so that's why you almost always need to tweak from the published number. Once you do that it can be really close. Last time I filled up some tanks it said we burned 29.0 gallons and the fuel pump put in 29.042.

Also, be careful calibrating fuel flow with anything but normal flying. Pouring fuel into a bucket runs way higher fuel flow rates than the real plane and that has an effect. You really want to just fly the plane, keep a log of how much fuel you put in, and then when you have put in about 100 gallons, tweak the K factor. Don't chase it over one or two fill-ups.
Click to expand...

Aha! That all makes sense. I'll give it a try. Thanks!

By the way, just kidding about texting and WOW. ;-)
 
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