Fuel tank calibration - measured

[meridian]

has anyone calibrated their 36-DC fuel tanks. If so, do you have the measurements? ie site tube showing 14" from bottom of tank equals x gallons left. I know the tanks are irregular in shape.

[psneeld]

it's pretty simple math if you have the dimensions of the tank...the hardest part to measure is the triangular section at the bottom of the tank.

[jleonard]

And even if you can't measure "exactly" who really cares? It will be close enough for practical purposes.
I have twin tanks and I can use a "calibrated dowel" on one of the tanks. 1/4,1/2,3/4,full are the only marks I need. Then I figure that percent of the 360 total capacity.
I can "guestimate" the amount left, or the amount I can take on within 10% which is more than close enough for cruising.
By the way my stick always agrees with the float type gage that I have on one tank.

[meridian]

So I got the measurements for the tanks and did the following calculations. I used 4" increments although you could use any you want. The tank is 60" wide, 36" deep, and 24" tall in the front and 12" in the back from the top of the tank then slanted to reach the bottom of the tank in front. All numbers are approximate but the calculations come out close to 150 gallons.

Starting from the top of the tank

in/gal used/gal remaining
24 0 150
20 33 116
16 66 83
12 99 50
8 127 22
4 143 6
0 bottom 150 0

I'll put the numbers on a plastic strip and post it next to the sight glass. Remenber, this is just one tank, be sure to multilpy by two.


Please let me know if my calcs are off. I had to dig out my high school trig.

[DougSea]

Hi Terry,

So I was sitting in my hotel room and decided to engage my brain a bit. I broke the problem in half. Effectively you have a retangular tank measuring 36x60x12 sitting on top of a right triangular tank also measuring 60x36x12.

First came the easy math. The "upper" tank holds 25,920 cu in, or 112.208 gallons. The "lower" tank holds half of that, 12,960 cu in, or 56.104 gallons.

I broke out my Google-fu and found a right triangle area calculator, used the big triangle (12x36) to figure out one of the other angles (18.4 degrees and 71.6 degrees if you're interested) and then figured out the areas of my decreasingly sized triangles.

My results, expressed as the total remaining, are:

24" - 168.313
20" - 130.910
16" - 93.507
12" - 56.104
8" - 24.987
4" - 6.234

All in US gallons.

The areas I got for the lower triangles were 216, 96.2, and 24 sq inches respectively.

[DougSea]

Here's my "worksheet"

Tank Calculations.jpeg

[RobS]

So Terry's number have a little cushion. Doug: What, no bar at the hotel?

[DougSea]

So Terry's number have a little cushion. Doug: What, no bar at the hotel?

Yup! And it's good (with fuel) to know you erred on the conservative side! (Of course this means you then trust MY numbers as well...)

And of course there's a bar ...but if I spent all of my free time there it would be a problem. I'm already over 120 nights this year...

[RobS]

Ouch, you spend more on hotel rooms and diesel fuel than most people make in a year!

[meridian]

don't forget the thickness of the tank walls

[DougSea]

don't forget the thickness of the tank walls

You mean you didn't give me the interior dimensions?? Next you'll tell me the corners are radiused!

[jleonard]

Doug, did you subtract the volume of the pick up tubes, and add the volume of the sight tubes?
Barometric pressure? Temperature?

[DougSea]

Doug, did you subtract the volume of the pick up tubes, and add the volume of the sight tubes?
Barometric pressure? Temperature?

Crap! More math!!

[RobS]

Next time just go to the hotel bar...

[meridian]

you're makin my head hurt!