Tank Chart Builder

How to build a tank chart

You need two numbers off the tank monitor before you start: the tank diameter in inches, and the total capacity in gallons.

1. Enter the diameter.
2. Enter the total capacity.
3. Enter your best estimate of the tank length in feet.
4. Press Recalculate.
5. Look at the last row of the chart. Adjust the length and recalculate again until that last row reaches both the full tank diameter and the total gallons. Fit length to capacity does this in one step.

When the bottom of the chart lands on the diameter and the capacity together, the chart is fitted to your tank.

Why the length has to be adjusted

The chart is built from a perfect cylinder. A real underground tank has dished heads, so the ends bulge outward and hold volume a straight cylinder does not. Stretching the length is how you make the cylinder hold the same gallons as the real tank.

That gets the total and the overfill heights right. It also means the middle of the chart can drift from the manufacturer's strapping chart, most noticeably toward the ends. Treat this as a working field chart, and verify against the manufacturer's chart before anything goes on a compliance record. For published manufacturer charts, see the tank charts at USTex Guide.

About the 95 and 90 percent heights

Those two follow the diameter alone and will not move when you change the length. That is correct, not a fault. Volume is cross sectional area times length, so the length cancels out when you ask what height holds 95 percent of the volume. On a horizontal cylinder, 95% of the volume sits at roughly 90% of the diameter, not 95% of it, which is why the overfill shutoff point has to be read from a chart.