> The question is - how do the tech inspectors for the IBA> figure out how much
> gas a tank will hold? Weight? Volume? Chicken entrails?
>
If the aux tank is a simple shape, I measure the external dimensions, make a conservatively low estimate of the wall thickness and then calculate the volume based on the interior dimensions for length times width times height. The volume in cubic inches divided by 231 is the number of gallons.
If the estimated aux tank volume based on estimated interior dimensions cause the total volume to exceed 11.5, or if the shape of the tank is too complex for a volume calculation, then I determine the capacity using the following steps:
1. empty the aux tank or determine that is empty.
2. fill a 5+ gallon container with gasoline (I also fill a second container with the same fuel if the aux cell is expected to be larger than 5 gallons).
3. pour a sample of the gasoline into a clear plastic graduate and measure its specific gravity with a high precision hydrometer (a device that looks like a big glass thermometer that floats in the gasoline). (How much of the hydrometer floats above the surface of the gasoline is a function of the density/specific gravity of the gasoline. The portion of the hydrometer that floats above the liquid surface is marked with a scale. The "specific gravity" of the gasoline is its density realative to water; e.g., a gasoline with a specific gravity of 0.720 weights 72% as much as water.)
4. pour the sample back into the 5 gallon container.
5. weight the full 5-gallon container with an electronic scale that is accurate to about 0.01 pounds.
6. fill the aux tank.
7. weight the empty/partially empty 5-gallon container again.
8. determine the weight of gasoline it took to fill the cell by subtracting the weight measured in step 7 from the weight measured in step 5.
9. determine the density of the gasoline in pounds per gallon by multiplying the specific gravity determined in step 3 by 8.337 (which is the density of water in pounds per gallon at 60°F). (I don't need to know the temperature of the gasoline as long as the temperature doesn't change between the time I measure the specific gravity and the time I fill the cell. The hydrometer is calibrated to read specific gravity relative to water at 60°F.)
10. divide the weight calculated in step 8 by the gasoline density calculated in step 9.
The final calculation give me the volume of the aux tank in gallons.
Tom Austin