Here are the answers to practice problem number three. 

1. If a pump tank has the following dimensions, 42 inches by 60 inches by 58 inches deep, what are the gallons per inch?

Answer
For gallons per inch we need to determine the volume in cubic feet of a 1-foot depth in the tank, divide by 12 to get the volume for an inch of depth and then multiply by 7.5 gallons per cubic foot to come up with the gallons per inch.

Remember you first have to convert the measurements in inches to feet.

So 42 inches ÷12 inches per foot = 3.5 feet, and 60 ÷ 12 = 5.0 feet.

We then calculate the volume: 3.5 feet x 5.0 feet x 1.0 feet/12 inches = 17.5 cubic feet/12 inches. Then we multiply by 7.5 gallons/cubic foot.

17.5 cubic feet ÷ 12 = 1.4 cubic feet/inch x 7.5 gallons/cubic foot = 10.9 gallons per inch since the cubic feet cancel and we are left with gallons per inch.

2. If an off float is set 52 inches from a surface reference point and the on float is activated at a depth of 46 inches, what is the dose volume?

Answer
If the floats are 6 inches apart then the dose volume is 6 inches x 10.9 gallons/inch; the inches cancel and we are left with 65.6 gallons for the dose volume.

3. From the reference point, where should the high-water alarm float be set?

Answer
This depends on the swing of float on the float tree but at minimum it is 3 inches, so from the reference point at the surface it would be 43 inches from the surface.

4. If there is 6 inches of sludge in the tank, what does this tell us?

Answer
If there is sludge in the tank, solids are getting through and the septic tank should be pumped more often. If there is scum in the pump tank it means there is something wrong with the outlet baffle of the septic tank.

This article is part of a series of practice problems for installers:

• Onsite Arithmetic: Is This Tank Large Enough?
• Onsite Arithmetic: Cylindrical Tanks
• Onsite Arithmetic: Setting Floats
• Onsite Arithmetic: Flow
• Onsite Arithmetic: Flow - Part 2
• Onsite Arithmetic: More O&M Practice
• Pointers for Onsite Arithmetic