Here are the answers to practice problem No. 4. 

1. If the pump tank is 49 inches by 60 inches by 70 inches deep, calculate the gallons per inch.

Answer
Convert length and width to feet.

49 inches ÷ 12 inches/foot = 4.1 feet    

60 inches ÷ 12 inches/foot = 5 feet

Calculate volume (cubic feet) in 1 foot of depth

4.1 feet x 5 feet x 1.0 foot = 20.5 cubic feet

Convert to volume in gallons.

20.5 feet  x 7.5 gallons/cubic foot  = 153.75 gallons/foot

Calculate gallons per inch.

153.75 gallons/foot ÷ 12 inches/foot = 12.8 gallons/inch

2. The OFF float is at 62 inches and the ON float activation is at 57 inches. What is the dose volume?

Answer
Subtract OFF minus ON

62 inches – 57 inches = 5 inches

Multiply total inches by gallons per inch

5 inches x 12.8 gallons/inch = 64 gallons

3. If the supply line is 80 feet of 2-inch SCH 40 PVC, what is the drainback volume in gallons?

Answer
Calculate drainback. To do this requires knowledge of the gallons per foot in a 2-inch pipe. This can be obtained from tables available through pipe supply companies or calculating the volume of a cylinder by taking the area of a circle times the length of the pipe. It’s much easier to look it up on a table!

0.17 gallon/foot x 80 feet = 13.6 gallons

4. If the cycle counter reads 46,490 today and read 45,850 the last visit 120 days ago, what is the number of cycles recorded?

Answer
PCR – LCR = Number of cycles

46,490 – 45,850 = 640 cycles

5. What is the total flow and average flow for the 120 days?

Answer
Calculate total flow.

640 cycles x 64 gallons/cycle = 40,960 gallons

Calculate average daily flow.

40,960 gallons ÷ 120 days = 341 gallons/day

6. If the design flow was 300 gpd for the system, what does this tell you?

Answer
The system is exceeding design flow by 41 gpd. Over the entire 120 days, total average is 120 days x 41 gallons/day = 4,920 gallons of additional flow; the system will eventually break if this continues.

First check for leaks and extra water sources such as runoff inflow, etc., and if there are no plumbing leaks, look at flow reduction options or enlarging the system.

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