Pool Calculations and Formulas Flashcards

The standard weight of water used in pool chemical dosage calculations is:

Answer: 8.34 lbs per gallon

Water weighs 8.34 lbs per gallon, which is the standard value used when calculating parts per million in pool water.

Using the formula: lbs = (ppm × gallons × 8.34) / (% × 10,000), how many pounds of calcium hypochlorite (65% available chlorine) are needed to raise a 10,000-gallon pool by 2 ppm?

Answer: 0.26 lbs

lbs = (2 × 10,000 × 8.34) / (65 × 10,000) = 166,800 / 650,000 ≈ 0.26 lbs of cal hypo.

A pool has combined chlorine (chloramines) measuring 0.5 ppm. To achieve breakpoint chlorination, the free chlorine level must reach at least:

Answer: 5.0 ppm

Breakpoint chlorination requires free chlorine to reach 10 times the combined chlorine level: 10 × 0.5 = 5.0 ppm.

Which chemical is typically used to LOWER pH in a swimming pool?

Answer: Muriatic acid (hydrochloric acid)

Muriatic acid (HCl) or dry acid (sodium bisulfate) is used to lower pH because both are acidic compounds.

Sodium hypochlorite solution at 12.5% strength weighs 10.0 lbs per gallon. How many ounces of actual (available) chlorine are in 1 gallon?

Answer: 20 oz

12.5% of 10.0 lbs = 1.25 lbs of available chlorine × 16 oz/lb = 20 oz of available chlorine per gallon.

When adding sodium carbonate (soda ash) to raise pH, which secondary effect must the operator monitor?

Answer: It also raises total alkalinity

Soda ash raises both pH and total alkalinity simultaneously, so alkalinity must be monitored to avoid it exceeding 120 ppm.

A 20,000-gallon pool needs its free chlorine raised from 0 to 3 ppm using calcium hypochlorite (65% available Cl). Using the formula lbs = (ppm × gallons × 8.34) / (% × 10,000), approximately how many pounds are needed?

Answer: 0.77 lbs

lbs = (3 × 20,000 × 8.34) / (65 × 10,000) = 500,400 / 650,000 ≈ 0.77 lbs of calcium hypochlorite.