2.4 Problem water & chemical-adjustment calculations
Key Takeaways
- Dosing charts are written per 10,000 gallons; scale each dose by desired ppm ÷ chart ppm and by gallons ÷ 10,000.
- Soda ash raises pH; muriatic acid or dry acid lowers it; sodium bicarbonate raises alkalinity; calcium chloride raises hardness.
- Adjust total alkalinity before pH because alkalinity buffers pH; always add chemical to water and never mix products.
- Superchlorinate to breakpoint (about 10× combined chlorine) and shock in the evening to avoid sunlight loss.
- Colored water usually means dissolved metals; a strong chlorine smell means chloramines, cured by breakpoint chlorination, not less chlorine.
Dosing by the 10,000-Gallon Rule
Every chemical adjustment starts with two numbers: the pool volume and the desired change. Dosing charts are written per 10,000 gallons for a stated ppm change, so the working method is always the same:
- Find the chart dose (amount per 10,000 gallons for a given ppm change).
- Scale it for your actual change (multiply by desired ppm ÷ chart ppm).
- Scale it for your actual volume (multiply by gallons ÷ 10,000).
dose = chart amount × (desired ppm ÷ chart ppm) × (gallons ÷ 10,000)
Estimating Pool Volume
Every dose depends on knowing the gallons, and volume is rarely printed on a plaque. For a rectangular pool, volume (gallons) = length × width × average depth × 7.48, because one cubic foot of water holds about 7.48 gallons. Average depth is (shallow depth + deep depth) ÷ 2. For example, a pool 40 ft long and 20 ft wide with depths of 3 ft and 9 ft has an average depth of (3 + 9) ÷ 2 = 6 ft, so volume = 40 × 20 × 6 × 7.48 = 35,904 gallons, which you round to about 36,000 gallons for dosing. Round volume estimates conservatively so you dose gradually and retest rather than overshoot a target.
Which Chemical Does What
| Goal | Chemical | Notes |
|---|---|---|
| Raise pH | Soda ash (sodium carbonate) | Also nudges alkalinity up |
| Lower pH | Muriatic acid or sodium bisulfate (dry acid) | Add acid to water, never water to acid |
| Raise total alkalinity | Sodium bicarbonate (baking soda) | ~1.5 lb / 10,000 gal raises TA ~10 ppm |
| Raise calcium hardness | Calcium chloride | ~1.25 lb / 10,000 gal raises CH ~10 ppm |
| Raise cyanuric acid | Cyanuric acid (stabilizer) | Dissolves slowly |
| Raise free chlorine / shock | Cal-hypo, liquid, dichlor | Dose to breakpoint or ~10 ppm |
Worked example — raising alkalinity
A 30,000-gallon pool tests total alkalinity 70 ppm; you want 100 ppm, a rise of 30 ppm. Sodium bicarbonate raises TA about 10 ppm per 1.5 lb per 10,000 gallons.
- Scale for the change: 30 ppm ÷ 10 ppm = 3, so 3 × 1.5 lb = 4.5 lb per 10,000 gallons.
- Scale for the volume: 30,000 ÷ 10,000 = 3, so 4.5 lb × 3 = 13.5 lb of sodium bicarbonate.
Add it in portions with the pump running, retest after a full turnover, and top up if short rather than overshooting.
Worked example — lowering pH with acid
Suppose the 36,000-gallon pool above reads pH 8.0 with total alkalinity near 120 ppm, and you want pH 7.5. Muriatic acid is dosed from a chart by volume, based on both the pH drop and the alkalinity. A typical chart might call for roughly 1 quart of 31% muriatic acid per 10,000 gallons to move pH about 0.2 unit at that alkalinity, so a full 0.5-unit correction across 36,000 gallons is several quarts. Because acid also lowers alkalinity, recheck both readings after a turnover and add in small steps — never pour a full calculated dose at once into a busy pool.
Worked example — superchlorination (shock)
A 20,000-gallon pool has combined chlorine 0.5 ppm, so breakpoint needs a free-chlorine rise of about 10 × 0.5 = 5 ppm. Suppose a cal-hypo product raises FC about 1 ppm per 1.25 lb per 10,000 gallons. For a 5 ppm rise you need 5 × 1.25 = 6.25 lb per 10,000 gallons, and for 20,000 gallons that is 6.25 × 2 = 12.5 lb of cal-hypo. Shock in the evening so sunlight does not burn off the chlorine before it can work.
Order of Operations
Adjust total alkalinity first, then pH, because alkalinity buffers pH — correcting pH first tends to drift once alkalinity is changed. Add soda ash to raise a low pH, sodium bicarbonate to raise low alkalinity, and acid to lower a high pH or alkalinity. Always add chemicals to water, in solution where the label directs, and never mix products together.
Troubleshooting Problem Water
| Problem | Likely cause | Fix |
|---|---|---|
| Cloudy / hazy water | Poor filtration, high pH, high CH, imbalance | Check filter and flow, balance pH and LSI, use clarifier |
| Green water, slippery walls (algae) | Low free chlorine, high CYA, poor circulation | Superchlorinate to breakpoint, brush, add algaecide |
| Strong "chlorine" odor, eye/skin irritation | Combined chlorine (chloramines), low pH | Breakpoint chlorinate, correct pH to 7.4-7.6 |
| Water turns green/brown after chlorinating | Dissolved metals (iron, copper, manganese) | Add sequestering agent, balance pH, avoid over-chlorinating |
| Scale on tile / heater | High LSI (high pH, CH, TA) | Lower pH and TA, rebalance toward LSI 0 |
| Etched plaster, corroded metal | Low LSI (aggressive water) | Raise pH, CH, and TA toward balance |
| Stains on surfaces | Metals or scale | Sequestrant for metals; balance the water |
Key Habits
Balance the water to an LSI near zero, keep free chlorine in range with pH at 7.4-7.6, and treat every dosing decision as arithmetic: known volume, known target, charted dose, scaled twice. Retest after a turnover before adding more chemical. Remember the two diagnostic shortcuts that appear again and again: colored water almost always means dissolved metals, and a strong chlorine smell almost always means chloramines — whose cure is more chlorine dosed to breakpoint, not less.
A 30,000-gallon pool has total alkalinity 70 ppm and you want 100 ppm. Sodium bicarbonate raises TA about 10 ppm per 1.5 lb per 10,000 gallons. How much do you add?
In what order should you adjust total alkalinity and pH, and why?
A pool turns green-brown shortly after you add chlorine, but no algae is present. What is the most likely cause and fix?