Pool Water Chemistry Basics
Understanding pool water chemistry is essential for maintaining a safe, clear, and comfortable swimming environment.
Quick Reference: Ideal Ranges
| Parameter | Ideal Range | Test Frequency |
|---|---|---|
| Free Chlorine | 2-4 ppm | 2-3x per week |
| pH | 7.4-7.6 | 2-3x per week |
| Total Alkalinity | 80-120 ppm | Weekly |
| Calcium Hardness | 200-400 ppm | Monthly |
| Cyanuric Acid | 30-50 ppm | Monthly |
Free Chlorine (FC)
Free chlorine is the active form of chlorine that sanitizes your pool by killing bacteria, viruses, and algae. It's called "free" because it's available to do its job, as opposed to combined chlorine which has already reacted with contaminants.
Too Low (below 1 ppm):
• Bacteria and algae can grow
• Pool becomes unsafe for swimming
• Water may turn cloudy or green
Too High (above 5 ppm):
• Eye and skin irritation
• Swimsuit fading
• Generally safe but uncomfortable
pH Level
pH measures how acidic or basic your pool water is on a scale of 0-14. A pH of 7 is neutral. Pool water should be slightly basic at 7.4-7.6, which matches human eye and skin pH for comfort.
Too Low (below 7.2):
• Water becomes corrosive
• Damages pool equipment and surfaces
• Eye and skin irritation
• Rapid chlorine loss
Too High (above 7.8):
• Chlorine becomes less effective
• Scale formation on surfaces
• Cloudy water
• Skin and eye irritation
Total Alkalinity (TA)
Total alkalinity is a measure of your water's ability to resist changes in pH. It acts as a "buffer" to prevent pH from bouncing around. Always adjust TA before adjusting pH.
Key relationship: High TA makes pH drift upward and harder to adjust down. Low TA causes "pH bounce" where pH changes rapidly.
Calcium Hardness (CH)
Calcium hardness measures dissolved calcium in your water. Proper calcium levels protect your pool surfaces from damage.
Plaster pools: Need 250-350 ppm to prevent calcium from being drawn out of the plaster.
Vinyl/Fiberglass: Can operate at lower levels (150-250 ppm).
Cyanuric Acid (CYA/Stabilizer)
CYA protects chlorine from being destroyed by UV light. Without stabilizer, sunlight can destroy 90% of free chlorine in just a few hours.
Important: CYA also "locks up" some chlorine, reducing its effectiveness. As CYA increases, you need higher chlorine levels. The FC to CYA ratio should be at least 7.5%.
Note: CYA can only be lowered by dilution (draining water).
Order of Adjustment
When multiple parameters are off, adjust them in this order for best results:
1. Total Alkalinity (affects pH stability)
2. pH (affects chlorine effectiveness)
3. Calcium Hardness (protects surfaces)
4. Cyanuric Acid (protects chlorine)
5. Chlorine (sanitization)