Module 02
Water chemistry
Seven readings. You will never dose a pool, but you will explain these numbers to customers every week, so they need to be yours.
The seven readings
| Reading | Target | What it does |
|---|---|---|
| Free chlorine | 2 to 4 ppm | The sanitizer that is still available to kill things |
| Combined chlorine | Under 0.5 ppm | Chlorine already used up and now just smelling bad |
| pH | 7.4 to 7.6 | Decides how much of the chlorine actually works |
| Total alkalinity | 80 to 120 ppm | Holds pH steady so it stops bouncing |
| Calcium hardness | 200 to 400 ppm | Protects surfaces from being eaten or coated |
| Cyanuric acid | 30 to 50 ppm | Sunscreen for chlorine. 60 to 80 is common on salt pools |
| Phosphates | See below | Algae food, not an algae cause |
Salt pools carry one more, salt itself, at 2500 to 3500 ppm. Never quote a single salt number without knowing the brand, because manufacturers differ and the wrong figure can trigger cell warnings or corrosion.
pH is the one that matters most
If you only remember one thing from this module, remember this.
pH decides how much of the chlorine is actually working.
At pH 7.2, roughly two thirds of the chlorine is in its active form. At pH 7.8, that drops to about one third. The test shows the same chlorine number both times. The killing power is half.
This is why a customer can say "but my chlorine reads fine" and still have a green pool. The chlorine is there. It is just not working, because pH drifted up.
The chlorine reading on the test never changes as you drag. Only how much of it is able to work.
What high and low pH actually do
| Low pH, under 7.2 | High pH, over 7.8 | |
|---|---|---|
| Damage | Water turns corrosive. Etches plaster, eats grout, attacks metal parts | Scale on tile and inside the salt cell, cloudy water |
| Chlorine | More active, but the water is eating the pool | Only about a third active |
The chlorine smell is backwards
A customer calls and says the pool reeks of chlorine and their eyes are burning, so obviously there is too much chlorine.
It is almost always the opposite.
That smell is chloramines, which is chlorine that has already bonded to sweat, sunscreen, and organics and been used up. A strong chlorine smell means there is not enough free chlorine left to do the work.
This is exactly what the combined chlorine reading measures. Over 0.5 ppm means a shock is due.
Say that with confidence and you will sound like you have been doing this for years.
Stabilizer, and the trap inside it
Cyanuric acid is sunscreen for chlorine. Outdoors it is not optional.
Without stabilizer, direct summer sun destroys 75 to 90 percent of free chlorine within about two hours. You could dose a pool perfectly at nine in the morning and have almost nothing left by lunchtime.
But it cuts both ways.
Chlorine lock starts earlier than most people think.
The binding effect becomes significant at 80 to 100 ppm of cyanuric acid. At that point chlorine still shows up on the test but it cannot work. Dilution is the only fix, meaning a partial drain and refill, because cyanuric acid does not break down on its own.
Alkalinity, and why order matters
Alkalinity is what stops pH bouncing around. Too low and pH swings unpredictably. Too high and pH creeps up and resists every correction.
The target is 80 to 120 ppm, but aim at the lower end, 80 to 100, on salt pools, cal hypo, or liquid chlorine. Those sanitizers push pH up on their own, so starting low in the band keeps you ahead of it.
Alkalinity first, then pH, then sanitizer. Correcting out of order means correcting twice. A tech who fixes pH before alkalinity watches the pH drift straight back.
Calcium hardness
Calcium protects surfaces. Under 150 ppm the water gets hungry and starts pulling calcium out of plaster and grout. Over 500 ppm it starts depositing scale instead. Plaster pools want the middle of the 200 to 400 range.
Phosphates, and a conversation you will have often
Phosphates are algae food. Here is what makes them different from every other reading on this list.
Phosphates are never zero, and chasing zero is a waste of everyone time.
Municipal water suppliers add phosphate to drinking water on purpose, to stop pipes corroding and leaching lead. Federal rules put typical dosing between 500 and 3000 ppb. So a pool filled from a tap can start life already above what some charts call the ideal, and it can never get under it.
HOAs and municipalities are the source. Not the customer, and not a failure of service.
Palmway thresholds
| Reading | What happens |
|---|---|
| Above 1000 ppb | Monitor it |
| Above 2000 ppb | Treat it |
So phosphate treatment is real service when a reading justifies it, and an upsell when it does not. Never present phosphate removal as an algae cure.
What to hold on to
- pH decides how much chlorine works. Two thirds active at 7.2, one third at 7.8.
- Low pH corrodes. It does not sting eyes.
- Strong chlorine smell and burning eyes means too little free chlorine, not too much.
- No stabilizer outdoors means most of the chlorine is gone within about two hours.
- Chlorine lock begins at 80 to 100 ppm stabilizer, and only dilution fixes it.
- Alkalinity first, then pH, then sanitizer.
- Phosphates are never zero. Monitor above 1000, treat above 2000.
- Never advise dosing over the phone.
Knowledge check
Seven questions. You need six to pass. Wrong answers explain themselves, so read them.
1. A customer says the pool smells strongly of chlorine and their eyes are burning. What is actually happening?
2. Chlorine tests fine, but the pool keeps going green. Stabilizer reads 95 ppm. What is going on?
3. A pool reads good chlorine but pH has drifted to 7.8. What should you tell the customer?
4. A customer in a city with treated tap water is upset that their phosphates are not zero. What is true?
5. Which order do corrections go in?
6. A customer asks whether removing phosphates will get rid of their algae. What is the correct answer?
7. A customer asks you how much acid to add to bring their pH down. What do you do?