The three numbers I check before anything else on every pool
Every pool I walk into — and I have been in hundreds — I check three numbers before I touch anything. Not the equipment room. Not the filters. Not the chlorine stock. Those three numbers.
They tell me more about the health of a pool in five minutes than an hour of equipment inspection. Because if these three numbers are wrong, everything else is just window dressing.
1. Saturation Index — the foundation everything else sits on
The Saturation Index (LSI — Langelier Saturation Index) measures whether your pool water is trying to dissolve calcium out of your plaster and metal surfaces, or whether it is trying to deposit calcium scale on your pipes and heat exchangers.
Water wants to be in balance. When it is not, it will take calcium from your surfaces or deposit it on them — and neither direction is good.
The LSI formula: LSI = pH + TF + CA + AL − 12.1 — where TF is water temperature factor, CA is calcium hardness factor, and AL is total alkalinity factor. Most test kits have an LSI chart or calculator built in.
What you are looking for:
- LSI between −0.3 and +0.3: balanced water
- LSI below −0.3: corrosive water, etching plaster, dissolving calcium from grout and metal
- LSI above +0.3: scale-forming water, calcium deposits on pipes, reduced heater efficiency
LSI is the number most operators never check. That is a mistake. If your LSI is off, you can dose chlorine and pH correctors every day and still be fighting a losing battle. The water is working against you.
Every week I get a service call from an operator wondering why their heater is failing or why their plaster is pitting. Almost every time, the LSI was out of range for months before the symptom showed up.
2. pH — the master variable
pH controls the effectiveness of almost every other chemical process in your pool. It is the most important single number on any chemistry log.
Why it matters:
- Chlorine effectiveness drops sharply as pH rises. At pH 7.2, chlorine is roughly 70% effective. At pH 7.8, it drops to about 40%. At pH 8.0, you are running at about 20% effectiveness for the same chlorine dose.
- Swimmer comfort deteriorates rapidly above pH 7.8 — eye irritation, skin dryness, that "chlorine smell" that is actually chloramines.
- pH below 7.2 corrodes metal equipment, etches plaster, and causes skin irritation in the other direction.
The target range: 7.4 – 7.6 for commercial pools. Most health codes allow 7.2 – 7.8, but the sweet spot is the middle — where chlorine works best and swimmers are most comfortable.
Most operators let pH drift high because they do not test frequently enough. A pool with moderate bather load and no automatic feeder will drift 0.2 – 0.4 pH points per week. You need to test at least twice a week to catch it in time.
When pH is high and chlorine demand seems excessive — that is not a chlorine problem. It is a pH problem. Get pH to 7.5 and watch chlorine demand drop.
3. Chlorine — whether it can actually do its job depends on the two numbers above
Chlorine is your sanitizer. But here is what most operators miss: chlorine is not a standalone number. Its ability to sanitize is completely dependent on what is happening with pH and LSI.
Free chlorine vs. combined chlorine: Free chlorine is the available chlorine ready to do work. Combined chlorine (chloramines) is what is left after chlorine has already been used. High combined chlorine means your free chlorine reserve is depleted and you are not getting adequate sanitization.
Calculate combined: Combined Chlorine = Total Chlorine − Free Chlorine. If combined is above 0.5 ppm, you need to shock. If it is above 1.0 ppm, swimmer comfort is already compromised.
Cyanuric acid (CYA) and chlorine effectiveness: CYA is added intentionally through dichlor and trichlor tablets to protect chlorine from UV degradation. But most operators never test CYA, and that creates a silent problem.
- 30 ppm CYA: chlorine effectiveness drops to about 50%
- 60 ppm CYA: chlorine effectiveness drops to about 25%
- 100 ppm CYA: chlorine effectiveness drops to about 10%
At 100 ppm CYA — the maximum allowed in most states for commercial pools — your test kit can show 2 ppm free chlorine while your actual available sanitizer is barely above the minimum safe threshold. You think you are covered. You are not.
The fix for high CYA is dilution. There is no chemical treatment that reduces CYA without replacing water.
The First Visit Order: LSI → pH → Chlorine
When I start with a new facility, I check them in exactly this order — not because chlorine is less important, but because if LSI is wrong, everything downstream is fighting uphill.
- Calculate LSI — is the water balanced, or is it trying to attack or deposit calcium?
- Test pH — is it in the 7.4 – 7.6 range? If not, chlorine effectiveness is compromised regardless of dose.
- Test free chlorine, total chlorine, and CYA — do the combined chlorine calculation, check if CYA is approaching 80 ppm or above 100 ppm.
Then look at chemistry logs going back three months. Patterns over time tell you more than a single reading.
Why the Order Matters
You can have perfect chlorine readings and still have a pool in trouble. If pH is 8.0, your chlorine is barely working. If LSI is off, your calcium is dissolving into the water or your pipes are getting scaled up.
These three numbers — Saturation Index, pH, Chlorine — in that order, are where every water chemistry conversation starts. Everything else is downstream.
Run them first.
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