Episode 2 · April 28, 2026
The 7.5% Rule — The Most Important Number You’ve Never Heard Of
Solo Teaching · Target Runtime: 42–49 minutes
Episode Overview
Your test strip shows chlorine. The water looks clear. But there’s a question most pool owners have never been asked — and the answer might change how you manage your pool from this day forward.
In this episode, John Cooper breaks down the relationship between cyanuric acid and free chlorine: what it is, why it matters, and the single ratio that determines whether your chlorine is actually working. It’s called the 7.5% Rule, and it’s the foundation of everything that follows in this show.
In This Episode
John starts with a question worth sitting with: your test shows chlorine present — but is that reading actually telling you anything useful? He walks through the scenario that exists in a surprising percentage of outdoor pools right now, and names the one chemical at the center of it.
Before the problem comes the chemistry. John builds the case for why cyanuric acid belongs in your pool — the UV degradation math, the genuine protection it provides, and the tradeoff hiding inside that benefit. CYA isn’t the villain. Understanding it is the job.
Then the rule itself: clear, concrete, and built on real numbers. John works through CYA levels from 30 ppm to 100 ppm, calculates the minimum free chlorine floor at each level, and explains why the guidance printed on most chlorine packaging isn’t enough. He covers trichlor’s role in the accumulation story, why dilution is the only fix, and how salt pool owners fit into the same equation.
The episode closes with a look at what failure actually looks like — and why you can’t always see it. A pool can be visually perfect and chemically compromised at the same time. John explains invisible failure, gives you three specific actions to take this week, and plants a flag on a future episode about something most pool owners have never considered: running a pool with little or no CYA at all.
What You’ll Take Away
You’ll leave this episode knowing your actual free chlorine target — not the number on the packaging, but the number calculated for your specific pool. And you’ll know exactly what to do if your pool isn’t there yet.
The 7.5% Rule — Reference Table
FC minimum = CYA × 0.075. These are floors, not targets.
| CYA Level | Minimum Free Chlorine | Status |
|---|---|---|
| 30 ppm | 2.25 ppm | Acceptable |
| 40 ppm | 3.0 ppm | Acceptable |
| 50 ppm | 3.75 ppm | Ceiling — monitor closely |
| 60 ppm | 4.5 ppm | Above ceiling — plan partial drain |
| 70 ppm | 5.25 ppm | Above ceiling — act now |
| 80 ppm | 6.0 ppm | Above ceiling — partial drain required |
| 100 ppm | 7.5 ppm | Critical — immediate action |
CYA standard: 0–20 ppm preferred · 0–30 ppm acceptable · 50 ppm absolute ceiling, never exceeded. At 0 ppm CYA, maintain FC at 0.5–2.0 ppm.
Resources Mentioned
- Pools Scientific ebook series — Volume 2 covers water balance, CYA, and LSI in full detail: payhip.com/poolsscientific
- Show notes, tools & free resources: poolsscientific.com
- Weekly episode emails — subscribe free at poolsscientific.com
- Instagram: @poolsscientific24
Next Episode — May 5
Ep. 3: LSI Explained — The Langelier Saturation Index Without the Math Headache
Your pool water is either slowly dissolving your plaster or slowly building scale on your equipment. The LSI is the number that tells you which — and every pool owner should know how to read it. Next Tuesday, John makes the math approachable and the stakes clear.
About the Host
John Cooper holds an M.S. in Nuclear Engineering, a B.S. in Chemical Engineering, served as a U.S. Navy Nuclear Operator on submarines, and holds five PHTA certifications: CPO, CPI, CMS, CST, and CSP. He is the founder of Pools Scientific LLC and has managed pool chemistry across thousands of pools throughout his career.
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8. Schedule — Set to publish Tuesday, April 28, 2026 at 6:00 AM CT — same time as the Buzzsprout release.