Beyond the Pool Store

How do you actually know your pool is safe? Not I think it's fine. Not it looks clear. Not I haven't heard anything's wrong. How do you know it? With a number. With a reason behind that number you could explain to somebody else if they asked. By the end of this episode, you're gonna understand something you didn't understand 30 minutes ago. And that understanding should outlast any rule I could give you.

Welcome to the Pools Scientific Podcast, elevated cool character through science and technology. I'm John Cooper, and whether you own a food, service them, or run a food company, this show is built for you. Real Chemistry, Real Answers every Tuesday.

Is my pool safe? It's a bigger question than it sounds like, because it's really three questions stacked on top of each other, and most people have only ever thought about one of them, if that. The first is whether it's actually disinfecting right now. Something gets into the water every single day, sweat, saliva, whatever a kid tracked in from the yard. And the question is whether your sanitizer is strong enough right now to deal with it. The second is whether it's safe for the pool itself, the plaster, the liner, the equipment. That's a slower, quieter danger. Water balance can spend months working against every surface it touches before any of it becomes visible.

The third is whether it's physically dangerous to be around. Drains, suction, barriers like fences. And there's a real code out there, the Model Aquatic Health Code, the industry's one attempt at a unified chemistry standard. It wasn't designed for and never applied to a residential backyard. It was for public and semi-public facilities, like your community pool or the local water park, and on purpose. So nobody's checking any of this for you, because the one real code that exists was never built with your pool in mind. Which means the only real standard that applies to your pool actually is the one you build yourself.

Question one is my pool sanitized? If you recall back in episode 12 when we first got into chloramines, this is where that comes back around. That sharp chemical smell at a pool gets blamed on too much chlorine every time. It's actually the opposite, and the chemistry behind why is quite interesting. Free chlorine, hypochlorous acid, reacts with nitrogen compounds the moment it finds them sweat, urea, sunscreen residue, anything with an available nitrogen atom. That reaction isn't a one-time event, it's a chain with three real steps and three real molecules. It's not a euphemism for used up chlorine. Step one, hypochlorous acid reacts with ammonia and forms monochloramine, and sanitizing power falls off a cliff, nowhere near what free chlorine does.

Step two, if more hypochlorous acid is available to react further, monochloramine becomes dichloramine. Sanitizing power drops again, even further, along with the first real appearance of that sharp smell and eye burn. Step three, dichloramine becomes trichloramine, otherwise known as nitrogen trichloride. It's functionally spent, with no meaningful disinfecting capacity left at all. And it's volatile, meaning it doesn't want to stay dissolved. It escapes the water into the air directly above the surface, concentrated right where a swimmer's face happens to be. That's the molecule behind what people call chlorine smell. It has been studied in indoor aquatic facility air quality research because it's a genuine respiratory irritant with real health consequences, not just an unpleasant odor.

Combined chlorine as a whole, all the chloramines together, sit roughly 25 to 100 times weaker at disinfecting than free chlorine. That's a huge gap, and I want the range stated as a range rather than a scary single number. A pool reading three parts per million total chlorine with most of it tied up as combined chlorine isn't quietly doing three parts per million of work. It's doing a small fraction of that, even though the number looks reassuring on the test strip. So that number in your hand actually matters. Total chlorine minus free chlorine gives you combined chlorine. Combined chlorine near zero and almost nothing's tied up in that weakened chain.

When combined starts to show up, you're watching real sanitizing capacity fall 25 to 100 times in real time. This is also why shocking a pool works the way it does. Adding enough chlorine, or roughly two to three times your combined chlorine reading and not the old 10x rule, doesn't just add more chloramines to the pile. Past a certain point, called the breakpoint, additional chlorine stops forming more chloramines and starts tearing them apart entirely, releasing the nitrogen as an inert gas that leaves the water. Chloramines go away, and you're back to full strength sanitization again. Knowing the chain exists and knowing how to manage the chloramine ladder tells you if your water's disinfecting.

It doesn't tell you how fast it's working, however, because not everything that ends up in your water dies at the same speed once chlorine gets to it. That's a real thing called contact time, and it's worth its own episode in the future. We'll get there.

Question two. Is my pool investment safe? The second question has an equally real mechanism underneath it, first worked out almost a century ago by an old friend of the show. In 1936, Wilfred Langalier, the same name behind the index we've referenced all the way back to episode three, was trying to solve a corrosion problem for municipal water utilities, pipes eating themselves from the inside, or scale building up enough to choke flow. He worked out a way to calculate whether a given water chemistry would dissolve calcium carbonate or deposit it, based on how saturated that water already was relative to its actual solubility limit. Decades later, the pool industry adapted his index almost unchanged because a swimming pool and a municipal pipe work from the same fundamental chemistry.

Calcium carbonate can only stay dissolved in tap water up to a certain concentration, and that limit shifts with pH, carbonate alkalinity, calcium level, and temperature, and to a smaller degree, total dissolved solids. LSI is a single number built from all five of those factors, and it tells you which side of that limit your water currently sits on. If the water's undersaturated, a negative LSI, it doesn't sit there quietly wanting more dissolved calcium carbonate. It aggressively pulls it from the nearest available surface, your plaster, your grout lines, any exposed metals like heaters and ladders. It's a real equilibrium correcting itself at your pool's direct expense. If the water is supersaturated instead, a positive LSI, it's holding more dissolved calcium carbonate than the current conditions can keep in solution, and the excess has to leave the water somehow.

It deposits everywhere, on a heater element, inside a filter, along a tile line, or wherever a surface offers it a place to land. That's scale, and it's the same equilibrium working in the opposite direction. Temperature's role in this is sharper than most people realize, and one of the pools that I managed proved it. A pool on my route measured at a minus 0.11 LSI under normal conditions, which is within range and safely balanced. Their auto cover was left closed during a heat wave, and the top several inches of water, which was insulated from evaporative cooling, climbed to roughly 120 Fahrenheit or about 49 Celsius. I burned my hand so I can confirm it.

At that temperature, the water's surface LSI crossed significantly positive to over 1.0, and active calcium carbonate precipitation was visibly happening in real time while water below the thermocline boundary stayed clear. Same pool, same day. Two completely different equilibrium states, driven entirely by temperature. Negative 0.3 to positive 0.3 on the LSI scale keeps that equilibrium close enough to neutral that neither direction wins in a meaningful way. Outside it, your water is actively pulling calcium out of your investment or actively dumping it back onto your equipment for as long as it stays out of range. And there's another, that's how it's always been done, hiding inside this whole discussion, one that has nothing to do with real chemistry and everything to do with habit.

Close the pool for the season. Forget about it until spring. That's what nearly everyone does, but the equilibrium we just walked through doesn't take the winter off. It's still active in that water every single day the pool sits closed, whether anyone's checking it or not. And falling water temperature actually makes it worse, not neutral. Lower temperature means a lower temperature factor in the LSI calculation, so as your water cools into winter, LSI drifts negative corrosive, even if nothing else about the water changes at all. A pool balanced perfectly at closing in October can and will be actively etching itself by December chemically, and then run the rest of the winter under those conditions with nobody there to notice until the cover comes off in spring and the damage is already done.

That's not a flaw in the pool. That's an entire industry convention that says walk away for six months. Don't worry about what the chemistry is actually doing the whole time nobody's looking. The answer to question two? No. Your investment is not safe without proper chemistry. I had a conversation a couple of weeks back with a customer, and it's the clearest example of everything this episode is actually about. Dan is a circle member. He's a good guy. I like him. I visit his pool once a month for a chemistry and equipment check. He's good at managing his own pool, so I just show up to keep a second eye on his numbers.

Dan's an engineer, and he has the same itch that I do, the one that can't leave that's just how it's done alone without asking why. He asked me a simple question. Why does everyone say you have to run cyaneuric acid in your pool? The mechanism behind that rule is one almost nobody's ever walked through. Cyanuric acid doesn't just sit next to your chlorine passively, it forms a real chemical equilibrium with it. Chlorinated cyanuric compounds holding a reserve of chlorine that continuously releases free hypochlorous acid as it's consumed, while keeping the concentration of unbound UV vulnerable hypochlorous acid low at any given instant. That's the actual reason CYA protects chlorine from sunlight, not a coating or a shield, an equilibrium that trades total chlorine concentration for stability against UV breakdown.

Murphy and his research team that did the research behind the 7.5% ratio itself established that roughly 0.05 parts per million of that active unbound hypochlorous acid is the real minimum needed for reliable disinfection. The number, the whole ratio exists to protect. Once you understand it that way, Dan's question answers itself. That entire mechanism exists to solve one specific problem. Chlorine getting destroyed by sunlight faster than a human can manually replace it by hand dosing. A properly sized salt cell doesn't have that problem. It's generating chlorine continuously, so whatever the sun burns off gets replaced within the same cycle rather than waiting for tomorrow's dose. The reservoir CYA provides isn't needed because there's never a supply gap for it to bridge.

I had asked myself the same question a few years ago, so I ran the experiment. 90% of the salt pools my team manages run zero CYA, going on two years now in full Kansas City sun. Outside of a handful of salt cell failures, not one of those pools has turned green, with some holding free chlorine as low as half a part per million. That's two years of field data, not a peer-reviewed study, and I won't dress it up as one, but over that time the number of green pools that experiment produced is zero. Dan didn't question the rule because rules are annoying. He asked what the rule was actually solving for, realizing his specific setup didn't have that problem, and applied a better answer to his own pool.

That's the method this episode has been teaching, applied out loud in front of you by somebody else who does it for a living. This lands somewhere concrete using the two things you just learned. Take the flat advice, keep your chlorine between one and three parts per million, apply it to a pool setting at 50 parts per million CYA near the ceiling this show teaches. Three parts per million passes that flat one to three rule easily. But run the actual ratio, 7.5% of 50, and the real number needed is closer to 3.5 to 4. That pool is underprotected while looking fine on the generic rule. Now apply that same flat rule to Dan's zero CYA salt pool holding half a part per million.

It fails the one to three rule outright, flagged as dangerously low, while the actual mechanism, continuous generation of replacing chlorine as fast as the sun removes it, says that the pool's been fine for two years. One flat number, two real pools, wrong in opposite directions on the exact same rule. LSI does the same thing to a fixed range. Take a pool sitting comfortably inside 80 to 120 parts per million alkalinity, textbook normal, checked once in the morning. Seal an auto cover over it during a heat wave, the same real case from a few minutes ago, and that same water's LSI swings from safely balanced to significantly positive by afternoon, actively scaling equipment without a single input number on the test kit changing except for temperature.

A rule checked once in the morning said everything was fine. The equilibrium underneath it had already moved. That's the standard you're actually building right now, and it's not a rule memorized once and repeated forever. Yours, built out of knowing instead of guessing, and understanding applied instead of instructions followed. That combination is what real confidence looks like, not the borrowed kind that comes from trusting somebody behind a counter or somebody else's authority. It's the earned kind of confidence that comes from understanding your own water well enough to trust yourself. No inspector walking through once a year can hand you that. No certification hanging on a wall can hand it to you either.

You earn it one episode at a time, one piece of real mechanism absorbed and applied to your specific pool. Once you have it, it's yours in a way no inspection ever could be. A written rule is a snapshot calibrated to whatever typical situation its author had in mind, and it has no way of knowing when your pool has quietly stepped outside that situation. It just keeps repeating the same instruction regardless. A mechanism doesn't work off of a snapshot. It recalculates the real answer for whatever's actually in front of it, whether that's a salt pool at zero CYA, a pool cooking under a sealed cover, or a closed pool drifting negative through the winter while nobody's watching.

So how do you know your pool is safe? A flat chlorine range would have called Dan's pool dangerous and called an underprotected 50 part per million CYA pool safe, both wrong on the same rule. You now know why. The chemistry of an equilibrium reserve releasing hypochlorous acid as it's consumed, not a number to memorize. A flat alkalinity range would have called a pool safe in the morning and missed it turning against its own equipment by afternoon under a sealed cover, or missed it quietly destroying itself all winter behind a closed cover nobody thought to check. You now know why. A solubility limit that moves with temperature, whether anyone's watching or not.

A rule tells you what to do and breaks the moment your pool doesn't match its assumptions. What you're building here recalculates the real answer for your specific water every time because it's made of mechanism instead of memorization. The next time somebody in your life gets a number back from a test strip and has no idea what it means, you won't need to look it up. You'll know why. Thank you for listening. See you next Tuesday. The Pools Scientific Podcast is an independent production by John Cooper and Pools Scientific LLC. Find us at poolsscientific.com. If this show has been useful to you, share it with someone who takes their pool seriously.

New episodes are every Tuesday.