LSI Outside the Bubble — What the Scientists Really Say

Rudy does something different this week: he takes the Langelier Saturation Index (LSI) out of the pool industry echo chamber and walks it through the lens of academics, civil engineers, groundwater chemists, and industrial water experts who don’t care about plaster warranties, brand marketing, or trade show politics.

If you’ve ever argued about “which LSI is right” in a Facebook group, this one is for you.

Episode Overview

Rudy kicks off with a news analogy:
 How the same event looks completely different on U.S. news versus international coverage. Same facts, different framing.

He uses that setup to shift how we look at LSI:

• We already know the pool industry story about LSI: PHTA charts, app calculators, Orenda talks, trade folklore.
• But this episode asks:
   “What does the scientific community think about LSI when they’re not talking to pool people at all?”

Rudy dives into how universities and researchers actually use and define LSI in research on:

• Groundwater
• Drinking water stability
• Industrial cooling systems
• Desalination plants
• Boiler operations
• Cement leaching
• Environmental engineering

Key Concepts Covered

1. LSI as the Outside World Sees It

• Academics consistently define LSI = pH − pHs, where pHs is the calculated saturation pH, not something you measure with a test kit.
• pHs is derived from:
  • Calcium hardness
  • Carbonate alkalinity
  • Ionic strength (modeled via TDS / activity coefficients)
  • Temperature
• LSI is treated as a thermodynamic index:
  • It shows which direction water wants to move with respect to calcium carbonate.
  • It does not tell you how fast scale forms, how thick it gets, or how quickly surfaces dissolve.

2. What Negative, Zero, and Positive LSI Actually Mean (Academically)

• Negative LSI
  • Water is under-saturated in calcium carbonate.
  • It is capable of dissolving calcium carbonate if given the chance.
  • Researchers call it “aggressive” or “decalcifying” water—but they do not say it guarantees corrosion.
• LSI ≈

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Chapters

• 9:30 [Ad] Roof Top Innovations the leading Roofing Company in Waco, Tomball surrounding areas
• 10:16 (Cont.) LSI Outside the Bubble — What the Scientists Really Say

Transcript

Rudy Stankowitz (00:00)
Welcome to Friday. I'm Rudy Stankiewicz. This is the Talking Pools podcast. I hope you're doing well.

I'm thrilled that you've taken the time out of your day to chat with me.

I wanna do something a little bit different today. Probably different than you've heard anybody else do in the past.

just roll with me on this one. Picture this for a second. You're sitting in your living room somewhere in the United States, wherever it is that you live, watching one of the big news networks, pick your flavor, doesn't matter. They run a story about something happening here. Politics, protest, an environmental issue, whatever. Then later that night,

You happen to be overseas. I know it's a stretch, but that exact same story covered by an international outlet. Maybe it's BBC, it's Al Jazeera, maybe it's CNN International being broadcast to everyone except the United States. And suddenly you're looking at two versions of the same event. Same facts on the ground, same timestamp, same location.

but the framing, the emphasis, the tone, even what's left out versus what's highlighted, completely different. That's really what I wanna do with the LSI today. In our industry, you've heard my take on the LSI before, the Langelier Saturation Index, more than once. You've heard the PHTA's version in their manuals and courses. You probably even heard the episode,

where I was discussing LSI with Eric Knight of Watershape University back when he was with Arenda Vihassa and with Jared Morgan of Arenda Vihassa. And I know a lot of you have swapped opinions with other pool professionals and trade groups, forums, classes, whatever. Within that bubble, it's a lot like watching US-based news about the US. We're all repeating and remixing the same internal narrative, but

Here's the question that almost nobody asks. What does the scientific community think about LSI when they're not talking to poor people at all?

What does the LSI look like when it shows up in peer reviewed papers by civil engineers, environmental chemists, groundwater researchers, and university water treatment labs that don't care about plaster warranties, trade show booths, or which pool app you're using? That's the outside the country view, if you will. Same index, same chemistry, just different framing. And that's exactly what I want to talk about today.

I mean, you already know the industry story. You know how the PHA packages LSI. You've heard me rant about misuse and overuse. You've heard the or render crowd talk about balanced water with their own twist. Now we're going to step outside that echo chamber and look at how LSI is treated in research gate. Peer reviewed studies.

At major universities, we're going to treat LSI the way an academic does, as a thermodynamic index used in groundwater science, industrial cooling systems, desalination plants, cement leaching studies, and municipal water stability, not as a magic good-bad dial for swimming pools. And along the way, we're also going to address a point that confuses a lot of pros.

Why do different LSI formulas exist at all? You've probably noticed that not every chart or calculator gives you the same exact answer. One table uses one constant, another adjusts more heavily for TDS, another seems to exaggerate temperature effects. It's tempting to think someone must be wrong, but...

If you read the scientific literature, you find out that those differences usually come from how each formula approximates ionic strength, activity coefficients, and the equilibrium constants behind pH, ready? S, pH S.

Which, when it comes right down to it, PHS, is the formula, the calculation, that Wilfrid Langelier actually came up with. This thing we call LSI is a bastardized version of PHS that we chose to name after the guy who came up with PHS. So, in other words, the arguments you hear in pool groups about which LSI is correct

often people fighting over which shortcut they like while the underlying science is quietly explaining why multiple approximations can coexist. That's the mission today. We're to look at LSI the way the outside world looks at it. No marketing, no trade group spin, no brand agenda, just what the research community actually publishes about this index we've all been so obsessed with over the last decade.

can set your own opinion next to mine, next to PHTAs, next to our renders, next to WaterShape universities, next to what the Ivy League universities are saying and decide how you want to use LSI in your own practice going forward.

Because when you filter out marketing, rule of thumb folklore, and common practice shortcuts, you're left with the actual chemistry. And isn't that really what you'd like to examine? The Langelier Saturation Index has been around for, gosh, at least a century. It was developed in the 1930s, mentioned before by Dr. Wilfrid Langelier of the University of California at Berkeley, and it continues to appear in modern academic.

papers on groundwater chemistry, drinking water stability,

industrial cooling systems, boiler operations, corrosion, cement leaching, and environmental engineering. here's the key point. academics rarely use the LSI the way the pool and spa industry uses it.

their interpretation is more precise, more nuanced and rooted in thermodynamics, solubility theory and carbonate equilibria. So today's talk does not rely on trade magazine simplifications. This is the LSI as it actually exists in the academic world.

Why?

because we need to look at it from the outside to get the real picture because you deserve the truth. Across ResearchGate publications, LSI is consistently defined as a thermodynamic index, an estimate of water's saturation state with respect to calcium carbonate. The core equation is always the same. LSI equals the actual pH of the water minus the theoretical saturation pH called pH

Sub s or ph s the saturation pH is not a measured value It is calculated from the carbonate systems equilibrium relationships using calcium hardness carbonate alkalinity ionic strength and activity coefficients commonly approximated via TDS and temperature so every peer reviewed description boils down to this LSI equals pH minus pH s

That's it. That's the equation used outside the pool industry. This equation though hides a tremendous amount of complexity underneath it. Academic authors emphasize that LSI is not a measure of how much scale will form or how fast corrosion will happen. It is only a measure of the direction in which water wants to move in the calcium carbonate equilibrium system. Think of it this way. LSI tells you whether water wants to dissolve or deposit

calcium carbonate, but it doesn't tell you how aggressive that process will be, how thick scale will grow, or how much material will actually dissolve. Universities repeatedly stress one phrase, LSI indicates the driving force, not the outcome. This is extremely important because misinterpretation is rampant outside academic circles. Many industries, including ours, tend to treat LSI as a yes-no answer for balance.

LSI is always framed as a qualitative indicator, not a quantitative prediction tool.

In peer-reviewed papers, generally classify LSI values like this. A negative LSI means the water is under-saturated with respect to calcium carbonate. It will tend to dissolve calcium carbonate from the surfaces if given the opportunity. Academically, this condition is often referred to as aggressive water or corrosive water, we use the same terms, or water with a decalcifying tendency. But it is crucial to understand

that academic authors rarely claim a negative LSI guarantees corrosion. It only means the thermodynamics allow it. A zero LSI, or very close to zero, means the water is theoretically at equilibrium, neither dissolving nor depositing calcium carbonate. But equilibrium is a theoretical construct. Real water systems

rarely sit at perfect equilibrium because flow, turbulence, bather load, aeration, and chemical additions constantly disturb the system. Several university studies point out that even a water sample with an LSI value of zero can still show either slight scaling or slight dissolution depending on kinetic factors.

Positive LSI means water is supersaturated with calcium carbonate and has a tendency to form scale, but again this does not guarantee scale will actually deposit. It only means that forming scale would be thermodynamically favorable.

Some papers go further and provide classifications such as slight scaling tendency, scale forming, or strongly scale forming, but even in those papers the authors repeatedly caution that LSI cannot quantify the extent of scale deposition.

Rudy Stankowitz (18:42)
You ever have one of those pools where you just can't stop algae from coming back in that one spot over and over again, always in that one damn spot. The rest of the pool is fine, but it just creeps back so much so that, know, every time you're heading to that pool, you know, you're going to have to scrub that area to get rid of what started. That's not you. That's not on you.

Algae is opportunistic. There's a reason why people think, I killed it, but it came back, or I didn't kill it all because it came back. And that's not the truth. You probably did kill it all. It's just something about that spot makes it more conducive to colonization for different algae.

What is this telling me? most likely, you're dealing with a pool that was built wrong or you have badly pitted plaster in an area where it needs to be resurfaced. In either case, you have a dead spot in circulation, which means that water is not being circulated as frequently. The water is not moving as quickly. It's not being chemically treated as often as it should be. But that doesn't get you off the hook because you still have to deal with this mess. That's

exactly the type of pool that I want to add a Blu-ray XL to.

Yeah, add them to your other pools. Do it. Definitely. If it's a part of your protocol of care, nice. But if it's not, but you're thinking you might want to try it somewhere, get that pool with the dead spot in circulation. I don't care if it's in the step area, because we know the circulation always starts, it sucks there. That's where, you know, black algae always starts in those step areas. I don't care where it is. Go ahead, brush it. Add your Blu-ray XL.

And watch the magic happen. You should not have this headache again in that pool for six months. How freaking cool is that?

Rudy Stankowitz (20:34)
Some papers go further and provide classifications such as slight scaling tendency, scale forming, or strongly scale forming, but even in those papers the authors repeatedly caution that LSI cannot quantify the extent of scale deposition.

So the interpretation is directional, not absolute. The scientific literature treats LSI as a map showing which way the water is leaning, not a prophecy that must come true. One of the most consistent observations is that PHS, again, PHS, not LSI, must be calculated rather than measured. The equation,

for PHS in academic papers is typically derived from the calcium carbonate solubility product and acid-based equilibria of carbonate species. Different papers may present slightly different formatting, but the core components are always the same. calcium hardness, alkalinity, carbonate alkalinity, TDS or ionic strength and temperature.

The typical academic formula looks something like this. PHS equals PK2 minus PKSP plus PCA plus PALK plus constant, where PK is the second dissociation constant of carbonic acid. PKSP is the solubility product constant for calcium carbonate, and PCA and PALK are negative logs of calcium and carbonate alkalinity concentrations.

respectively.

accounting for ionic strength. That is where TDS enters the picture. And this is important because you'll see it is repeatedly highlighted that most trade literature glosses over

is that TDS does not directly change LSI. Instead, ionic strength influences activity coefficients, which alter the effective saturation pH. That means that two water samples with identical calcium and alkalinity values can have different LSI values if their dissolved solid content is different. University researchers routinely calculate PHS

using extensive tables of activity coefficients or software like PHREEQC, which models carbonate equilibria with much greater accuracy. In these simulations, PHS is not treated as a single number, but as a function of the chemical environment. This is a reoccurring theme in academic literature.

LSI is only as accurate as the precision of its input data and the accuracy of the Ionic Activity Model.

There's a lot of studies where LSI is calculated for real water, groundwater, drinking water, irrigation water, industrial cooling tower water, desalination plant output, geothermal water, natural lakes. These studies consistently show a surprising level of variability. Many natural water sources have LSI values ranging from negative 1.5 to positive 1.5, depending on the season, geological substrate, rainfall, and temperature swings.

In groundwater studies, water that originates from limestone aquifers often show positive LSI values because they are naturally saturated or supersaturated with respect to calcium carbonate. Surface waters, on the other hand, often show negative LSI values because they have lower alkalinity and lower mineral content. One particular study from Iran available on ResearchGate

analyzed dozens of water samples and found that average LSI values hovered somewhere around positive 0.18, which is slightly scale forming. Yet many water samples still had signs of corrosive behavior. This contradiction underscores one of the major conclusions the scientific community emphasizes, is that LSI alone cannot predict corrosion or scale formation. It only indicates the thermodynamic direction, period.

It's so bad.

Another study from Poland examined seasonal variations in lakes and found that LSI fluctuated dramatically throughout the year. Spring meltwater diluted mineral content drove LSI downward, while summer evaporation increased concentrations and actually pushed LSI upward. These seasonal shifts produced water that cycled between negative and positive LSI states without ever reaching equilibrium for any length of time.

Academic studies in the industrial water sector show that systems with turbulent flow, high velocity,

disturbance often fail to form protective scale films even when the LSI is slightly positive because shear forces prevent

stable deposition. Conversely, very slow moving waters can form thin protective films even when LSI is slightly negative. This is one of the most fundamental takeaways from the peer-reviewed literature. LSI is only one piece of a much larger puzzle and real-world outcomes depend heavily on kinetics, flow, turbulence, and surface materials.

You know, corrosion science is a whole field with a massive number of variables, surface chemistry, redox conditions, oxygen availability, dissolved oxygen, chloride concentration, sulfate ion concentration, carbon dioxide equilibrium, biofilm formation, hydraulics, temperature gradients, and more.

LSI does not include any of those variables directly, so academic authors repeatedly warn that it is an oversimplification to treat LSI as a corrosion index.

which is pretty much exactly what we do.

What does a negative LSI guarantee? Nothing. It guarantees the water is under saturated in calcium carbonate and therefore thermodynamically capable of dissolving calcium carbonate from concrete or calcium bearing minerals. But what it does not guarantee is that pipes will corrode, heaters will fail or metals will dissolve. Corrosion is an electrochemical process and is influenced

far more by the chloride concentration, dissolved oxygen and water velocity than it would ever be by LSI alone. explicitly say LSI, again, is not a corrosion index and should not be used as one. However, LSI does a good job predicting the dissolution of calcium carbonate scale and leaching of concrete. A paper on cement leaching demonstrated that's

Water with an LSI of negative 1.0 caused notable material loss in concrete samples, while water with an LSI of positive 0.5 caused very little. But even in this controlled study, kinetic factors such as agitation, flow, and temperature all alter the outcome.

Researchers emphasize that LSI describes the chemical environment, not the rate of dissolution or deposition.

So on the scaling side, papers examining industrial cooling towers, reverse osmosis membranes, geothermal wells, and potable systems. In these studies, water with significantly positive LSI values, somewhere between 1.0 and above, often produced notable scale formation under static or low turbulence conditions. However,

Under high flow rates or turbulent mechanical systems, scale formation did not correlate strongly with LSI values. Instead, scale deposition correlated with boundary layer stagnation, microenvironments with reduced velocity, and localized temperature differences. In reverse osmosis systems, water with high positive

was often kept from scaling by adding antiscalents or acidification.

because water couldn't form scale, but because operators intentionally suppressed its ability to do so. This, again, highlights the importance of underscoring LSI as a thermodynamic indicator rather than some standalone tool. One of the most interesting findings actually comes from geothermal system studies. Calcium carbonate scale formation accelerates in zones where water rapidly depressurizes or degasses

as carbon dioxide is removed from solution. pH increases, saturation pH drops, which instantly increases LSI. These changes occur too rapidly for manual testing to detect. Academic authors emphasize that these dynamic LSI shifts can only be understood using predictive modeling tools, not handheld tests.

So there is a ton, a ton of research on LSI available. And I'll tell you not much of it is on swimming pools, but the chemistry is identical. University studies on recreational water, mainly in Europe, confirm the same pattern seen in industrial and groundwater literature. Calcium carbonate equilibrium controls whether water deposits or dissolves calcium carbonate. But...

Other variables control corrosion and scale kinetics. Some academic papers critique the use of LSI in pools Because pools operate far outside the chemical equilibrium conditions Langelier originally assumed. Dr. Langelier's model was developed for municipal drinking water in unheated distribution lines, not for warm water, chlorinated water.

or conditions with massive aeration and mechanical turbulence. Because of that, researchers argue that LSI remains useful, but should be interpreted with caution. They note pool industry versions of LSI charts often oversimplify the chemistry by ignoring ionic activity models and

TDS approximations that may not match real activity coefficients.

the peer-reviewed recommendation is not to abandon LSI, but rather to ensure it's used only in the capacity originally intended.

as a guide to calcium carbonate saturation, not as a universal measure of water balance.

Every academic paper includes limitations. These are the major ones. LSI assumes a closed equilibrium system, but most real-world systems involve aeration, turbulence, pressure changes, and gas exchange. It assumes that calcium and carbonate alkalinity are the only scaling species, ignoring magnesium, sulfate, silicate, phosphate, and organic inhibitors.

It assumes that activity coefficients can be approximated from TDS, but high TDS waters with diverse ionic species may deviate significantly. It does not account for chloride or sulfate ions, which dramatically influence corrosion. It does not consider redox chemistry, disinfectants, chloramines, or oxidizers. And finally, it does not model kinetics.

This means water can be super saturated without forming scale if flow velocities are high and can be under saturated without dissolving surfaces if protective films are present. These limitations do not mean LSI is useless, only that it must be interpreted as one tool among many, a small piece of the big puzzle, not the whole freaking forest.

Despite its limitations, university studies consistently highlight three areas where LSI is extremely powerful. First, LSI accurately predicts whether water will dissolve or deposit calcium carbonate in steady state conditions. In laboratory experiments where temperature, flow, and chemistry are controlled, LSI correlates tightly with the formation or dissolution of calcium carbonate films. Secondly,

is extremely useful in

studies. Several academic papers document that water with moderately negative LSI values

calcium hydroxide and

LSI plays an important role in predicting scaling potential in systems with stable temperatures and laminate flows such as water storage tanks, passive plumbing sections and geothermal pipes. In short, it's an excellent equilibrium indicator, just not a comprehensive water behavior model. So based on the scientific consensus,

LSI should be used to understand calcium carbonate saturation state, but not a measure of corrosion or safety. Keeping LSI near zero is useful, but zero does not guarantee neutrality, nor does slight deviation guarantee problems. Kinetic conditions, flow, turbulence, pressure, gas exchange override LSI predictions. Chloride concentration, sulfate levels, disinfectant type, temperature shocks, and real-world hydraulics must

be evaluated alongside LSI. And more importantly, academic researchers consistently recommend pairing LSI with additional indices such as the Reissner Stability Index or even better, full carbonate modeling tools like PHREEQC. The most accurate interpretation of LSI from the scientific world is this. LSI is the starting point for understanding water stability, not

the final answer.

It is one of the oldest and most widely studied water indices in existence, but peer-reviewed literature makes it clear its value lies in its role as a thermodynamic indicator of calcium carbon equilibrium, not as a universal gauge of balance or safety. LSI is a powerful and scientifically grounded tool. But when used simplistically, like we do in the pool industry,

it becomes misleading. The academic world treats LSI with nuance, precision, and respect. And that's exactly how we should use it in the field because when you understand what the index is telling you and what it doesn't tell you, more importantly, you're operating in alignment with nearly a century of scientific research.

That's all I have for you for this week.

Next week, gonna be a surprise. It will be something chemical.

It will be real talk.

and it will be with me on Friday.

So until next time, be good.

Be safe.