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The most misunderstood chemistry in the pool business


Calcium impacts all types of pool surfaces

Pick your surface, fielder’s choice: plaster, pebble, tile grout, fiberglass/gelcoat, or vinyl. What do they all have in common?  They are all—without exception—dramatically impacted by the calcium hardness level in the water.  This article will attempt to explain the urgent need to understand and manage proper levels of calcium hardness in our pools.

Having begun my career in the pool service industry in 1984, I have a confession to make. Call it an admission of guilt. A common belief–which I shared–was “I do pool repair, not chemicals”.  This is more common than not, so let’s explore why it’s a problem. How many of us pool repair people even bothered to carry a pool chemical test kit on our truck? Not me. How about calcium chloride? Nope, me neither.

Alright, so you say “That is not true! I have a 4 in 1 test kit to check the pH.” And maybe if you’re feeling generous, you checked the chlorine and alkalinity too. Well guess what: when you discovered a hole in the heat exchanger, if your instant conclusion was just “low pH”, think again. I used to think that, and I was painfully mistaken. At the expense of my customers.

Water chemistry is very dynamic and constantly changing.

The measurement you get at one moment may have nothing to do with the current condition of your surface or equipment. It is more likely that a water balance problem presented itself over a long period of time; not the moment you are testing the pool. Furthermore, the most important overriding chemical question was “what was the LSI reading?

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Simply raising the calcium changes the LSI significantly.

On this illustration of our LSI Calculator App, notice the only difference in the chemistry between the right and left levels is the calcium level. All other measurements are identical. The difference between a pool that is corrosive or not simply relies on a higher calcium hardness level.

By the way, the aggressive reading on the left of the screen has a industry-acceptable level of 200ppm Calcium Hardness. And we all thought we were within range.

Another benefit and point of fact about calcium hardness is that it is very slow to change, and less dynamic than pH or alkalinity.  What would you rather control first? A chemistry that fluctuates by literally minutes or hours (pH and alkalinity)? Or one that is very stable and slow to change (calcium hardness)? In my opinion, it’s a very easy decision.

LSI first. Range chemistry second.

So why does the industry generally regard calcium as a detrimental chemistry? We instantly assume it will cause scaling at most levels. Why? Because we have been taught to think so. But in reality, the truth about scale and corrosion lies with the balance of the LSI, not just calcium. In my view, the LSI is the primary measure of balanced chemistry, not range chemistry.


This is a very critical point, so let me repeat myself. LSI first. Range chemistry second.

How may times have you heard someone say “But my pool chemistry is good”?  Perhaps the chemistry is in proper ranges acceptable to the industry. But as the earlier screenshot shows, so much for “good pool chemistry”.  Bye bye to the plaster and equipment over time. By simply elevating the calcium level we can protect equipment and surfaces.

Water stops at nothing to find balance

So you say to yourself “But Harold, I treat vinyl liner/fiberglass pools…the surface is not at risk.” Wrong. Both surfaces are impacted identically to a plaster pool with one exception. Neither vinyl nor fiberglass has available calcium to donate to the water like a plaster pool. That is to say, in calcium deficient water, a plaster surface gives up necessary surface calcium to attain equilibrium with the water (causing premature surface failure).

So yes, liner and fiberglass pools may not etch the way plaster does, but they will degrade.

Equally important to note: surface degradation is gradual and will escape visual evidence for quite some time…perhaps leading you to believe there is no damage taking place.

Low calcium water in vinyl or gelcoat/fiberglass surfaces continues to create more corrosive water. There is no mechanism to improve the buffering power of calcium and will therefore cause plastic to become more brittle and gelcoat to chalk and degrade or etch. Truth be known, vinyl liner and fiberglass pools are likely to decline faster in a low-calcium environment.

Is high calcium hardness always a problem? No.

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If other factors are adjusted properly, 500ppm calcium hardness is easily managed, without scale formation.

Look at the screenshot of our LSI calculator. I show you this to burst the myth that 500ppm calcium hardness is always scale forming, and therefore a problem. We have been taught for so long that we must drain the pool to correct high calcium hardness. I argue that’s not necessary.

My question to the industry is a simple one: do you take issue with the LSI or not?

The LSI standard is cut and dried. So if the LSI is balanced—which by the way, at 500ppm on our calculations, it is—what are we to think?

If the LSI is the Gold Standard for corrosion and scale formation it seems to me it should trump a range guess of balanced water.

Pool chemistry and balance is always changing

Remember when I stated previously that water chemistry is dynamic and constantly changing? Take that into account in this conversation…the LSI can change day to day, week to week. Manage the water chemistry accordingly. One can assume in the course of a week, the pH in a plaster pool will rise, and in vinyl or fiberglass pools the pH will drop in most cases.

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Most of the etching and corrosion that takes place in the pool occurs during the winter. Why? Temperature is a critical factor in the LSI.

Lastly be VERY AWARE of the water temperature! Water temperature has a dramatic impact on the LSI. This will be discussed in a later article at length. It was a painful epiphany for me as a service professional of 30+ years. Play around with the temperature dials on our app and brace yourself for some brutal truth.

Conclusion: Calcium Hardness is your friend

In conclusion, I hope this information has been helpful. I encourage you to use our Orenda App to always test and measure LSI balance. Never guess, or base your actions on old habits.  Lastly, maintain a minimum of 200ppm calcium within 24 hours of filling a pool, and use the great asset of calcium hardness as your friend—not your enemy.  Help me change the negative view of calcium hardness levels and use it to our advantage as the positive factor it is.  Together we can change the way the world treats and thinks about water.

Harold Evans began his pool service/repair company, The Pool Surgeon, in 1984. Frustrated with problems he could continue to fix–but not prevent–he searched for a water chemistry solution to resolve many of the problems he faced as a pool professional. Harold is now the CEO/owner of Orenda Technologies, and focuses on educating pool professionals on how to make the swimming pool safer, long-lasting and enjoyable for all.

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