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2-4 skinner artBy Kim Skinner

Decades ago, people in the pool industry started becoming aware that there was more to pool maintenance than just adding sanitizers to the water (to kill algae and bacteria) and filtering it (to help keep it crystal clear).  Gradually, we learned that even properly sanitized and filtered pool water could become unbalanced.  

Further, we figured out that unbalanced water could be either scale-forming, in which case a layer of calcium scale would form on every surface of a pool – or aggressive, where uniform etching (that is, dissolution) of quality pool plaster and other cementitious surfaces (including quartz and pebble) would be observed.  

As an industry, we needed a way to determine whether the water in our pools was potentially scale-forming or aggressive – and found the answer in what is known as the Langelier Saturation Index.  Originally published by Dr. W. F. Langelier in 1936, it has been improved through the years to produce results with greater accuracy and is now a familiar approach to water assessment both within and beyond the pool industry.

In past articles I’ve written for WaterShapes on a variety of subjects related to water chemistry, I have referred to the Saturation Index almost every time – but without explaining what it is.  In this article, I’ll fill that gap and dig into key bits of information everyone who works with water should have in mind from the design phase through to routine maintenance.

In his research, Langelier found that there were five key parameters involved in a body of water’s status – that is, pH, calcium hardness (CH), total alkalinity (TA), water temperature and total dissolved solids (TDS) – and that various relationships among these factors determined whether a pool’s water would be balanced, scale-forming, or aggressive.  (Note that a sanitizer residual plays no role in these calculations.)  

The pH value of the water, he observed, is perhaps the most important factor affecting the Saturation Index (SI), with alkalinity and calcium hardness following close behind.  This is so because, as pH lowers, the more aggressive the water becomes – or, in other words, the more likely it will be to dissolve calcium carbonate (a main ingredient in pool plaster) from a pool’s surface.  (Note that calcium hydroxide, another key plaster component, is not involved here.)

On the flip side, as the pH rises, the more scaling-forming the water potentially becomes – or, in other words, the more the water may precipitate calcium carbonate out of solution as a solid powder (such as plaster dust) or as uniform crystalline surface deposits.  

The same occurs with alkalinity and calcium hardness:  The lower they are, the more aggressive the water potentially becomes; the higher they are, the more scale-forming.

Water temperature follows a similar pattern:  The higher the temperature, the more scale-forming the water; the colder the water, the more aggressive it becomes.  With TDS, however, it’s just the opposite:  The higher the TDS, the more aggressive it makes the water (which is why TDS is a cause of concern in saltwater pools).

The key point to master here is that all of these individual water parameters or values can be offset by the other water parameters.  This is why referring to the water’s balance is so appropriate.  A low calcium hardness of 100 parts per million, for example, would by itself tend to make the water more aggressive, but it can be offset and neutralized by a high (or higher) pH, a higher alkalinity, or a higher water temperature.  Similarly, a high calcium level can be compensated for by a slightly lower level of pH and/or alkalinity.


If that all sounds complicated, that’s because it is – and that’s why Dr. Langelier’s equations are so handy:  All you need to do to work with the Saturation Index is plug in water-test values and, with a bit of figuring, derive a number that should hover in a range in the vicinity of 0.0.  

After years of conversation, the Association of Pool & Spa Professionals set limits to that range:  For water to be considered balanced, the value derived using the SI should fall between -0.3 to +0.5.  Let’s look at a few examples to show what that means and help us all see the sort of wiggle room that comes into play.

Let’s start with all-around “good” values:  If testing shows that the water has a pH of 7.6, TA at 100 ppm, CH at 250 ppm, TDS at 1,000 ppm, a temperature of 78 degrees Fahrenheit, and a cyanurate (stabilizer) level of 50 ppm, the SI of this water will be 0.0.

Basic Water Values
The Association of Pool & Spa Professionals (APSP) has established minimums and maximums for tested water values:  
pH:  7.2 to 7.8
Alkalinity:  60 to 180 ppm
Calcium hardness:  150 to 1,000 ppm
Water temperature:  Maximum 104 degrees F
TDS:  Maximum 1,500 ppm over start-up water level.  (This includes salt added for saltwater pools.  Thus, if 3,000 ppm of salt is added to tap water that contains, for example, TDS at 500 ppm, then the maximum would be 5,000 ppm.)
-- K.S.

Let’s look at what happens when a lower pH reading or a higher TA level is offset by the other water values:  If the pH checks in at 7.2, the TA at 140 ppm, the CH at 450 ppm, the TDS at 1,000 ppm, the water temperature at 78 degrees, and the cyanurate level at 90 ppm, the SI is still 0.0 and the water is in balance.

Again, look at how these figures offset one another:  If the pH is 8.0, the TA is 65 ppm, the CH is 200 ppm, the temperature is 78 degrees, the TDS is 3,000 ppm and the cyanurate level is 20 ppm, the SI is still 0.0.

Note that I introduced a sixth factor into the equation:  When pool water contains cyanuric acid as a chlorine stabilizer, one-third of the cyanurate content needs to be subtracted from the total alkalinity amount before any SI calculation is made.  Helpfully, there are online SI calculators that will determine SI and also make adjustments for you when cyanurate levels are known.  

When you consider all that’s going on in the water, plus and minus, it’s easy to start seeing value in automatic control systems that monitor water values and can be used to dose pool water with chemicals that keep things in balance.  As suggested above, this is why water values are important even to pool designers as they specify equipment sets and various other control systems:  Why put a high-quality plaster or any other cementitious surface at risk by leaving the water to balance itself, perhaps to the detriment of the finish?   


If there’s anything you take away from the test readings listed above, it’s that water is an amazingly dynamic medium and that it is always seeking balance.  This is why the Saturation Index has been mentioned in my past articles on plaster finishes and in discussions of the sorts of damage that can be inflicted on them with thoughtless start-up and maintenance procedures.  This is also why I think everyone involved in pool design, engineering and construction should know a thing or three about water chemistry, because what you don’t know in this respect can seriously compromise the finished product – and client satisfaction.

It’s a bit of an exaggeration, but it might be helpful to look at a swimming pool as a dynamic, ever-changing cauldron.  That is, if a pool’s water is either aggressive or scale-forming right now, it may not stay that way for very long:  When pool water has high calcium hardness and total alkalinity levels, for instance, this causes calcium to precipitate out as calcium carbonate scale, and the resulting loss of calcium carbonate from solution will cause the calcium hardness and alkalinity of the pool water to lower, thereby making the water more balanced and less likely to continue scaling.

By the same token, if aggressive water etches, dissolves and solubilizes some of the plaster surface (in the form of calcium carbonate), the calcium hardness and alkalinity of the pool water will increase – thereby edging the water toward balance – and it will stop being as aggressive as it once was.  Therefore, unbalanced and aggressive water can correct itself and become balanced, but only at the expense of dissolving some of the plaster surface.

Want SI Help?

At onBalance, which is dedicated to studying and understanding factors involved in managing pool-water chemistry, we’ve developed an SI calculator that’s available on our website.  For information, go to  (You may need to download some software to gain access.)

The calculator is also available as a mobile-phone app.  It costs a couple bucks, but it makes on-the-road calculations much easier.  For details, visit

-- K.S.

It’s a feast-or-famine cycle, and there’s clear merit to trying to avoid the out-of-range conditions that cause all the damage.

Consider as well that the pH in most pools rises during the hours and days that pass after a watershape has been chemically treated and adjusted by a technician on a visitation basis.  In these cases, that rise in pH will make the water less aggressive or more potentially scale-forming as time passes.  Those who maintain the pool water should take the above information into consideration for proper water balancing and understand what can occur between chemical treatments.  (On the design side, this might be part of a discussion about including automatic control systems in the equipment package.)

The good news is that research has shown that the water can move off 0.0 in that range (cited above) of -0.3 to +0.5 without causing any damage to a pool’s plaster, quartz or pebble finish.  While perfection is a worthy goal, in other words, it’s not a minute-by-minute necessity:  There is some room to maneuver in working things out.

It’s a complex situation, and the Langelier Saturation Index is the key to understanding and keeping all of the factors involved in water treatment in focus and sorted out.  In my book, that’s huge.

Kim Skinner began his work in the pool industry 45 years ago, starting as an employee and eventually becoming manager of Skinner Swim Pool Plastering in Sun Valley, Calif.  He later became president of Pool Chlor, a chemical service firm with offices throughout the Southwest.  He is also a partner in onBalance, a consulting firm that performs both laboratory and field research on pool-water chemistry and on the relationships between water chemistry and pool plaster surfaces.

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