Reprinted with permission from Kim Skinner of OnBalance:
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.
We figured out that unbalanced water could be either scale-forming, in which case a layer of calcium scale would form everywhere – or aggressive, where etching (uniform dissolution) of quality pool plaster/cement surfaces would occur.
As an industry, we needed a way to determine whether the water in our pools was potentially scale-forming or aggressive – and found it in what is known as the Langelier Saturation Index (SI). That Index has been improved in recent years to produce better accuracy.
In his research, Langelier found that there were five key parameters involved in a body of water’s status – that is, pH, calcium hardness, alkalinity, water temperature and total dissolved solids (TDS) – and that various relationships among these factors determined whether a pool’s water would be balanced, or have an active tendency toward scale-formation, or toward aggressiveness against cement surfaces. (Note that the sanitizer residual plays no role in these calculations.)
The SI is applicable to calcium carbonate only, which is what a major percentage of the pool plaster surface is made of. Calcium hydroxide, another plaster component (about 20% of fresh plaster), is somewhat soluble and can be dissolved, even by balanced water. (More on that issue in a future post.)
The pH value of the water is perhaps the most important factor affecting the Saturation Index (SI), with alkalinity and calcium hardness following close behind. As the pH lowers, the water becomes more aggressive, and as the pH rises, the water becomes more scale forming. The same follows for alkalinity and calcium hardness, the lower they are, the more aggressive the water becomes, and 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 makes it more of a concern in salt pools. But TDS is a minor player overall.
The key point to master here is that all of these individual water parameters or values can be offset by the other water parameters – that’s 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 (carbonate) alkalinity.
The Association of Pool & Spa Professionals set limits to the SI range: For water to be considered balanced by APSP standards, the value derived using the SI should be 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” water values: If the water has a pH of 7.6, alkalinity at 90 ppm, CH at 300 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 and is balanced.
Let’s look at what happens when the pH is on the low side, but which can be offset by a higher alkalinity and calcium level: With a lower pH at 7.2, the alkalinity at 120 ppm, the CH at 500 ppm, the TDS at 1,500 ppm, the water temperature at 82 degrees, and the cyanurate level at 30 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 high at 7.9, it can be offset by a lower alkalinity of 60 ppm, a CH of 500 ppm, a lower water temperature of 60 degrees, the TDS at 3,000 ppm, and the cyanurate level at 30 ppm, the SI is still 0.0.
Note that there is a sixth factor in 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.
It also might be helpful to look at swimming pool water as dynamic and often changing. 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, 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 (which includes calcium ions), 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, aggressive water can correct itself and become balanced, but at the expense of dissolving some of the plaster surface, which of course we are trying to avoid.
However, let’s also consider that the pH in most pools rises during the hours and days after a pool 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, potentially non-etching, and result in scale-forming water 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.
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 damage to a pool’s plaster, quartz, or pebble finish.
At onBalance, we’ve developed an SI calculator that’s available on our website. For information, go to http://sic.poolhelp.com/. The calculator is also available as a mobile-phone app. For details, visit https://www.facebook.com/onbalancepools.