Since the 1960s, the pool industry has relied on the Langelier Saturation Index to achieve balanced water. The traditional method has been to land right at 0.0 of an SI with an allowable variance in either direction. But the subtleties of pool chemistry call for both a studied approach and a modified SI range.
Over the years, experts have seen two things: mistakes in calculating SI and a need for the Index itself to be adjusted. Here, we examine some common errors techs make in calculating each component of the Index, as well as the advantages of skewing SI balance toward a positive number.
Balanced water will go a long way in preserving the life of the plaster and the clarity of the pool.
Measuring pH is the least problematic portion of the SI. There are no conversions necessary since the pH level is actually the number you’re using to compute SI. Just be aware of anything introduced to the pool that may have an affect on the pH.
The most obvious pH changes will be the result of introducing sanitizers into the pool. Acidic sanitizers like trichlor and dichlor will lower the pH. However, you should be cognizant of CO2 loss, which can be especially dramatic in spas.
“People forget that when you turn the aerator on … immediately the pH goes up,” says Joan Vienot, owner of Pool Pal Inc. in Freeport, Fla.
It’s best to tabulate the water balance in a spa after the unit has already been used in order to account for these changes, she adds.
As most techs know, alkalinity refers to the pool’s ability to buffer the water against pH change. When calculating the Saturation Index, however, we are only interested in alkalinity’s ability to keep calcium carbonate in solution.
To calculate the correct SI factor, techs must isolate carbonate (or, in actuality, bicarbonate) alkalinity from the pool’s total alkalinity.
Though a host of buffers contribute to total alkalinity, carbonates and cyanurates make up the majority of alkalinity in most pools. This is particularly true in pools that are sanitized with dichlor or trichlor, which continually feed cyanuric acid in the vessel.
In order to isolate carbonate- from cyanurate alkalinity, experts generally recommend taking a third of the cyanuric acid level from the total alkalinity reading. For example, at a pH of 7.5, a pool with 90 ppm of CYA would have approximately 30 ppm of cyanurate alkalinity, which you would have to subtract from the total alkalinity reading to find the necessary SI factor. Note that cyanurate levels at a higher pH will be more than a third of the cyanuric acid level.
But remember, isolating the carbonate alkalinity is not correcting any kind of interference. The cyanurate is still buffering against pH bounce.
“There are people applying that ‘correction’ to the alkalinity even when they’re reporting total alkalinity,” says Que Hales, a manager for Pool Chlor in Tucson, Ariz.
This mistake is especially troublesome because every time cyanuric acid is added to the pool — whether directly or through stabilized sanitizers — carbonate alkalinity actually is being transformed into cyanurates. Even with the rapid transformation of bicarbonates to cyanurates, the total alkalinity should remain a relatively stable pH buffer.
—or in some cases 1,500 ppm — in the water. However, these tables were largely put together before salt-chlorine pools became popular.
With salt levels of 3,500 ppm, many pools with chlorine generators have from 4,000- to 5,000 of TDS in them. In these cases, even using the 12.2 number is hardly being accurate. Your water is much more corrosive than you may think.
“If everyone will use 12.4 for the TDS factor, then the corrosive effect of the salinity will be accounted for,” Vienot says.
While Vienot uses a standard value of 12.4, chemist John Wojtowicz published an equation in 2001 to explain the changing SI values as the TDS rose over 2,000 and above. As demonstrated by the chart, a normal salt pool will be improperly balanced if a tech uses a 12.1 or 12.2 constant.
The calcium component of saturation chemistry is often misunderstood. The actual contributing factor to saturation issues is the amount of calcium in the water. Calcium test kits give that value. However, some industry test methods give total harness instead, which includes calcium but also other components. To correctly calculate saturation values, use a calcium hardness rather than a total hardness kit.
Although the calcium hardness level is a direct indication of how much calcium is available in the water to fall out of solution, it is not the primary indicator of whether calcium actually will precipitate. pH is the primary factor, with alkalinity in second place.
And although lower calcium levels do limit the amount of precipitate that can be formed, you can’t go overboard and maintain too little calcium.
“If you don’t have calcium added to water, it’s going to leech the calcium out of that system,” explains Greg Garrett, technical advisor for the National Plasterer’s Council in Port Charlotte, Fla.
This is especially true of newly plastered pools that have not had adequate time for internal cement compound transformation, he adds.
Temperature is as straight-forward as pH, but perhaps because of its relative simplicity, many techs don’t pay attention to the seasonal differences in pool and spa maintenance.
Temperature is key to the SI because as the water heats up, calcium becomes much less soluble. This is why pools can deteriorate over the off-season and many spas are ridden with scaling problems in the summer.
“A perfectly balanced pool in the summer may be very corrosive in the winter,” Vienot says. “In spas is probably where you get the most common water balance problems. If you balance them for cold water and then heat them up 60 degrees, you’ve got some real problems.”
Even if all the other SI factors are within an accepted range, an unaccounted for temperature can quickly tip the water’s balance.
The NPC’s new start-up guide is now recommending an ideal range of all positive indices for maintaining pools and spas. This approach is to ensure that the calcium in the pool plaster is not etched out into the water.
“Why tolerate negative indices, which indicates the water is undersaturated by [at least] one parameter?” Garrett says.
Even APSP’s recommendations call for an ideal range of 0.0 to +0.5, with an allowable variance on the negative side.
Service techs generally agree.
“With most pools, we intentionally ride the Saturation Index a little bit toward the positive side to where, if something goes wrong, we’re more likely to scale it than we are to etch it,” Hales says.
However, some pools — those with exposed aggregate, waterfeatures that continually raise pH through aeration or sanitizers that require constant adjustment — may need the full range of the “neutral zone” of -0.3 to +0.5, he adds.
•Saturation Index Calculator
Visit this site to figure out the saturation index for your pool.