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.

pH
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.

Alkalinity
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.

TDS
—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.

Calcium hardness
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
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.

Proof positive
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 under saturated 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, water features 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.