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Credit: USFilter

Common problems with pool-water chemistry can easily be corrected through careful monitoring, testing, and managing.
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Credit: USFilter

The only reliable free-chlorine testing method is a diethyl-p-phenylene diamine kit.
ALKALINITY AND CALCIUM HARDNESS

Total alkalinity and calcium hardness cannot be considered separately, as one's target influences the others.

Temperature, pH, total alkalinity, and calcium hardness are four major factors used to calculate the saturation index, a measure of water's tendency to scale or corrode surfaces that it touches. The saturation index must be considered in picking a target for total alkalinity and calcium hardness. However, in pools maintained between 78° and 85°F and at pH levels of 7.4 to 7.6, temperature and pH can effectively be considered constants, not variables. This somewhat simplifies calculating the saturation index.

In general, total alkalinity should be 50 to 125 ppm, and calcium hardness 200 to 500 ppm. To ensure that the levels of these factors fall within the appropriate range, total alkalinity multiplied by calcium hardness in ppm should equal the number 25,000, give or take a thousand or two. For instance, if total alkalinity is 50 ppm, calcium hardness should be about 500. Note that this “25,000 rule” only works if pH is between 7.4 and 7.6 and temperature is between 78° and 85°F.

Makeup water characteristics, bather load (the number of people using the pool), and chemicals used affect precisely where in this range of total alkalinity and calcium hardness a pool is operated. Under normal operations, total alkalinity tends to drop in most pools because it is reduced by gas chlorine feed and muriatic acid feed. To counter these constantly declining levels, total alkalinity's lower levels should be maintained between 50 and 80 ppm and countered with higher calcium hardness.

Calcium hardness, on the other hand, is a comparatively stable factor in pool water chemistry. If total alkalinity falls below 50 ppm, operators can feed calcium chloride at the appropriate dosage to achieve a calcium hardness level that satisfies the 25,000 rule. New pools that have fresh plaster, grout, or concrete can be maintained at a saturation index level closer to 30,000 or 35,000 for the first six to 12 months while the surface finish is still new.

— Jim Tanner is aquatics national sales manager for Warrendale, Pa.-based USFilter, a Siemens Co.

Other critical chemistry factors

Some experts believe that temperature, total dissolved solids (TDS), and chloride levels should also be considered as critical factors.

Pools used competitively tend to be a little cooler than pools used primarily for recreation or special-education purposes. The temperature range that swimmers prefer is so narrow that it does not affect mechanical equipment, swimmer safety, or other factors. The correlation between water temperature, air temperature, and air humidity level matters more than a pool's exact water temperature.

TDS comes into play only when some of the five critical factors fall outside their respective target ranges. Like TDS, chlorides are not an issue when the other five factors are managed properly. Contrary to popular belief, high chloride levels alone do not cause corrosion problems. Rather, chloride affects pool water taste and swimmer buoyancy.