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Salt Chlorine Generators:  Water Testing Still A Must

Salt chlorine generators are becoming ever more popular among pool owners because of the convenience of not having to transport, store, and handle chlorine. The promise of less maintenance and lower long-term costs also makes this method attractive. Unfortunately, some customers do not regularly test water chemistry once the equipment is installed because they falsely assume that "low" maintenance means "no" maintenance. Here we'll set the record straight to help you and your customers avoid the unnecessary consequences of poor water quality.

How They Work

The most widely used type of salt chlorine generator is the plumbed in-line system, designed for inground pools. Small submersible units are also available for aboveground pools. The process begins when non-iodized salt (usually food-grade salt or water softener pellets) is added directly to the pool water. The concentration of salt needed to produce sufficient chlorine for germ killing varies depending on the model of the chlorinator but is commonly 3,000 to 5,000 ppm. (By comparison, the salinity of seawater is about 35,000 ppm.)

Chlorine is produced as the salted pool water moves through an electrolytic cell, which emits an electric charge that frees the chlorine from the salt molecule.

2NaCl + 2H2O →2NaOH + Cl2 + H2
Salt + Water + Electricity produces Sodium Hydroxide + Chlorine + Hydrogen Gas

Just like any other chlorine product added to water (gas, pool bleach, cal hypo tablets, dichlor granules, etc.), the chlorine made through the electrolysis of salt water forms hypochlorous acid (HOCl). HOCl acts to kill bacteria and other microorganisms and to oxidize organic contaminants; then it reverts to salt (chloride). Only some salt is lost to splash-out, carry-out, and backwashing filters. Most eventually feeds back through the cell to begin the cycle again. Thus, sanitization is practically self-perpetuating.

Testing Salt

The salt level does not need to be tested often, but you will need to manually check it when the system is first installed and periodically thereafter. Also, automatic salt-level indicators sometimes fail, especially if a cell has scaling. A cell that is not functioning properly may result in a false indication of low salt, so confirm the readout with a manual test before adding more. Stand-alone salt (sodium chloride) tests are sold as drop-count titrations or test strips.

Testing Chlorine

In a traditional pool, free chlorine should be monitored daily to ensure effective sanitizing (target 2–4 ppm). There are easy-to-use color-matching tests available (DPD reagents used with a color comparator, or test strips), or you can use the FAS-DPD titration method, which involves an obvious color change in the water sample and provides more precise results. The most precise method of testing will be using a colorimeter, which electronically reads the pink color developed by the DPD reagents and reports the free chlorine levels. NOTE: OT tests do not test free chlorine, only total chlorine.

As water is recycled through the salt chlorine generator, any combined chlorine—the irritant that causes red eyes and the unmistakable "chlorine odor" of a poorly maintained pool—is eliminated as it passes over the electrolytic cell. Therefore, under normal conditions, there should be little combined chlorine in a pool with a salt chlorine generator. But after a pool party, heavy rain or windstorms, or if there are signs of an algae bloom, test for combined chlorine. Choose a DPD test with a color comparator that allows you to determine total and free chlorine, so you can subtract the free reading from the total to get the combined chlorine level. You can also use the popular FAS-DPD titration, which determines both free and combined chlorine directly without color matching, or a high-end test strip that measures both total and free chlorine. Colorimeters also may be used for this testing.

If the combined chlorine level exceeds 0.2 ppm, you will need to breakpoint chlorinate the water. Some salt chlorine generators are equipped with a "boost feature" that enables you to increase the chlorine level over a short period of time (usually within 24 hours). Or, use the traditional method of superchlorination and add more chlorine directly to the water yourself. The combined chlorine reading is used to calculate the amount of chlorine to be added to achieve breakpoint dosage.

After breakpoint chlorination, the chlorine level should be tested again before the pool is reopened to swimmers. EPA guidelines suggest 4 ppm is a safe re-entry level. Reagents in the DPD chlorine test tend to bleach out at chlorine levels around 10 ppm, making it difficult to test water that still has a high level of sanitizer. An alternative is the FAS-DPD method. The reagents used in this method won't be affected by chlorine under 20 ppm. Test strips generally only offer 5 or 10 ppm as their maximum.

Testing Chlorine Stabilizer

An outdoor pool being chlorinated with salt water will require the addition of cyanuric acid "stabilizer" to slow the rate UV rays in sunlight destroy the free chlorine residual. (The industry-recommended ideal range is 30–50 ppm). It is particularly important to test the cyanuric acid level regularly during the summer months when the sun's rays are strongest. Many liquid kits and some multiparameter test strips include a stabilizer test. Colorimeter tests are also available for cyanuric acid.

Testing pH, Total Alkalinity, and Calcium Hardness

As in any other pool, the Big Three water balance factors should be checked regularly in pools with salt chlorine generators. When pH is too high, chlorine's sanitizing ability is lessened and scale tends to form on pool surfaces and equipment. When pH is too low the water tends to be corrosive, pitting the concrete and dissolving the metal it touches. A pH outside of the acceptable range (7.2–7.8, but ideally 7.4–7.6) is also potentially irritating to bathers.

Manufacturers say their salt chlorine generators produce chlorine that is closer to pH neutral—i.e., 7 on the pH scale of 0 to 14—than most other forms (see chart), so this chlorine has less overall effect on pool pH. However, a byproduct of the salt to chlorine reaction is sodium hydroxide (NaOH). The production of sodium hydroxide will push the pH higher, requiring more frequent addition of acid in salt pools. The consequences of improper pH are the same, regardless of the method of sanitation. We recommend testing pH daily in pools with salt chlorine generators. Easy color-matching tests (with liquid reagents or strips) are available for this purpose, and a pH meter or colorimeter may be used.

 Chlorine Product pH (in a 1% solution)
 chlorine gas 0
 trichloroisocyanurate 2.8–3.5 
 dichloroisocyanurate 6.7
 chlorine from salt chlorinator 7.6–7.8
 calcium hypochlorite 8.5–11.8
 lithium hypochlorite 10.8
 sodium hypochlorite 11–13

The total alkalinity of the water influences the pH. When adjusted to within the recommended range, total alkalinity acts as a buffer for pH. The target for alkalinity is 80–120 ppm. According to major manufacturers, there is little fluctuation in the alkalinity due to chlorine produced from a salt chlorine generator; however, the generator tends to keep a steadier pH with an alkalinity between 90–100 ppm. Total alkalinity can be checked with a simple drop-count titration. Test strips containing a total alkalinity test are widely available but are less useful when adjustments are needed because of their limited range. Colorimeter tests also exist for total alkalinity.

When the calcium hardness level is too low, water can become aggressive even if the pH is within the recommended range. It may damage plaster, concrete, and grout and could lead to equipment corrosion. A high calcium hardness level causes the water to deposit scale, also regardless of pH. This crusty buildup is unsightly and rough to the touch. Deposits can clog filters and piping as well as cause heaters to fail. Calcium hardness is tested with a drop-count titration. Test strips will measure total hardness (calcium plus magnesium hardness), and colorimeters may be used for the calcium test.

Because the chlorine from a salt chlorine generator is pure and is not combined with additives found in solid products, this form of chlorination does not affect total alkalinity or calcium hardness directly. However, rain- and windborne contaminants, splash-out and carry-out, and treatment chemicals all affect these levels. Therefore, to prevent balance-related problems, we recommend alkalinity be checked weekly and hardness be tested at least monthly.

The Moral of the Story…

As we've seen, a pool equipped with a salt chlorine generator offers several benefits, but it doesn't eliminate the need for routine water analysis. Balanced water and correct salt and stabilizer levels will extend the life of the chlorinator and provide your customers with the best possible swimming experience. Advise them to test regularly and always follow the recommendations of the generator manufacturer.