calculations to figure out how to bring their customer’s pool and spa water into balance. Today, while it isn’t necessary to get out a pencil and paper to know which chemicals will be required to provide balance, it is still a good idea to understand the mathematical operations that are going on behind the scenes.

So, for those who are curious, the following is a discussion about the Langelier Saturation Index.

The LSI provides the mathematical relationship between whether the water might be damaging to surfaces and equipment as a function of the pH, total alkalinity, calcium hardness, temperature and total dissolved solids.

To use the Langelier Saturation Index, the following formula is applied to the water balance parameters:

SI = pH + F(T) + F(TA) + F(CH) – F(TDS)

Temperature, total alkalinity, calcium hardness and total dissolved solids are all given a factor, as shown in the accompanying chart. The pH value is not given a factor and is used asmeasured at the pool. When the various water parameter factors are used in the formula, calculating zero indicates perfectly balanced water. Negative values below -0.3 indicate the water is corrosive. Positive values greater than +0.5 indicate scale forming conditions. It is better to have slightly positive values than slightly negative values because a small amount of scale provides some protection and is less damaging than corrosion.

While the Langelier Saturation Index is a useful formula for obtaining balanced water, it’s possible to obtain an LSI that indicates balance by using water balance parameters that are outside the industry recommended concentrations.

For example, if a pool had a pH of 8 with an average temperature of 84 °F, calcium hardness of 250 ppm and a total carbonate alkalinity of 25 ppm, with the total dissolved solids at 1000 ppm, the Saturation Index would calculate balanced water: SI = 8 + .6 + 1.4 + 2.2 – 12.1 = .1 Here, the LSI indicates balanced water but the total alkalinity is well below the minimum standard, while the calcium hardness and pH are slightly higher than generally advised. Whether this represents balance is therefore debatable.

There are those who argue that such conditions would be damaging to pool plaster.

A total alkalinity that low would be considered aggressive by many industry experts. On the other hand, experiments have been conducted on plaster coupons in such “aggressive” conditions to examine whether calcium is leached from the plaster when the alkalinity is low, and found no change as long as the LSI is near zero.

For commercial accounts, there are regulations in place that determine the range of acceptable chemical parameters. However, when it comes to residential service, in the absence of regulations, service pros can simply decide for themselves whether or not they want to take the risk.

Factors for Langlier Saturation Index, Source material: APSP Service Tech Manual, 4th Edition.