Keep pool water balanced using LSI
By Marcelle Dibrell
The onset of spring on March 20 generally marks the time to open up pools in preparation for the swim season. After this year’s unprecedented winter storms, it may become a practice performed by service techs over a much greater regional area.
In temperate climates, many pools are not closed for the winter and for those that experience occasional freezes, in times past, it has made sense to simply run the filtration system with the heater on if freezing conditions are expected. However, as folks in Texas discovered this year, that may not always be possible.
Where freezing temperatures are normal, pools and spas are winterized by cleaning, treating water and protecting the surfaces and equipment from weather related problems. That means lowering the water level and fully draining the plumbing and equipment of any water that would otherwise expand when freezing and lead to burst pipes and broken equipment.
Service techs who perform winter closings and spring openings can command hefty profits, and for good reason. It’s a lot of work. According to recent Service Industry News surveys, the average charge to open or close a basic pool for the season is about $330.
And it can run a lot more if the pool has extra features like waterfalls, sheer descents, spas and multiple pumps. Or if the pool is beyond a certain square footage. Or if opening the pool requires multiple visits. Or if it requires extra chemicals. Or extra vacuuming. See how this works?
While this might seem like a lot of money to many pool owners, it is a lot less expensive than replacing a pump, filter, heater, deck, etc. Because you don’t need to open a pool unless it was closed to protect it from a hard freeze.
After this year, it is highly likely more pool owners will be willing to pay for the peace of mind to ensure their backyard investments will not be destroyed by an unexpected freeze.
As they say, hindsight is 20/20, and it is a lot easier to winterize and then reopen the pools than replace nearly every component of them. And that means that service professionals in some of the southern states are going to want to learn how it’s done.
Opening a pool after it has been closed for the season requires readying both the equipment as well as adjusting the water chemistry to operational levels.
After a long winter’s neglect, the single most important aspect of adjusting the water chemistry is likely going to be a heavy dose of sanitizer.
But bringing the pool water into balance is also essential to prevent irrevocably damaging pool surfaces and equipment.
When it comes to pool and spa water balance, the Langelier Saturation Index is the tool that is used to prevent corrosion of surfaces and equipment, as well the formation of scale. Nowadays, most service pros have a favorite app, website or LSI calculator on their smart phones that can do the calculation for them.
Before the advent of such pocket knowledge, service pros were required to perform back-of-the- envelope 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.