When it comes to water balance, one item that is frequently underappreciated is the importance of water temperature. With all of the various chemicals that we add to pools and spas to achieve sanitation and sparkle, it is easy to underestimate the role that temperature plays in water balance. But all other things being equal, temperature alone can provide the catalyst to whether a pool has experienced etching or whether a spa has scale deposits.
In the pool and spa industry, water balance is measured using the Langelier Saturation Index, which is a measure of a solution’s ability to dissolve or deposit calcium carbonate. Calculating the LSI is relatively easy and involves simply adding a set of water parameter factors to arrive at a number between -0.3 and +0.5, which indicates that the water is in balance. If the number calculated is outside of this range, the water is out of balance, with negative numbers indicating aggressive, corrosive conditions, and positive numbers outside of this range indicating scale-forming conditions. In general, it is better to err on the positive side, because a small amount of scale provides some protection and is less damaging than corrosion.
The water balance parameters that are used in calculating the LSI are: pH, total carbonate alkalinity, calcium hardness, total dissolved solids, and importantly, temperature.
Only the pH value is used as measured; the other water balance parameters are given factors, dependent upon their measured values.
See accompanying table.
To calculate the LSI from the provided table, simply measure the parameters, use the corresponding factor, and plug the numbers into the following formula:
SI = pH + F(T) + F(TA) +F(CH) – F(TDS)
Where F(T) is the temperature factor; F(TA) if the adjusted total alkalinity factor (see article about cyanuric acid’s and borate’s effect on total alkalinity); F(CH) is the calcium hardness factor; And F(TDS) is the factor for total dissolved solids.
In milder climates and when the pool is in use, regularly testing these water balance parameters is a normal part of pool maintenance and service.
But in areas of the country that experience significant seasonal changes, it is common to close pools and spas for the winter, covering the water with pool and spa covers. That means the water won’t likely be tested again until springtime, when it’s time to open them up again for the season.
This is an important thing to consider, because temperature plays a major role in LSI water balance, and this role is all too often overlooked.
It is generally true to say that more damage occurs to pools during winter months, and a lot of that comes from the effect that temperature has on the water’s balance.
The reason for that is that temperature causes the very chemistry of the water to change.
Most of us have an intuitive understanding of the effect of temperature on dissolving stuff in water.
Add salt to water and it dissolves much better at a boil.
Factors for Saturation Index Sugar dissolves more easily in a steaming cup of coffee than in coffee served over ice.
But when it comes to the dissolution of calcium in pool water, the situation is actually reversed. Hot water brings calcium out of solution – not in.
That’s why heaters are susceptible to scale buildup.
Conversely, low temperatures dissolve more calcium into solution, and if there isn’t enough calcium already present in the pool, the water will seek the calcium from any available source. The water is hungry for more calcium, and it will leach it from the pool surface itself. That is what is meant by aggressive water.
This is also why many pool industry experts prefer to call water with solidly negative LSI’s “aggressive” rather than “corrosive,” because corrosive implies the destruction of metals, whereas there are a lot more items in a pool that can be damaged by aggressive water conditions.
When the LSI is low due to lowered temperatures, the resulting low calcium hardness can cause pitting and etching of concrete and plaster surfaces, dissolving of grout, and pitting of concrete pool decks.
To demonstrate the importance of water temperature, let’s look at two scenarios where everything in the pool is the same except the water temperature.
With the pH at 7.4, the water temperature at 85 ° F, the calcium hardness at 200 ppm, and the adjusted alkalinity at 80 ppm, the LSI is calculated at -0.2 indicating that the water is balanced.
LSI = 7.4 + 0.7 + 1.9 + 1.9 – 12.1 = -0.2
Take this same pool and drop the temperature to 45 ° F. The LSI is now -0.7, which makes the water naturally quite aggressive.
LSI = 7.4 + 0.2 + 1.9 + 1.9 – 12.1 = -0.7
This water will have a naturally high propensity to achieve its own balance by taking what it needs from pool surfaces to re-achieve equilibrium, which means etched and pitted plaster and dissolved grout at the tile line.
When closing a pool for the winter, remember to consider the winter temperature of the water. Then, adjust the water balance to what it will be when the temperature is in that range to cause an acceptable effect on the LSI.
You can adjust your alkalinity and pH, but the easiest item to change is the calcium hardness. Pool temperature, on the other hand, is not something that is not easily adjusted, unless you plan on cranking up the heater for the season.
Experts recommend predicting the worst-case scenario for the off-season.
If you are in a climate that is guaranteed to freeze, it is a good idea to set the LSI to be balanced at 32°F, so that when the temperature drops that low, there is sufficient calcium and alkalinity to keep the water balanced and not damaging to the expensive backyard investments service pros are charged with maintaining.
Otherwise, there will be consequences.
CYA and Borates
When cyanuric acid and/or borates are used, an additional adjustment must be made to the LSI. That’s because when measuring total alkalinity, one is really interested in carbonate alkalinity, because it is high or low carbonate levels that cause corrosion or scaling respectively.
Cyanuric acid increases the total alkalinity of the pool water, but not the carbonate alkalinity, which is the alkalinity that matters in water balance.
Borates, which are also sometimes used in swimming pools, have a similar effect.
Total alkalinity is actually a measure of carbonate, bicarbonate, hydroxyl, cyanurate ions, and borate ions, if present.
But because neither cyanurate or borate ions play any real role in corrosion or scaling, they should be subtracted out of the measured total alkalinity to get a number that is useful for understanding the water’s balance.
In the case of cyanuric acid, this is especially important when the cyanuric acid concentration becomes particularly high. When this occurs, the measured total alkalinity may fall within normal ranges, but the true carbonate alkalinity might be exceedingly low, causing an extremely aggressive environment. This circumstance would cause etching to plaster surfaces as well as corroded metals.
In the case of borate use, its effects become more impactful at higher pH values.
At normal pool pH levels, to calculate carbonate alkalinity, the rule of thumb is to subtract out one third of the cyanuric acid concentration.
In other words, total alkalinity is corrected for cyanurate alkalinity by the following equation to yield carbonate alkalinity:
Alkalinity corrected = Alkalinity total – 1/3 Cyanuric Acid
Thus, if the test for total alkalinity shows 100 ppm, while the test for cyanuric acid shows 90, the carbonate alkalinity is roughly 70. 100 – (.33 x 90) = 70 It is important to note that the one third correction factor is pH dependent. That is because the cyanuric acid and cyanurate ion concentrations are pH dependent. Therefore, it is best to use correction factors that account for this pH dependency.
The cyanuric acid correction factors may be found in the accompanying table. Borate Correction
The borate correction factor is pH dependent. The borate correction factors may be found in the accompanying table.
To obtain the carbonate alkalinity from the measured total alkalinity in the presence of borates, the following procedure can be used.
1. Select a borate factor based on the measured pH: F(B) 2. Calculate the carbonate alkalinity from the measured total alkalinity, the measured borate concentration (B), and
the selected borate factor (F(B))
Alkalinity corrected = Alkalinity total – (B x F(B))
For example, if the measured total alkalinity is 100 ppm, the pH is 7.6, and the measured borate concentration is 50 ppm, then: Alkalinity corrected = 100 – (50 x 0.17) Alkalinity corrected = 92 ppm
Putting it all together
As will soon become apparent, calculating the LSI in the presence of cyanuric acid and borates is a bit complicated, which is why many of today’s pool service techs simply install smart phone LSI calculators to save time.
However, for those interested in learning how to calculate the saturation index by hand, the following formula is applied to the water balance parameters: SI = pH + F(T) + F(TA) + F(CH) – F(TDS)
However, a calculated carbonate alkalinity must be used prior to selecting F(TA) For example, consider the following measured water parameters pH = 7.6; Temperature = 76; Total Alkalinity = 100 ppm; Cyanuric Acid = 90 ppm; Borates = 50 ppm; Calcium Hardness = 200 ppm; Total Dissolved Solids = 800 ppm First, calculate the carbonate alkalinity, or Alkalinity corrected: The Alkalinity corrected is obtained using the cyanuric acid and borate correction factors and the following formula:
Alkalinity corrected = TA – (CYA X .33) – (B x F(B))
Alkalinity corrected = 100 – (100 x .33) – (50 x 0.17) Alkalinity corrected = 58.5 ppm Next, find the factors for the Langelier Saturation Index from the accompanying table and simply plug them into the general formula:
LSI = pH + F(T) + F(TA) + F(CH) – F(TDS)
LSI = 7.6 + 0.6 + 1.8 + 1.9- 12.1 LSI = -0.2 The LSI is within range.
Cyanuric Acid Correction factors as a function of pH
Borate Correction Factors as a function of pH