In the 1930`s, Professor Wilfred F.Langelier was commissioned to discover how to lay down a thin layer of scale on the distribution piping of a Large City. Such a thin layer of scale would protect the cast iron pipes from corrosion. About 40 years ago this calculation was adjusted and amended to suit swimming pool water conditions and made into a calculation that determines whether the water is neutral, balanced or unbalanced ( corrosive or scale forming )
The Langelier Saturation Index, often called the “LSI,” the “saturation index,” or the “stability index,” is a numerical value indicating whether or not water is balanced. It is calculated using the formula shown below.
SI = pH + TF + logCH + logALK – CONSTANT, where:
SI = saturation index
pH = measured pH
TF = temperature factor
CH = measured calcium hardness
ALK = measured alkalinity minus cyanurate alkalinity
CONSTANT = combined factor for temperature and ionic strength correction, and concentration conversions
An SI value of zero means water is properly balanced. No chemical adjustment is necessary if the SI is within ±0.5 units of zero; however, conditions producing a value greater than 0.5 may lead to cloudiness and scaling, while conditions producing a value less than -0.5 may cause corrosion of concrete or metal surfaces. (Note: While the cited SI range is commonly taught, the trend now in the pool/spa industry is to recommend a more restrictive range of -0.3 to 0.5 or even -0.3 to 0.3.)
The Langelier Index is used to determine the tendencies of water toward corrosion or scaling based upon the pH (potential of hydrogen) of calcium carbonate (CaCO3). In other words, The Langelier Saturation Index is used as a guide to achieve stable water. Several factors; pH, total alkalinity, total hardness, total dissolved solids and temperature are calculated to determine the potential “aggressiveness” or scale forming potential of the water. The LSI is used to pinpoint water balancing problems.
Temperature F ° = TF | Calcium Hardness = CF | Total Alkalinity = AF |
32° = 0.0 | 5 = 0.3 | 5 = 0.7 |
37° = 0.1 | 25 = 1.0 | 25 = 1.4 |
46° = 0.2 | 50 = 1.3 | 50 = 1.7 |
53° = 0.3 | 75 = 1.5 | 75 = 1.9 |
60° = 0.4 | 100 = 1.6 | 100 = 2.0 |
66° = 0.5 | 150 = 1.8 | 150 = 2.2 |
76° = 0.6 | 200 = 1.9 | 200 = 2.3 |
84° = 0.7 | 300 = 2.1 | 300 = 2.5 |
94° = 0.8 | 400 = 2.2 | 400 = 2.6 |
105° = 0.9 | 800 = 2.5 | 800 = 2.9 |
128° = 1.0 | 1000 = 2.6 | 1000 = 3.0 |
Saturation Index = pH + TF + CF + AF – 12.1 | ||
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While the SI is a useful calculation, crunching it out poolside could prove troublesome. To simplify the process, Taylor developed the Watergram®, a sliderule-like device used to determine whether water is balanced without performing tedious calculations. Just follow these four simple steps:
- Using test kit, determine pH, Calcium Hardness, and Total Alkalinity of sample water.
- Using Watergram® Water Balance Calculator, set Calcium Hardness opposite Total Alkalinity.
- Hold Calcium Hardness against Total Alkalinity and set arrow to measured pH in window.
- Read Saturation Index opposite Water Temperature. Note: If temperature is not known, use 27°C for pools or 38°C for spas and hot tubs.
See our Pool & Spa Water Chemistry booklet (#2004B) for proper water-chemistry adjustment procedures when you discover scaling or corrosive tendencies.
The Taylor "Watergram"
Every swimming pool owner should have this very useful and informative tool at their finger tips. Is your pool shell showing early signs of corrosion or erosion?
Balanced water with these 3 readings / points intersecting
Every swimming pool owner should should strive to get a straight line intersecting the 3 lines on the Watergram above. The readings/values can be different as long as they intersect in a straight line as shown above.
This indicates balanced water. Try to get to the Industry standard readings as per the original Watergram