The Langelier Saturation Index (LSI) is a numerical value indicating whether or not water is balanced. It is calculated using the formula shown below.
LSI = pH + TF + logCH + logALK - CONSTANT, where:
LSI = Langelier 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
A Langelier Saturation Index, or LSI value, of zero means water is properly balanced. No chemical adjustment is necessary if the LSI 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 -0.3 may cause corrosion of concrete or metal surfaces.
While the LSI is a useful calculation, computing it poolside is not an easy task! To simplify the process, Taylor developed the Watergram® Water Balance Calculator (#6026), an ingenious device used to determine whether water is balanced without performing tedious calculations. Just follow these four simple steps:
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Using a test kit, determine pH, Calcium Hardness and Total Alkalinity of sample water.
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Using Watergram®, set Calcium Hardness opposite Total Alkalinity.
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Hold Calcium Hardness against Total Alkalinity and set the arrow to measured pH in the window.
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Read the Langelier Saturation Index opposite Water Temperature.*
If you discover scaling or corrosive tendencies, see our Pool & Spa Water Chemistry booklet (#2004B) for proper water-chemistry adjustment procedures.
*Note: If water temperature is not known, use 76°F for pools or 104°F for spas and hot tubs.