Understanding the Saturation Index - hot tubs and pools
What is the Saturation Index?
The saturation index, or Langelier Index, was developed by Wilfred E. Langelier,
a professor of Civil Engineering at the University of California-Berkeley. It was
originally developed for use in the boiler and steel pipe industry to determine
whether water would tend to form scale or corrode surfaces. The index was modified for the swimming pool and hot tub industry to determine the overall mineral balance of water as it relates to calcium carbonate equilibrium.
Pool and hot tub professionals use the saturation index as a quick guide to determine if water in a pool or hot tub is adequate or balanced. Water that is low relative to the saturation index (a negative number) will tend to corrode surfaces, damaging them. Water that is high relative to the saturation index (a positive number), will tend to form scale on surfaces, clogging pipes, damaging heater elements and providing a place for bacteria to grow. An acceptable saturation index calculation provides a result between -.5 and +.5. The saturation index takes into account the pH, total alkalinity (TA), calcium hardness (CH), temperature and total dissolved solids (TDS) in the calculation. The pH is
used as its own value while the total alkalinity, calcium hardness, temperature and TDS are all factors of the actual water test result. Therefore, the calculation is:
SI = pH + FTA + FCH + FT – FTDS
This shows the importance of having perfectly balanced water. You don't want acidic water (low PH) and you also don't want high PH as it damages your gear.