Saturation Index = pH + calchd + totak + tmpc - 12.1
In the above formula:
calchd is the Calcium Hardness factor. Optimum range 75 to 150 mg/liter.
totak is the Total Alkalinity factor. Optimum range 100 to 150 mg/liter.
tmpc is the Temperature in Degrees Centigrade. Suggested water temperature is between 27 and 29.5 Degrees C in commercial pools.
pH. Optimum range 7.2 to 7.6.
If the result of this calculation is zero, the target goal, then the water is balanced. If the answer is less than 0, the water leans toward being corrosive. If it is greater than 0, then it is indicative of scale forming. An acceptable range is -0.5 to +0.5, with the optimum goal of 0 (zero). If the result is outside this range, adjustments should be made to the hardness, alkalinity and pH to bring it as close to zero as possible and ideally within the optimum ranges shown above. Be aware however, that while the water may be balanced from a corrosion and scaling point of view it does not necessarily mean that it is ideal for chemical efficiency and bather comfort.