The Langelier Scale, later to become the Langelier Saturation Index (LSI) , was originally developed by Dr Wilfred Langelier. It is a very accurate way of determining water balance and is often used by builders and poolshops.
The LSI is mathematical calculation that takes into account four factors: pH, Alkalinity, Calcium Hardness and Temperature. A minus figure is under-saturated and corrosive while a positive figure is over-saturated and has a tendency for scaling to occur. The accepted limits are -0.5 to 0.5 . Perfectly balanced water has an LSI of 0.
The LSI can be interpreted as the pH change required to balance the water.
A positive LSI means you should lower your pH (add acid).
A negative LSI means you should increase your pH (add soda ash or buffer, depending on alkalinity).
NOTE: Buffer primarily increases alkalinity and as a side effect increases pH, while Soda Ash will only increase your pH.
We only adjust these variables because you cannot adjust temperature and calcium hardness easily.
To make it easy for you to calculate the LSI for your pool we have provided a FREE program that will do it for you. By adjusting the various levels you can test and then correct your balance. Try adjusting your figures and see what works best.
The Pool Clinic 