LSI - Langelier saturation Index

Your LSI is the overall state or balance of your pool water.

Water (in a pool) will always try to get back to its natural balance. Means if it has too much calcium (over saturated) or high positive LSI, it will deposit the excess calcium on the pool surfaces, until its back in balance, or with LSI of zero. So, if you see a scale forming this is the reason.

By comparison if the LSI is negative, or undersaturated, then its short of calcium and will do whatever it can, to find it. This can be the cement-based surfaces, (which it will eat) or if painted – fibreglass, will cause attack on metal fittings, in the pool and the plumbing – heating system. And the water being out of balance and having trouble finding enough calcium, loses its “buoyancy” and can no longer hold dissolved solids in suspension, and they drop out “overnight”, leaving a dull grey – brown, dirty looking deposits on all pool surfaces. Often known as TDS dropout, (Total Dissolved Solids)