By The Zagosa Team. Published .
Reviewed or updated .
The Langelier Saturation Index, or LSI, tells you whether your water is about to lay down scale or strip calcium off your equipment. A figure between -0.3 and +0.3 means the water is balanced. Step outside that and it is either etching or scaling. Running it takes a minute and uses numbers you already have.
What the calculation needs
Four figures: pH, total alkalinity, calcium hardness and water temperature. A routine test usually gives you all of these.
A simple LSI calculator does the rest. There are free apps, websites and printable charts. The full formula needs lookup tables for temperature and TDS factors, but a calculator hides that from you.
Reading the answer
Between -0.3 and +0.3: balanced. Calcium and your equipment are in equilibrium.
Below -0.3, say -0.5: corrosive. The water strips calcium from grout, heaters and pumps. The usual fix is more pH, more alkalinity or more calcium hardness.
Above +0.3, say +0.6: scaling. Calcium drops out as a cloudy haze and scale on the waterline and heater. The fix is lower pH, or careful dilution if calcium and alkalinity are high too.
When it earns its keep
At every refill. Run LSI on the fresh water before you do anything else. If your fill water is naturally outside the band, your dosing has to compensate from the start.
When you have an unexplained problem, such as etching, scale or persistent cloudiness. LSI often reveals the cause when the individual readings all look fine on their own.
For any tub on a high-end mineral cartridge or salt system. Both need LSI to be right for the cell or mineral release to work as intended.
FAQ
Is LSI the same as the Ryznar Index?
Same idea, slightly different formula. LSI is the one the spa and pool trade tends to use. Ryznar shows up more in industrial water treatment.
Must I test TDS for the LSI sum?
Most calculators accept TDS as an input but fall back on a default if you leave it blank. For typical hot tub ranges the TDS factor is small, so the default is usually close enough.