The Langelier Saturation Index (LSI) and and Stiff & Davis Stability Index (S&DSI) are a measure for the saturation of CaCO3 in water. The LSI should be used for low TDS ranges (<10,000 ppm) and the S&DSI is used for high TDS ranges (>10,000 ppm). In general, the Langelier Saturation Index (LSI) is used for brackish waters and the Stiff & Davis Stability Index (S&DSI) for sea waters. At the pH of saturation (pHS), the water is in equilibrium with CaCO3. The definitions of LSI and S&DSI are: LSI = pH – pHs (TDS <10,000 mg/l) S&DSI = pH - pHs (TDS >10,000 mg/l) To control calcium carbonate scaling by acid addition alone, the LSI or S&DSI in the concentrate stream must be negative. If a high quality scale inhibitor is used, the LSI in the concentrate stream can be < 1.5. This will reduce or eliminate the acid consumption. Many inhibitors allow operation up to an LSI of < 1.8 in the concentrate. Please refer to the inhibitor manufacturer's own literature for reference points.