This page is for informational purposes. Often you will hear the term "balanced water". This term is almost always used to talk about pool and spa water. "Balanced water" is a simplified way of saying your water has been tested and is within the parameters of the Langelier Saturation Index. The Langelier Saturation Index is a pool and spa industry accepted standard for water quality. For a more detailed description, please read the below passage, which is a direct copy and paste from Wikipedia.
The Langelier saturation index (sometimes Langelier stability index) is a calculated number used to predict the calcium carbonate stability of water. It indicates whether the water will precipitate, dissolve, or be in equilibrium with calcium carbonate. In 1936, Wilfred Langelier developed a method for predicting the pH at which water is saturated in calcium carbonate (called pHs). The LSI is expressed as the difference between the actual system pH and the saturation pH: LSI = pH (measured) − pHs
For LSI > 0, water is super saturated and tends to precipitate a scale layer of CaCO3.
For LSI = 0, water is saturated (in equilibrium) with CaCO3. A scale layer of CaCO3 is neither precipitated nor dissolved.
For LSI < 0, water is under saturated and tends to dissolve solid CaCO3.
If the actual pH of the water is below the calculated saturation pH, the LSI is negative and the water has a very limited scaling potential. If the actual pH exceeds pHs, the LSI is positive, and being supersaturated with CaCO3, the water has a tendency to form scale. At increasing positive index values, the scaling potential increases. In practice, water with an LSI between -0.5 and +0.5 will not display enhanced mineral dissolving or scale forming properties. Water with an LSI below -0.5 tends to exhibit noticeably increased dissolving abilities while water with an LSI above +0.5 tends to exhibit noticeably increased scale forming properties. The LSI is temperature sensitive. The LSI becomes more positive as the water temperature increases. This has particular implications in situations where well water is used. The temperature of the water when it first exits the well is often significantly lower than the temperature inside the building served by the well or at the laboratory where the LSI measurement is made. This increase in temperature can cause scaling, especially in cases such as hot water heaters. Conversely, systems that reduce water temperature will have less scaling. Water Analysis: pH = 7.5 TDS = 320 mg/L Calcium = 150 mg/L (or ppm) as CaCO3 Alkalinity = 34 mg/L (or ppm) as CaCO3 LSI Formula: LSI = pH - pHs pHs = (9.3 + A + B) - (C + D) where: A = (Log10[TDS] - 1)/10 = 0.15 B = -13.12 x Log10(oC + 273) + 34.55 = 2.09 at 25 °C and 1.09 at 82 °C C = Log10[Ca2+ as CaCO3] - 0.4 = 1.78 (Ca2+ as CaCO3 is also called Calcium Hardness and is calculated as=2.5(Ca2+)) D = Log10[alkalinity as CaCO3] = 1.53