Del. Admin. Code tit. 16, Department of Health and Social Services, Division of Public Health, Health Systems Protection (HSP), 4464, app A
All chemicals in pool water affect the tendency of the water to be corrosive or to deposit a calcium carbonate scale. It is possible to control these two (2) conditions through the use of the Langelier Index (L.I.). The index was developed in 1936 by Professor W.F. Langelier at the University of California (Berkeley). It expresses the relationship between pH, alkalinity, calcium hardness and temperature. The resulting number (formula method - below) or position on the nomograph (nomograph method - Appendix B) indicates whether the pool water is chemically "balanced". In theory, pool water that is "balanced" will neither corrode nor deposit a scale.
LANGELIER INDEX/WATER BALANCE FORMULA METHOD
"Balanced" pool water has an L.I. between -0.3 and +0.3. An L.I. that is greater negatively than -0.3 indicates a corrosive water and an L.I. greater than +0.3 indicates a water that will deposit scale. After determining the pool water pH, alkalinity, calcium hardness and temperature, find the alkalinity factor(AF), calcium hardness factor(CHF) and temperature factor(TF) using the table below. Use the pH and these three (3) factors in the following equation:
L.I. = pH + AF + CHF + TF - 12.1
FACTORS NECESSARY TO CALCULATE THE L.I.
| Alkalinity Factor | - | Hardness Factor | - | Temp. (F) Factor | Calcium |
| 5 ppm | 0.7 | 5 ppm | 0.3 | 32 | 0.1 |
| 25 ppm | 1.4 | 25 ppm | 1.0 | 37 | 0.1 |
| 50 ppm | 1.7 | 50 ppm | 1.3 | 46 | 0.2 |
| 75 ppm | 1.9 | 75 ppm | 1.5 | 53 | 0.3 |
| 100 ppm | 2.0 | 100 ppm | 1.6 | 60 | 0.4 |
| 150 ppm | 2.2 | 150 ppm | 1.8 | 66 | 0.5 |
| 200 ppm | 2.3 | 200 ppm | 1.9 | 76 | 0.6 |
| 300 ppm | 2.5 | 300 ppm | 2.1 | 84 | 0.7 |
| 400 ppm | 2.6 | 400 ppm | 2.2 | 94 | 0.8 |
| 800 ppm | 2.9 | 800 ppm | 2.5 | 104 | 0.9 |
Sample Problem
A pool water has the following characteristics: pH-7.4; alkalinity-50; calcium hardness-50 ppm; temperature-78 F. Using the tables above, the factors are 1.7 for alkalinity, 1.3 for calcium hardness; and 0.6 for temperature. Using the formula:
L.I. = 7.4 + 1.7 + 1.3 + 0.6 - 12.1 = -1.1
This pool water is not balanced and is corrosive. The pH is within the required range and almost ideal, however, the alkalinity and calcium hardness should be much higher (see Appendix B). Alkalinity can be raised by adding sodium bicarbonate (baking soda) and lowered by adding muriatic acid or sodium bisulfate (dry acid). Calcium hardness can be raised by adding calcium chloride but can only be lowered by replacing part or all of the pool water with lower calcium hardness water.
Del. Admin. Code tit. 16, Department of Health and Social Services, Division of Public Health, Health Systems Protection (HSP), 4464, app A