High Calcium (Hard) water sources – the Langlier Saturation Index
The Langlier Saturation Index (Si) is a relationship between the Calcium Hardness, Total
Alkalinity, pH and water temperature. When the water is balanced correctly, the Si value
should be somewhere between -0.2 and +0.2. A value lower than -0.2 indicates the water
is corrosive, and a value higher than +0.2 indicates the water is scaling.
If you are in a situation where the water source for the pool contains a higher level of
calcium than is recommended for you pool, then this index can be very useful. It will allow
you to find a pH value that will not be corrosive or scale forming, and this pH value will
generally be slightly lower than what would normally be recommended for your pool.
The following formula and reference chart can be used to find the current Si for your water.
You will need to know the pH level, Total Alkalinity, Calcium Hardness and Temperature of
your water.
Use the chart to determine the values for Ti, Ci, and Ai, and insert these values into the
formula, along with the measured pH value to determine your Si value.
If you get an Si value higher than +0.2, then your water is scaling. In this case you should
reduce the pH in your water by the value of Si to ensure your water is neither scaling or
corrosive.
Si = pH + Ti + Ci + Ai – 12.1
Temperature
Degrees C
Ti Calcium
Hardness
Ci Total
Alkalinity
Ai
12 0.3 75 1.5 75 1.9
16 0.4 100 1.6 100 2.0
19 0.5 125 1.7 125 2.1
24 0.6 150 1.8 150 2.2
29 0.7 200 1.9 200 2.3
34 0.8 250 2.0 250 2.4
39 0.9 300 2.1 300 2.5
400 2.2 400 2.6
600 2.4 600 2.8
800 2.5 800 2.9
Example:
ph = 7.6, Temp = 24 (Ti = 0.6), Calcium = 400 (Ci = 2.2), Alkalinity = 125 (Ai = 2.1)
Si = 7.6 + 0.6 + 2.2 + 2.1 – 12.1,
Si = +0.4, so the water is scaling.
In this case we would take 0.4 away from the measured pH to find the ideal pH is 7.2. A
pH value anywhere between 7.0 and 7.4 would be acceptable in this case.
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