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Calcium Saturation Index - What it is and what does it have to do with my pool?
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Calcium Saturation Index is the measure of your water's ability to be saturated by dissolved minerals, particularly calcium carbonate.
When the CSI level is too high there will be excessive scale production, if it's too low - the water will start acting in a corrosive manner.
Either of these states will greatly reduce the life and effectiveness of your electrolytic cell.
If water is corrosive, it may also damage the finish of your pool, as it will leech calcium from it until the saturation point is reached.
This does not apply to fiberglass pools or pools with vynil liner (aboveground pools).
For these reasons a balanced CSI is an important consideration during your pool maintenance.
The value of the Saturation Index can vary greatly as does the resulting amount of calcium build-up produced.
The amount of calcium can vary from no calcium build-up in 12 months to a significantly calcified electrode within 5 to 6 days of operation, or in the other extreme corrosion of your cell.
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How is CSI calculated?
The formula for the Calcium Saturation Index is
Calcium Saturation Index (CSI) = pH + TF + CF + AF - 12.1
Where:
- pH - pH reading
- TF - Water temperature
- CF - Calcium Hardness
- AF - Total Alkalinity
- 12.1 - Constant
To calculate your CSI please use the following procedure:
- Test your water for PH, Calcium Hardness, Total Alkalinity and TDS
- Enter the values into the water balance calculator provided to calculate your CSI value or use the table below.
| Temperature Deg C / F |
Calcium Hardness |
Total Alkalinity |
| Deg C / Deg F = TF Factor |
PPM = CF Factor |
PPM = AF Factor |
| 0 / 32= 0.0 |
5 = 0.7 |
5 = 0.3 |
| 3 / 37= 0.1 |
25 = 1.4 |
25 = 1.0 |
| 8 / 46= 0.2 |
50 = 1.7 |
50 = 1.3 |
| 12 / 53= 0.3 |
75 = 1.9 |
75 = 1.5 |
| 16 / 60= 0.4 |
100 = 2.0 |
100 = 1.6 |
| 19 / 66= 0.5 |
150 = 2.2 |
150 = 1.8 |
| 24 / 76= 0.6 |
200 = 2.5 |
200 = 1.9 |
| 29 / 84= 0.7 |
300 = 2.5 |
300 = 2.1 |
| 34 / 94= 0.8 |
400 = 2.6 |
400 = 2.2 |
| 41 /105 = 0.9 |
800 = 2.9 |
800 = 2.5 |
What do all those strange words mean?
- pH - pH is the measure of the concentration of hydrogen ions in the water on a scale of 0 - 14, with 0 being the highest acidity and 14 being the lowest base. A reading of 7.0 is considered the most neutral. Note a drop in PH to 6.0 is 10 times more acidic than 7.0.
- Total Alkalinity (AF) - Total alkalinity is a measure of the buffering capacity of water with an acceptable range of 60 to 120 parts per million (PPM)
- Calcium Hardness (CF) - CF is a measure of the calcium carbonate content of the water with a range of 175 to 300 ppm.
When water is too saturated with calcium it will cause scaling, when the water does not have enough it will cause corrosion and etching.
- Water temperature (TF)- A water temperature range of 78 to 82 degrees F (26 to 28 degrees C) is the industry standard. High temperatures make calcium carbonate less soluble and contribute to scaling, while lower temperatures contribute to corrosiveness.
- Total Dissolved Solids (TDS) - TDS is the sum of all the dissolved metals and minerals in the water. TDS should not be a major concern until it reaches levels of 3000ppm. If this happens subtract 0.1 for each 1000-1500ppm.
If the CSI calculation is zero then the water is balanced. If the CSI value is less than 0 then the water will tend to be corrosive. If the CSI value is greater than 0, then it is said to be scale forming.
An acceptable range is -0.5 to +0.5. If the result is outside this range, adjustments should be made to the hardness, alkalinity and pH to bring it as close to zero as possible. Your local pool store should be able to help you with this.
Be aware however, that whilst the water may be balanced from a corrosion and scaling point of view it does not necessarily mean that it is ideal for sanitiser efficiency and bather comfort.
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