6-1.30 Saturation index for swimming pools. Saturation index is equal to the pH plus a temperature factor, plus a calcium hardness factor, plus an alkalinity factor, minus the constant 12.1.
S.I. = pH + TF + CF + AF - 12.1
The optimum saturation index is zero.
Tolerance limits are plus or minus 0.5.
If the index is positive (+), the water is super-saturated with CaCO3 and may deposit a protective coating or scale in the pipeline, particularly metal filters, valves and pumps.
If the index is negative (-), water will dissolve CaCO3 and may be corrosive.
|
TF |
Calcium hardness |
CF |
Total alkalinity |
AF |
|
|
32 |
0.0 |
5 |
0.3 |
5 |
0.7 |
|
37 |
0.1 |
25 |
1.0 |
25 |
1.4 |
|
46 |
0.2 |
50 |
1.3 |
50 |
1.7 |
|
53 |
0.3 |
75 |
1.5 |
75 |
1.9 |
|
60 |
0.4 |
100 |
1.6 |
100 |
2.0 |
|
66 |
0.5 |
150 |
1.8 |
150 |
2.2 |
|
76 |
0.6 |
200 |
1.9 |
200 |
2.3 |
|
84 |
0.7 |
300 |
2.1 |
300 |
2.5 |
|
94 |
0.8 |
400 |
2.2 |
400 |
2.6 |
|
105 |
0.9 |
800 |
2.5 |
800 |
2.9 |
|
128 |
1.0 |
1000 |
2.6 |
1000 |
3.0 |
Example: Given temperature 68 degrees, total hardness 200 mg/l, total alkalinity
20 mg//l, CaCO 3and pH = 7.8
S.I. = pH + TF + *CF + AF - 12.1 *(Calcium hardness –
0.70 x 200 = 140 mg/l.)
S.I. = 7.8 + 0.52 + 1.76 + 1.22 - 12.1 = (-)0.8
Therefore, the water is corrosive.
Note: Saturation index must be maintained slightly on the positive side within the tolerance limits.
Normal Control Levels:
|
free chlorine: 0.6 (minimum) |
|
|
temperature: 78 - 80 degrees (indoor) |
saturation index: (-)0.5 - (+)0.5 |
|
total alkalinity: 80 - 120/mg/1 |
calcium hardness: 180 - 250 mg/1 |
Alkalinity control:
to increase - 1-1/2 lb of sodium bicarbonate NaHCO 3baking soda will raise the alkalinity of 10,000 gallons of water by 10 mg/l.
to lower - add muriatic acid no more than one pint (1/8 gallon) per 5,000 gallons of pool water will lower alkalinity by 12 mg/l (or, add 1.25 lb of sodium bisulfate).
pH:
to increase - use soda ash.
to decrease - muriatic acid or sodium bisulfate.
Hardness Control: Calcium hardness is assumed to be 70% of total hardness. to increase - 1 lb of calcium chloride will raise the calcium hardness of 10,000 gallons of water by 11 mg/l. It should be added in small amounts. to lower, dilute with soft water.