Section 6-1.30 - Saturation index for swimming pools

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.

 

Temp. (in degrees F)

 

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:

pH: 7.4-7.8

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.

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