ARM 37.115.1314
ARM 37.115.1314. SATURATION INDEX TABLE
Cite as Mont. Admin. R. 37.115.1314
(1) The following is the saturation index table. If the total dissolved solids (TDS) are less than 1,000 pm then 12.1 is used as the total dissolved solids factor. If the total dissolved solids are greater than 1,000 ppm then 12.2 is used as the total dissolved solids factor.
(2) If a spa does not meet the total alkalinity (TA) level requirements for saturation index and chemical balance, the spa owner must either chemically correct the water chemistry or the spa water must be completely exchanged with fresh make-up water according to one of the following frequency standards:
(a) weekly; or
(b) when the total dissolved solids (TDS) reach 1500 ppm higher than the water supply's base TDS; or
(c) when required by using the following formula: number of spa gallons divided by 3 divided by the number of bathers in a 24 hour period = number of days the spa can be used before draining and refilling with fresh make-up water.
Table 7.
Temperature (Fº)
Temp Factor (TF)
Calcium Hardness as ppm
Calcium Factor (CF)
Total Alkalinity
Alkalinity Factor (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
1.7
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