With water maintenance, there are three broad areas. First is the safety of the pool user. Second is water clarity. And the third is the Calcium Saturation Index, or CSI. Its goal is to prevent destruction of the pool shell, deck, mechanical equipment and enclosures.
CSI is composed of five pool parameters — pH, temperature, bicarbonate alkalinity, calcium hardness and, to a small degree, total dissolved solids — combined into a single number. The concept itself is called pool water balance.
Water is considered scale-forming for a positive CSI, or aggressive (for nonmetallic surfaces) or corrosive (for metals) for a negative value.
This index of pool water balance was developed in the 1930s. But the tables for calculating the CSI apparently have been copied — incorrectly — for decades by one author after another, including the authors of the main pool operator manuals and handbooks. The CSI is defined as follows:
CSI = pH + TF + AF + CF – TDS correction
In this formula, pH is the measured value for the pool water. TF, AF and CF correspond to tabulated adjustments for temperature, bicarbonate alkalinity and calcium hardness.
But if you look at the temperatures of the existing CSI tables, something odd appears: The pattern goes up and down rather than in a smooth, rising curve. All pool operator manuals use the same scale, so the question is: Who published the first table (in pool antiquity) and how often has this scale been copied?
Taking a look at the first column in the chart, the discrepancy becomes clear. The temperature factor or correction should be a number that is gradually increasing with increasing temperature. The reason is that the original equation for calculating the temperature correction is a logarithmic function, a smooth curve; it does not allow for values jumping above or below the gradually increasing value.
In the first column, the tabulated temperature differences are: 5, 9, 7, 7, 6, 10, 8, 10, 11, 23. There is no consistency in these differences; they are jumping up and down. They should either be the same or slowly increasing. Using the original equation for the temperature compensation,
Bt = - 13.12 x log10 (oC + 273.15) – 34.55
the correct temperatures for compensation intervals of 0.1 were recalculated in Celsius (oC), and then converted to Fahrenheit (oF). The slight discrepancies in the increase are due to rounding of the values, which vanishes if the numbers are listed with four significant digits. The consistency and slight increase in the newly calculated values become quite clear.
Recalculation of the other two parameters, for bicarbonate alkalinity and calcium hardness, showed that they are tabulated correctly. It is my hope that future pool manuals will incorporate the corrected temperature scale. Though the adjustment is small (the old table has a factor of 0.7 at 84F, while the new table has 0.6 at 87F, resulting in an overall difference of approximately 0.1 in the range of pool and whirlpool temperatures), it deserves to be corrected.
If we use the CSI for our pools, why not do it right?