My pool:

21000 gallon in-ground gunite salt water pool, new October 2018, screened in.

Hayward PL-PS4 controller, Hayward T-CELL-9 salt cell, Hayward C3030 cartridge filter, Hayward Ecostar SP3400VSP pump.

Set to do a little over 2 full turnovers per day.

South Florida climate.

Brushed and vacuumed regularly.

Taylor K-2006 FAS-DPD titration test kit, fresh chemicals.

My recent Taylor readings:

pH: 7.6

total alkalinity: 90ppm

calcium hardness: 250ppm

stabilizer: 70ppm

water temp: 85F

salt: 3300ppm

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HOW I CALCULATE CSI:

I use a spreadsheet to log all my pool test values, and it auto-calculates CSI values.

Here's an example layout for the spreadsheet:

cell# value

A1 pH

B1 total alkalinity ppm

C1 calcium hardness ppm

D1 water temp F

E1 cyanuric acid ppm

F1 salt ppm

To determine the carbonate alkalinity, I use a vlookup function to derive the cyanurate factor from pH and cyanuric acid ppm, then multiply that factor by the cyanuric acid ppm. That gives me the cyanurate alkalinity ppm. Then I subtract that cyanurate alkalinity from the total alkalinity, to get the true carbonate alkalinity, which is used in the CSI formula.

Here's the lookup table I'm using in the spreadsheet (from TAYLOR "POOL & SPA WATER CHEMISTRY") for the vlookup to determine the cyanurate factor (this table is in spreadsheet cells P1:Q10):

pH CYANURATE FACTOR

6.5 0.11

7 0.22

7.2 0.26

7.4 0.3

7.6 0.33

7.8 0.35

8 0.36

8.5 0.38

Here's the spreadsheet formula I use to auto-calculate CSI:

**=A1 + (0.4341*LN((B1-(D1*(VLOOKUP(A1,$P$1:$Q$10,2)))))+0.0074) + (0.4943*LN(C1)-0.7661) + ((0.0105*E1)-0.2368) - 12.34**

It looks complicated with the logarithmic functions, but it's just 5 parts:

the Ph factor

the carbonate alkalinity factor

the calcium hardness factor

the cyanuric acid factor

the correction factor for salt pools (~3400ppm salt should be about -12.34)

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This calculates a CSI of -.29

When I enter my 6 measurements into the PoolMath, it gives me -.24

Close enough, but I am curious to know the formulaic details that PoolMath uses, and if I need to adjust any of the values I'm using in my spreadsheet.

I would have attached a sample spreadsheet to clarify the above info, but the .xlsx file format is not allowed for upload.

Thanks!