I have made notes to myself over the years as followed:
Cal-Hypo increase Calcium Hardness (CH). Trichlor or Dichlor increase CYA.
For every 10 ppm Free Chlorine (FC) added by Trichlor, it also increases Cyanuric Acid (CYA) by 6 ppm.
For every 10 ppm FC added by Dichlor, it also increases CYA by 9 ppm.
For every 10 ppm FC added by Cal-Hypo, it also increases Calcium Hardness (CH) by 7 ppm.
For ANY source of chlorine (except a saltwater chlorine generator, SWG) for every 10 ppm FC that is added, the result will be an increase in salt of 8 ppm when the chlorine gets consumed/used (since the chlorine becomes chloride). For sodium hypochlorite and lithium hypochlorite, there is an additional 8 ppm salt added upon addition so the net result is an increase of around 17 ppm salt (not 16, due to rounding and other by-products). For Cal-Hypo, the extra salt added is around 2-4 ppm so the net salt increase is around 10-12 ppm.
CYA .. Min FC3 .. Target FC .. Yel/Mstrd Min .. Shock FC .. Yel/MstrdShock
0 ....... 0.071 ....... 0.111 ......... 0.151 .............. 0.641 ....... 1.501
10 ...... 0.81 ........ 1.21 ........... 1.61 ............... 4.5 ......... 7.1
20 ...... 1.51 ........ 2.4 ............ 3.1 ................ 8.3 ........ 12.7
30 ...... 2.2 ......... 3.5 ............ 4.6 ............... 12.2 ........ 18.2
40 ...... 2.9 ......... 4.6 ............ 6.1 ............... 16.0 ........ 23.8
50 ...... 3.7 ......... 5.7 ............ 7.5 ............... 19.8 ........ 29.42
60 ...... 4.4 ......... 6.8 ............ 9.0 ............... 23.7 ........ 34.92
70 ...... 5.1 ......... 8.0 ........... 10.5 .............. 27.52 …...... 40.52
80 ...... 5.8 ......... 9.1 ........... 12.0 .............. 31.42 ........ 46.12
90 ...... 6.6 ........ 10.2 ........... 13.5 .............. 35.22 ........ 51.72
100 ..... 7.3 ........ 11.4 .......... 14.9 ............... 39.12 ........ 57.32
120 ..... 8.7 ........ 13.6 .......... 17.9 ............... 46.72 ........ 68.42
1A minimum FC level is needed as a "reserve" for usage so in practice at least 2 ppm FC is required even at low CYA levels. The table above shows the amount needed for disinfecting chlorine for equivalent killing power (rates), but does not take into account the amount needed in reserve to prevent getting used up as this varies by pool.
2The shock levels shown have equivalent disinfecting chlorine amounts (in a column) but at high CYA levels it may be impractical to use such high FC levels. A partial drain/refill to lower the CYA level is usually what is needed or one can shock at a lower level but will take longer to kill the algae.
3Most saltwater chlorine generator (SWG) pools appear to prevent algae at a minimum FC level of 4.5% of the CYA level as compared with the roughly 7.5% of the CYA level shown in the "Min FC" column for manually dosed pools.
NOTE: A reasonable approximation to the above table is the following:
.... "Min FC" is 7.5% of the CYA level
.... "Target FC" is 11.5% of the CYA level
.... "Yel/Mstrd Min" is 15% of the CYA level
.... "Shock FC" is 40% of the CYA level
.... "Yel/MstrdShock" is 60% of the CYA level.
Since chlorine is more effective at lower pH, it is normally recommended to lower the pH before shocking at a high FC level, especially for yellow/mustard algae where lowering the pH to 7.2 before shocking at the above levels is best (and remember that the pH test will not be valid during shocking due to high FC levels).
Pool Size is 21,470
FC level target is 3 PPM at 3 PM (1 Qt. = 2 PPM using 12.5) at 30 CYA
PH level target is 7.3 (1 Qt Muriatic Acid lowers PH by .2)
TA target is 80 to 120
CA target is 200 to 400
CYA target is 30 to 80
Borate level target is 50 (1 lb. Boric Acid raises Borate by 1, 25 Oz. 20 Mule team Borax raises Borate 1)
Pool School recommended as below:
Plaster with Bleach
FC 3-7
pH 7.5-7.8
TA 70-90+
CH 250-350
CYA 30-50
The following chart shows how much the CYA, CH and Salt would rise after 6 months if there were 2 ppm FC per day chlorine usage and no water dilution:
........................ CYA ... CH ... Salt
Trichlor .......... 220 ...... 0 .... 295
Dichlor ........... 325 ...... 0 .... 295
Cal-Hypo .......... 0 .... 255 ... 360
Bleach* ............. 0 ........ 0 ... 595
*Bleach, chlorinating liquid, lithium hypochlorite and chlorine gas all result in no CYA, no CH and the same amount of salt.
Note that a rise in CYA of 220 ppm is much worse than a rise in CH of 255 ppm which is worse than a rise in salt of 595 ppm.
My 21,470 gallon pool costs me just under $ 500.00 in Chlorine and Muriatic acid per year. This is with a target CYA of 40-50. My TA hover around 90-110, and my PH is always rising so I target the low end of the range at 7.2 and add Muriatic acid when it hits 7.8 because I do not like to mess with Muriatic Acid on a daily basis. I like to get my Chlorine at the local pool store in refillable jugs and keep 8 of them for the summer usage so I do not have to go to the store as often. (It gets expensive here in July and August but not so much in January and February) I test my CL level just before sunset and add CL then so it will have all night to do it job before the evil orb has its way with it.
Have faith, it’s just a pool and if you stare at the chemical aspect of it long enough it will make sense.