I have a 355 gal hot tub, and as a result of CC creeping up to 0.5 - 0.6, I've decided to shock the spa. My CYA is 35, and consulting the Cl/CYA chart, I see I need an FC level of 14 ppm. To be safe, I raise FC to 16, and test again later in the day. FC has dropped to 12 or so. I raise it back up to 16. This continues several times, over three days, but my CC is always exactly where it was when I started. I double-check the CYA level to be sure I have it right. The same thing happened to me last draining cycle when I tried to shock the spa mid-cycle. What should I be doing differently?
BTW, I'm testing FC/CC using Lelie's FAS-DPD kit (#81329). I normally use the 25ml sample (with 1 drop = 0.2 ppm), but occasionally I use the 10ml sample (with 1 drop = 0.5 ppm), especially when I know FC will be high. In connection with this kit, I do have a few questions:
1. The test calls for 2 little spoons of DPD powder (R-0870), regardless of whether I'm using the 25ml or 10ml sample. Does that mean that the test is insensitive to the amount of powder I add, as long as it's above some minimum threshold? The reading won't be off if my spoons of powder are a little bigger or smaller than usual?
2. The CC test requires me to add 5 drops of DPD Reagent #3 (R-0003) after finishing the FC test, again regardless of the sample size (and which again seems odd). What if I mistakenly add 6 drops instead of 5? Also, what if I accidentally add an extra drop of the titrating reagent (R-0871) when testing FC? Will that invalidate the subsequent CC test? I guess it's not possible to test CC without first testing FC.
3. Finally, I've noticed, when testing CC, that after adding enough drops of the titrating reagent to make the sample clear again and recording the CC level, the sample will become pink again after sitting a minute or two. Is that normal?