First, I want to Thank the TFP community for educating me with Pool School. I learned more reading hours of posts and reading the Pool School articles than I have EVER learned in all my years of pool care put together.
I always thought before coming here: Chlorine should be 3 ppm (ideal). Anything below 2 is not enough and above 3 is too much.
I always figured stabilized is superior to stabilized, though I did not realize that ANY dry chlorine is has an additive such as calcium or CYA. I had no idea what the difference was between Cal-Hypo and Tri-Chlor and that one leaves calcium and the other leaves CYA. I did not realize that Tri-Chlor, the stuff I had been using adds 6ppm of CYA for every 10 ppm of chlorine.
Hence, if my pool consumes 2 ppm per day, in 5 days I would have added 10 ppm FC to maintain levels, but left my CYA 6 ppm higher. Being the ideal numbers are 30 - 50 (except for salt water)... this would only take a few weeks to screw up the CYA level making it too high.
NOW ON TOPIC:
I read this and love it:
Pool School - Chlorine / CYA Chart
Here is my memorization technique:
Take the first number and add one (1). The first number in 20 is 2... 2 + 1 is 3. You could also look at it as 20/10 + 1, but that starts to look like a formula.
20 CYA is Target 3 ppm (2 + 1 is 3)
30 CYA is Target of 4 ppm (3 + 1 is 4)
40 is 5 ppm
Quits working at 100 (tipple digits), but water is usually screwed up to ever get that stabilized.
Anyway, this got me to notice:
Pool stores sell these:
Taylor K-1001 (DPD):
.5 ppm to 5 ppm (Nothing higher than 5. Hollow COW - useless if your CYA is > 40,)
Taylor K-1000 (OTO):
.5 ppm to 5 ppm (again same problem)
Can't be beat for the $6 the kit costs though.
How can they sell these maxing out at 5 ppm Chlorine in good conscious when 30-50 CYA is Recommended, and above 40 CYA you need MORE than you can measure! (i.e. at 50, you need 6 ppm). It's insanity! No wonder they sell so blasted much PHOSfree and Algesides.
After coming here, I can assure you that most days I WILL not use the FAS-DPD because I do not need perfection. If I know my CYA is 40 ppm for example, I know I need 5 ppm of FC. If I keep it around 6 ppm FC, I have a safety margin, and why count so darned many drops with FAS-DPD daily?
I look at FAS-DPD as a troubleshooting test when EXACT numbers are needed.
Hence, I am buying this because it is a REASONABLE range for CYA < 100
What I am saying is that in addition to my K-2006 test kit, I am buying a K-1000 to get OTO (to check for the presence of chlorine), and a K-1001 to accurately check anything up to 5 ppm (Probably +/- 1 ppm), but I am getting that extra comparator that goes up to 10 ppm