Trichlor tabs lose CYA effectiveness?

[Prav]

My parents have religiously used tabs through the years. I warned them about overstabilizing, but imagine my shock when CYA tested very low (20). Yes, there is significant dilution because of backwashing (sand filter) and fill-ups (evaporation), but if I was going to place a bet on their CYA level, I would have guessed closer to 100.

So, now I'm trying to raise CYA (carefully so as to not overdo it). Naturally, I would think the pucks would do it - but it's been 2 weeks and it's still in that 20 range. Is there anything that would make 3" trichlor tabs lose their effectiveness as CYA source? The only other possibility is that I'm adding it so slowly that it's really not registering. They do have about ½ gallon of liquid conditioner (>1 year old) that should in theory raise CYA by 15 ppm.

Funny thing is that their HOCl usage is about the same as mine: both of us are using 2½ gallons/week - me with 15K gallons/70 CYA/50 borates and them with 14K gallons and the aforementioned 20 CYA (I just added borates, so I'm discounting that). We're both in similar sun situtations (high - though mine may get an extra hour than they do due to shading). I would have thought they'd consume more.

Thoughts?

 

[JasonLion]

Most likely they lost all of their CYA over the winter, which happens often enough (just not when you want it to).

Since they are using trichlor, I would leave CYA alone. Give yourself as much headroom as you can get to allow CYA to rise.

 

[woodyp]

My CYA dropped from 50 to between 20ish over the winter but I didn't close it.

 

[anonapersona]

How are you testing that CYA? I found that strips told me CYA = 20 when it was actually more like 40.

 

[PaulR]

In my 18K gallon pool, each 8-oz tab works out to about 3ppm FC and 2ppm CYA--it would take about 10 tabs to raise CYA by 20. Tab consumption in my pool right now is less than 1.5 per week so it will be a couple months to bring up my CYA that way.
--paulr

 

[thunderkyss]

In my 18K gallon pool, each 8-oz tab works out to about 3ppm FC and 2ppm CYA--it would take about 10 tabs to raise CYA by 20. Tab consumption in my pool right now is less than 1.5 per week so it will be a couple months to bring up my CYA that way.
--paulr

So if you are only adding 4.5ppm of FC to your pool per week what are level are you maintaining FC at?

Or do you mean that an 8-oz tab supplies 3ppm of FC a day for 4 days?

 

[PaulR]

The pool calculator tells me 8 oz trichlor is worth what I said, when it has completely dissolved. I loaded 4 tabs in the floater a month ago, have put 2 more in since, and they're not all gone yet, so the floater is contributing < 1.5 tabs per week. I'm still using some bleach to keep FC in range, the tabs alone in my dinky floater aren't enough.

But the main point of my post was: Patience. Two weeks of tab use isn't enough to show a difference in CYA.
--paulr

 

[Prav]

Using 2.5 gals/wk liquid HOCl in addition to the tabs. Not concerned about a slow rise, more like surprise that continuous puck use like they have been doing would not have CYA>100.

BTW, not using strips but the dot test.