Using Trichloro tablets ocassionally to maintain CYA level

Fuldo

Well-known member
Nov 23, 2017
81
Port Orange, FL
#1
I have a 12,000 gallon plastered gunite pool that I've been maintaining almost exclusively with 11% liquid bleach for about a year now. After recovering from excessive CYA last year I'm very careful of monitoring and maintaining my CYA levels. I try to keep CYA at about 30, I haven't had a problem with it drifting lower but I can only measure it down to about 25-30 with the Taylor kit and I want to know just what it is.

The CYA level drops very slowly due to normal water losses so I must bolster it occasionally. I have some Clorox Active 99 3" Trichloro tablets I'd like to use-up and figure using them to bolster CYA may be their best use. My question is how to judge how effective they'd be at that role and how to judge how many to use for a particular adjustment. I found a thread on TFP where someone with a 12,000 gallon pool found an increase in CYA of 30 by using only a total of 5 pucks. That translates to an increase of 6 CYA per puck. That seems like a lot. Does that seem reasonable and if not can someone give me an idea of what to expect?
 

mknauss

Mod Squad
May 3, 2014
12,690
Laughlin, NV
#2
Use Effects of Adding Chemicals in PoolMath. You input your pool volume in the upper left and then down towards the bottom of the page you put in what you are adding and it will tell you the effects to the water chemistry.

For example, in your 12000 gallon pool, an 8OZ trichlor tablet will raise your water FC by 4.6ppm, your CYA by 2.8ppm, lower your pH by 0.24, and raise your salt by 3.7ppm
 

JoyfulNoise

TFP Expert
Platinum Supporter
May 23, 2015
14,633
Tucson, AZ
#3
As Marty said, the Effects of Adding Chemicals section in Pool Math can help you out. Most 3" trichlor pucks are about 8oz, give or take. Don't pay much attention to the pH changes as those are only very rough estimates and assumptions are being made about the TA of the water. As long as you have sufficient alkalinity in the pool water, the pH decrease from dissolving trichlor pucks will be unnoticeable. Just keep an eye on your TA and adjust as necessary.

The only downside to using pucks is this - they are very slow to dissolve (that's a feature, not a bug). So if you were looking to raise CYA quickly, then pucks are not the best method. Dichlor powder dissolves more quickly than trichlor and is less acidic (it also raises the FC by less).
 
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Fuldo

Well-known member
Nov 23, 2017
81
Port Orange, FL
#4
Thanks for the advice and guidance. I had no idea that the "effects" portion of the pool math existed. A very valuable reference I'll start using immediately.

The predicted FC increase of 4.6 looks suspicious to me however. I'm assuming that would assume no FC loss and the tablets take many days to dissolve. Can I assume if the rate of FC loss is normally about .7 per day then adding a Trichlor tablet will hold FC constant for about a week that it takes to dissolve?
 

triptyx

TFP Guide
Apr 12, 2016
1,485
Tucson, AZ
#5
FC loss per day in a healthy pool with no swimmers that's at swimmable temperature is usually 2-4ppm depending on sun strength and a few other factors. Colder water will lower the FC loss.

Start with one tab and test the following day to see where you are. Add a second if it's not keeping up, but do carefully watch that CYA level.
 
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Fuldo

Well-known member
Nov 23, 2017
81
Port Orange, FL
#6
Lately my pool is dropping only 1 ppm FC or less per day when trying to maintain it at about 4 ppm with CYA at about 25 and water temp about 70. Perhaps the full screen enclosure is partially why it drops so little this time of the year.

I added one tab last week when the FC measured 5.0. I checked it again 48 hours later and measured the same 5.0 FC. The tab hardly changed in size in those two days so I suspect it will last a full week and perhaps keep the FC fairly constant for that week. Unfortunately I had to leave town three days after adding the tab and I'll likely be away for a few weeks so I won't be able to observe the tab dissolving or measure the CYA directly afterwards. I'll keep in touch with my "pool caretaker" while I'm away and play it by ear.

It sounds like 3-4 tabs will get me back to about 35 CYA. The pool seems fine and doesn't use excessive amounts of chlorine even with 25 CYA so I'm not in a hurry to raise it. I'll check CYA level when I return and use the tabs sparingly when the pool has less attention and using the tabs will be an advantage.