Counterintuitive CYA level / chlorine consumption question

Steve14

Member
Mar 25, 2024
9
Venice, FL
Pool Size
8500
Surface
Plaster
Chlorine
Liquid Chlorine
First post - hi everyone!

Started maintaining my pool last fall when I bought the house. Previous owner had been using a puck dispenser with trichlor and CYA level was 220. I've managed to work the CYA level down to 50 with drain/fills and the help of a tropical storm. I keep excellent records of my pool chemicals using a spreadsheet. Here is my issue:

In May, June and July, when my CYA was 70-80 the pool was consuming approximately 1.8 - 2.2 ppm of chlorine per day at a FC level of about 8ppm. pH 7.5, TA 70, CH 250 - 300, CC 0.5.
In the last five days, after getting my CYA down to 50 it is consuming only 0.73ppm per day at a FC level of about 6ppm. pH 7.4, TA 70, CH 225, CC 0.5.
This is with a similar weather pattern, same water temp. the water is crystal clear. (I know my hardness is a little low - working on that, but it's consistent between the two periods in question.)
I've been using the same chlorine - 2.5 gallon jugs from Leslies.

My question is this, according to what I read on here, FC consumption should be lower at higher CYA levels - why am I seeing the opposite? What am I missing?
 
Welcome to TFP.

Have you been checking the date codes of the liquid chlorine you are using?

You could have been getting old chlorine that was degraded and not at full strength. Which now Leslie has restocked with fresh chlorine that is full strength.

 
Welcome to TFP.

Have you been checking the date codes of the liquid chlorine you are using?

You could have been getting old chlorine that was degraded and not at full strength. Which now Leslie has restocked with fresh chlorine that is full strength.

I thought about that and asked if anyone had mentioned weak chlorine. The clerk said no, but that doesn't mean he hadn't.
These are the refillable jugs, not retail packaging. It's a "Pro" designated Leslies store so I think their product turnover is pretty good.
 
Last edited:
Follow-up.

I did before and after adding chlorine tests. ajw22 was right, the issue is chlorine strength. The batch I have is nearly twice as strong as a batch I had in May. If I've estimated my pool volume correctly, this batch is nearly 20%.
 
Don't forget we are further from the longest day of the year than we were in June and July.

The sun is lower in the sky now as well.

It's tricky making comparisons as you did because they need to be real close in time.
 
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