Chlorine Lock Causes?

[MrcTag]

What is causing my chlorine to lock up? I have put A LOT of chlorine in my pool this year but still cannot get a FC chlorine reading. My testing over the last 2 weeks says my TC levels ranged from 3ppm to over 10ppm for a couple days at a time but nothing in the FC levels. I've taken my water to the pools store and they confirmed my TC and FC levels, they also did a chlorine demand test. The Chlorine Demand test recommends a partial drain might be my best way to solve this because the chlorine demand is so high (somewhere is the range of 88lbs. of Cal Hypo).

They also checked Phosphates and that level was 1000ppb, but they told me that level wouldn't be causing this problem.
What could be causing this Chlorine Lock, no matter how much or what from of chlorine I put in it just seems to lock up.

Thanks in advance Marc

I have a 20K gal. In Ground Pool with a white plaster finish.
ph 7.6
Total Alk. 120
Total Hardness 300
CYA 75
TDS 1000
Iron 0.0
Copper 0.0

 

[JohnT]

With your high CYA, you need more like 30ppm or higher to get rid of your Combined Chlorine. Sunlight can be a big help in clearing CC, so if you have a cover, leave it off.

 

[MrcTag]

Any Idea what causes the chlorine to lock up like that?

 

[JasonLion]

Chlorine lock is a very misleading phrase. The chlorine is not locked, it is getting consumed in the process of oxidizing some organic material. The most likely culprit is ammonia from some organic breakdown process over the winter, but fertilizer and algae will also have similar effects.

The proper treatment is to raise the FC level up to around 21 as frequently as is practical, up to once per hour, by testing the current level and adding more chlorine as needed.

 

[Guest]

You high CYA level requres a much higher FC level than you have been keeping to maintian a sanitized and algae free pool. In effect you have been underchloirnating your pool. The solution is to either run higher FC levels and shock to higher levels or drain some water to lower your CYA. you maximim level of 10 ppm is NOT enough to shock a pool with 75 ppm cya. Are you using trichlor tablets by any chance?

 

[chem geek]

Marc,

Was your CYA level higher at the end of last year? It's possible that some of it got converted to ammonia by soil bacteria (that got into the pool) when the pool was "let go" over the winter and that is the source of the high chlorine demand. When you add chlorine to ammonia you get monochloramine which shows up as Combined Chlorine (CC) and you will register zero Free Chlorine (FC) as all the chlorine very quickly combines with the ammonia. The monochloramine won't break down quickly unless there is an excess of Free Chlorine available (sunlight can also break it down, but somewhat slowly).

You might still have more ammonia in the pool. You can get an inexpensive ammonia test kit at a fish/aquarium/pet store. That way, you'll know how much more chlorine will be needed and whether it might just be easier to do a partial drain/refill to reduce the amount of all chemicals including CYA, combined chlorine, and ammonia. It takes a total FC addition of roughly 10x the ammonia level plus less than 1x the CC level to get rid of both.

Richard

 

[MrcTag]

You high CYA level requres a much higher FC level than you have been keeping to maintian a sanitized and algae free pool. In effect you have been underchloirnating your pool. The solution is to either run higher FC levels and shock to higher levels or drain some water to lower your CYA. you maximim level of 10 ppm is NOT enough to shock a pool with 75 ppm cya. Are you using trichlor tablets by any chance?

Yes I am using trichlor only because I need a slow release form of chlorine. I'm away a lot. My CYA does get rather high by the end of the season. A SWCG is my next investment to the pool.

Marc,

Was your CYA level higher at the end of last year? It's possible that some of it got converted to ammonia by soil bacteria (that got into the pool) when the pool was "let go" over the winter and that is the source of the high chlorine demand. When you add chlorine to ammonia you get monochloramine which shows up as Combined Chlorine (CC) and you will register zero Free Chlorine (FC) as all the chlorine very quickly combines with the ammonia. The monochloramine won't break down quickly unless there is an excess of Free Chlorine available (sunlight can also break it down, but somewhat slowly).

You might still have more ammonia in the pool. You can get an inexpensive ammonia test kit at a fish/aquarium/pet store. That way, you'll know how much more chlorine will be needed and whether it might just be easier to do a partial drain/refill to reduce the amount of all chemicals including CYA, combined chlorine, and ammonia. It takes a total FC addition of roughly 10x the ammonia level plus less than 1x the CC level to get rid of both.

Richard

OK this is starting to make sense to me. Yes my CYA level was very high when I closed last year. I thought I would lose some when I drained for the winter but I assume I didn't lose much. CYA breaking down into Ammonia and combining with chlorine fits well with the problems I am having each year. I'll check for ammonia and see if a partial drain is in order. Thanks Jason, Bear & Geek for helping out....I'll repost with the results

Marc

 

[chem geek]

Yes I am using trichlor only because I need a slow release form of chlorine. I'm away a lot. My CYA does get rather high by the end of the season. A SWCG is my next investment to the pool.

Marc,

In addition to looking at an SWCG, you can also consider using The Liquidator talked about in this thread and available at tftestkits.com. I'm not sure how long an 8-gallon sized container would last in your pool but in 20,000 gallons at 2 ppm FC per day that would be 4.5 gallons of 6% bleach per week so would last almost 2 weeks. If you used 10% or 12.5% chlorinating liquid, then it would last even longer.

Richard