Something is consuming my FC

[avish222]

Hi,
I have a SWG unit but have been running my pool without it using bleach and had no problem.
I wanted to start using the SWG so I have added salt to a level of 3000 ppm.
I have also added CYA to raise its level from 30 ppm to 40 ppm, but this one is not dissolved yet.
After few hours I have tested FC and was suprised to see that it dropped to 0, CC was 1.5 ppm.
I have added bleach to raise FC to 5 ppm but after few hours FC dropped back to 0.5 ppm.
I have added again bleach to raise it to 8 ppm, tested after 15 min and it was OK. 2 hours later FC dropped again to 1.5 ppm.
During all this time I had the SWG running and the additional CYA soaked in the skimmer.
pH is 7.4, TA is 400, CH is 360.
It is an outdoor pool but all this happened after 5pm so no direct sun light.
Any idea why FC levels are dropping?
Thanks,
Avi.

 

[jblizzle]

Sounds like something is living in your pool. Time to start the shock process ... remember:

[shock:3m6l57kf][/shock:3m6l57kf]

 

[chem geek]

(posted same time as jblizzle)

Sounds like nascent algae growth that isn't yet visible, but if you don't get ahead of it then it will become dull/cloudy and eventually a full-fledged green bloom. You should shock the pool immediately. Go to 16 ppm FC at least (using chlorinating liquid or bleach) and keep it there until your overnight chlorine loss drops down to <= 1 ppm.

What you are experiencing is what the pool stores classically call "phosphates eating chlorine", but it's really just algae growth that isn't yet visible. By getting ahead of that growth and killing off the algae, your chlorine demand should return to normal.

 

[avish222]

I was thinking about this option but:

1. Water is crystal clear
2. In the past when I had algae starting to grow I could touch the walls of the pool and it was becoming smooth, it is not the case now.
3. It has started just after I have put the salt and the CYA in and turned on the SWG.

Is there other possible explanation beside invisible algae? I didn't use the SWG since last year, can it be something that was growing inside the SWG?

Thanks,

Avi.

 

[Richard320]

[center:16w5zvac][youtube:16w5zvac]uijLXS-IJZM[/youtube:16w5zvac][/center:16w5zvac]

 

[avish222]

Have started the shock process. Started with FC of 18 ppm and within an hour it dropped to 15 ppm. I guess it will take some time...
Is it OK to swim in the pool while FC levels are that high?
What is the maximal safe FC level to swim in?
Thanks,
Avi.

 

[heikejohn]

Hi,
I have a SWG unit.
I have also added CYA to raise its level from 30 ppm to 40 ppm, but this one is not dissolved yet.

Avi.

What level of CYA is recommended for your unit? I was under the impression that it is about 70 or so, for them to work properly.
Definitely finish the shocking process before adding more.

 

[avish222]

The recommended CYA level of my unit is 40 ppm.
Does low level of CYA has to do anything with consuming so much FC at night time?

The FC consumption started few hours after I have added the salt and the CYA. I can't ignore the feeling that it is somehow connected to what I have put into the the water. I'm testing the water daily and had no signs of anything going wrong before that event.

Avi.

 

[ping]

Having a low level of CYA has nothing to due with your FC consumption at night. Something is living in the water and must be killed off with chlorine. The salt and CYA additions did not cause this problem. Follow the shock process jblizzle linked you to.

Your problem is very similar to mine when I first found this site and it took me a week to clear up whatever was in my pool. My pool was also clear with no hint of algae but was consuming way too much chlorine.

 

[chem geek]

It's possible that the batch of chemical you added had something bad in it that reacts with chlorine, but the solution would still be to shock until the chlorine demand was exhausted.