The chlorine gremlins stole 25ppm in 2 days

I've been having an interesting time managing my pool chemistry this season. I live in Arizona and we're regularly experiencing 100+ degree days. My family is pretty upset that the pool is either super-chlorinated or green. I have a 14500 in ground pebble-tech pool with a salt water generator and an ever-present green line at the water line. The SWG doesn't seem to keep the FC at a reasonable level.

After some research and professional advice, I was told to add 10 - 12 gallons of liquid chlorine to zap the algae and kickstart the FC. After 2 days, the water looks great, but I went from 25ppm to 1ppm in ~60 hours.

Here are the readings with my PS234.
6/24 (A.M.)
FC 25
CC .5
TC 25.5
pH 2.8
Alk --
Cya 30

6/25 (P.M.)
FC 7
CC .5
TC 7.5
pH 7.4
Alk 110
Cya 20

6/26 (P.M.)
FC 1
CC 0
TC 1
pH 7.5
Alk 110
Cya 0

I turned off the SWG based on advice from this forum. What's going on here? Is it normal to lose that much FC in so little time and my CC to remain fairly constant. Also, what about the drastic drop in Cya? Any help would be appreciated.

Please help me gain back the title of super-dad... My kids are looking at me with pathetic faces waiting to take a swim.

Three things to check:

1) Make sure your SWG is on! (Sounds stupid, but mine was somehow switched off for several days before I discovered the "cause" of my missing CL). Also, make sure you have your output set high until you are sure you are generating.

2) Check your salt level. It needs to be within (and preferably on the high side) the range specified by your SWG.

3) Get your CYA up to whatever the SWG recommends. This is really critical, especially with your situation, the sun is going to eat up your CL long before you have a chance to get it built up if you don't get your CYA into the range of 60 to 80 recommended by most SWG manufacturers.

Covering the pool, especially during daylight, may help as well.

It sounds as though you still have some organics (algae) consuming your free chlorine. One way to TEST this is to measure your overnight loss of chlorine.

I would suggest that you dose your pool to "best guess chlorine for CYA" "shock" level around 8 or 9 pm. Then check you CL around 11 pm or so (give it time to distribute) and then re-check in the am. If there is nothing bad (algea or others) in your pool then your nightime CL and morning CL should be the same (this is a negative test or a normal)

If there is a drop, that means that something undesireable is consuming your free CL (this is a positive or abnormal test).

I would suggest superchlorinating until your morning CL does not drop.

For this test (overnight chlorine loss test), you will need to turn your SWG off. Because if you leave it on, you may get a false negative test. I would run it the rest of the time.

I have no experience with SWG, but it seems that your CYA is too low. Most manufactures suggest a CYA in the 80 range.

I cannot explain your loss of CYA, but must consider a measurement error. How did you measure your CYA? Most kits cannot give a reading less than 30.

I hope this helps.

I second adesalvo's advice...you have algae or other organics to kill and that's evidenced by your green line even though you have clear water. Kill the algae first. The retesting and redosing a few times a day is the only way to be sure it's gone. I agree that for this stage, turn off the SWCG and use bleach or liquid chlorine.

Once the algae's gone, increase your CYA and follow tphaggerty's advice to be sure your SWCG is operating properly.

As for your decreasing CYA reading...I know that I can only get a +/- 10ppm on that reading when I do it. The same sample can seem like 30 but when I redo it, maybe it's 35, maybe it's 25, etc.

This is my first full summer with the SWCG and I can see that I need to go higher than the 45-50 CYA I had when the unit became operational last August. When we've had the intense sun and 90+ degree days, my FC was not holding where I wanted it to be, given my pump run time and my SWCG setting (both of which also influence the FC in the pool). I'm heading for 60, which, ironically, is the minimum CYA suggestion for my unit. I have to wait for all this rain to stop :mad: before I can tell if 60 will be the right number for my pool.

Thank you for the advice. I will repeat the chlorination process until the FC drop overnight.

The only thing I would add, if you're not doing already, is keep the filter running 24/7 especially when temps are high. Beyond that, increasing CYA would be prudent.

CaryB

Thank you to everyone for your help! After two rounds of heavy brushing and lots of bleach, my pool is back to normal.

Dad is a hero again!
- A loyal convert of the church of poolforum.com