Chlorine went up unexpectedly

Been using the BBB method since opening my 30000 gal inground concrete pool in May this year. Water has been clear and no algae problems. I have a DE filter and a Nature 2 system so been keeping chlorine low 1-2ppm with no problems and water has been crystal clear since May 2007 (it was clear when I opened the pool also. I last shocked the pool in mid June 2007. 4 days ago we had a big rain (2 inches), but my free chlorine levels (FC) held at 1-2ppm. I usually add 60oz of chlorox each night and FC has been maintaining 1-2 ppm without issue during the day. (my pool gets full sunlight). 2 days ago, my FC dropped during the day to 0.1ppm for no reason-I quickly added chlorox for a 1.5ppm increase and measured it the next morning. Again, the FC dropped to 0 during the night, so I added another 136 oz dose of chlorox plus a 1lb bag of CaHypo that morning (yesterday morning) and measured FC during the day-it was 5 ppm (as expected after this add) and slowly decreased to 3.3ppm by the end of the day (yesterday) (not too bad since sunny all day). So, that evening (last night) I shocked the pool with 4 large jugs (1.4gal) of chlorox or 5.6 gal total which should have been an 11ppm increase.

2 hrs later (last night) I measured FC and it was 22ppm and the pool was cloudy-barely able to see the main drain (8 ft of water). pH also had gone up to 8.2 from the chlorox add. The next morning (this morning) it had decreased down to 16ppm (and pH went down to 8.0). The water this morning has cleared up approx 50% as I can now see the main drain clearly although still some cloudiness in the water. At this point I back flushed the DE filter as the back press had risen from 21psi to 27psi during the night (went up 6psi after shocking).

Question is, where did the chlorine come from? The FC is way higher than I added, so it appears the chlorine came out of some "black hole" after shocking. I have been measuring FC with both OTO Taylor drops and my LaMotte DPD colorimeter, so I think the high chlorine was a true reading as they both compared before and after the shock (had to dilute 1 to 3 after the shock to get a reading within range).

Any ideas why my chlorine rose to 22ppm when I only added enough chlorox for 11ppm? (5.6 gal).

My latest chemistry shows pH a little high (it is coming down as the chlorine level comes down. Here is my chemistry before and after the shock. Pool temp = 85F. I expect pH to continue to decrease so I have not added for pH, plus the water is in balance at this point.

Any suggestions of what happened would be appreciated. I will be adjusting pH as needed after my chlorine comes down due the the sun



Before shock
FC 3.3/TC 3.5
pH 7.6
TA 75
CH 218
CYA 46
water clear, DE press 22psi

After shock
FC 16/TC 16
pH 8.0
TA 80
CH 218
CYA 46
water slightly cloudy, DE press 27psi

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My latest chemistry shows pH a little high (it is coming down as the chlorine level comes down. Here is my chemistry before and after the shock. Pool temp = 85F. I expect pH to continue to decrease so I have not added for pH, plus the water is in balance at this point.

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Your high pH reading is misleading as many pH tests are not accurate at high chlorine levels.

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Originally Posted by JohnT

Your high pH reading is misleading as many pH tests are not accurate at high chlorine levels.

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Ok, thanks, will wait until Chlorine in range before pH adjust. pH was 7.8 with Chlorine at 11.2ppm today at 1200 (6 hrs after the 16ppm sample), so your info is aligned with this. My OTO tester pH read 7.4, so I know that pH reading was affected by high Cl.

The Cal-Hypo is sometimes slow to dissolve so I suspect that is what happened. It wasn't fully dissolved and continued to dissolve and raise the FC after you added your Clorox. That's my best guess.

Richard

If you check the thread you started on www.troublefreepool.com I gave an answer there that could possibly explain what happened.

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Originally Posted by waterbear

If you check the thread you started on www.troublefreepool.com I gave an answer there that could possibly explain what happened.

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Thanks to waterbear and chemgeek. Appreciate your thoughts.
I did not consider analysis errors due to diluting my 5cc sample for the colorQ. I also did not consider the CaHypo dissolving slowly. It makes sense putting the two together, would account for the FC rise to 22ppm and also the pool water cloudiness. Makes me feel a little bit more comfortable.

The pool appears to be clearing up and the DE back pressure is staying at 22psi. Still do not know what hit the pool. I guess I need to super chlorinate more often? My shock in June was only a 2.5ppm increase since my CC was only 0.2ppm. The last super chlorinate (10ppm) was way back in May07. I also have been running chlorine a little low since I have the Nature2 system which is probably a risk, especially since I know I have a high phosphate level (2000ppm).

This is one of the big problems with Nature2. If you had kept your FC at about 4 ppm (about right for your CYA level) then you would have had enough reserve FC in the water and you probably would not have had this problem in the first place.