Question about breakpoint shocking

[Tmaragni]

Every time I try to breakpoint shock one of the pools I run (a 27,000 gal therapy pool), it seems the combined chlorine goes up not down after I shock it! This is the process I use to shock the pool:
1. Backwash the filters
2. use powdered Pulsar shock adding enough to bring the free chlorine up 15 to 20 times the combined chlorine reading at close
3. backwash filters again in the morning before opening.
4. bring chlorine into legal range using sodium thiosulfate

Every time I have done this, the combined chlorine goes up in the days after I shock not down! Is there something I am not doing properly?

 

[frustratedpoolmom]

What type of shock is it? If it's MPS this is normal. What is the active ingredient in the "pulsar shock" product?

Is this an indoor pool?

Please post a full set of results.

Your shock level is based on your CYA level, but if this is an indoor pool sometimes the advice changes.

 

[polyvue]

Every time I have done this, the combined chlorine goes up in the days after I shock not down! Is there something I am not doing properly?

Yes, as frustratedpoolmom said, post your test results and chlorine levels; please include pH, Total Alkalinity (TA), Calcium Hardness (CH) and Cyanuric Acid (CYA).

Here's a brochure describing the shock (it's basically Cal-Hypo): Pulsar Power Shock

 

[Aquaman95]

Do you have good air circulation?

 

[PaulR]

Is it possible that using the thiosulfate produces CC? I did a casual experiment and while I don't know that it's conclusive of anything (except liking to play with chem tests), I think it did produce some CC in my test tube.
--paulr

 

[chem geek]

The thiosulfate, as most reasonably strong reducing agents, would normally reduce both FC and CC so I can't explain your result.

 

[Tmaragni]

Here's the numbers I had before I last shocked:
pH: 7.5
Alk: ~80
Hardness: 450
Free Chlorine: 2.0
Total Chlorine: 3.0
(I don't test for CYA since it is illegal to use in NY for public pools)
The pool is a 27,000 gal indoor therapy pool kept at 89 degrees.
The shock I use is granular Calcium Hypochlorite 78%

 

[duraleigh]

(I don't test for CYA since it is illegal to use in NY for public pools)

I don't live in NY but I'm not sure that's the case.

Are you shocking on an "as needed" basis or on a regular schedule?

Thanks for posting those numbers. So, after you shocked, what did your CC's value rise to? Has it stayed there (higher than 1.0) or has it come back down?

 

[Aquaman95]

(I don't test for CYA since it is illegal to use in NY for public pools)

I don't live in NY but I'm not sure that's the case.

Are you shocking on an "as needed" basis or on a regular schedule?

Thanks for posting those numbers. So, after you shocked, what did your CC's value rise to? Has it stayed there (higher than 1.0) or has it come back down?

Unless something has changed recently, he's correct. CYA and products containing CYA have been banned in public pools for years in NY.

 

[frustratedpoolmom]

Yup, found this here:
http://www.health.state.ny.us/nysdoh/phforum/nycrr10.htm

I'm not surprised, but don't get me started on why governments make certain decisions...

 

[Tmaragni]

I shock as needed (when my combined chlorine reaches 1.0 or higher)
After shocking, the combined chlorine went up to 1.3 and has risen slowly since.

 

[chem geek]

You've probably got a lot of organics in the pool. Since they are forming CC that isn't decreasing, they are either persistent chloramines or a lot of urea in colder water that combines with chlorine to form chlorourea and takes days to oxidize (but oxidizes much faster in warmer water which is why it doesn't build up in hot tubs). You can do a bucket test with the pool water to see if chlorine alone will get rid of the CC. It might not. If that is the case, then you may need to use alternative techniques including non-chlorine shock (MPS) which unfortunately measures as CC itself (though there is a Taylor K-2042 for removing such interference), enzymes, UV or ozone, or dechlorination/rechlorination systems such as activated carbon followed by chlorine reinjection (i.e. the German DIN 19643 standard approach used in Europe).

Since you said this is a therapy pool, I would assume that the water temperature is warm, possibly 86-88F, is that correct? If so, then the problem is less likely to be urea unless the bather load is rather high or quite a few people urinated in the pool.

Whatever technique you want to try can be tested with a bucket test scaled accordingly. That way you don't waste your time on the bulk pool water adding things that end up not working.

By not using CYA in the water, the 2 ppm FC is harsher on swimsuits, skin and hair and theoretically the production of irritating nitrogen trichloride is higher, all by roughly a factor of 10 more than it would be if you had an FC that was 20% of the CYA level such as 4 ppm FC with 20 ppm CYA which is probably a good level for commercial/public pools. I understand this isn't allowed in your state. The only other alternative is to keep the FC as low as reasonably possible such as the German DIN 19643 standard I mentioned, but that's 0.3 to 0.6 ppm which can be hard to maintain and would also violate the New York standard which I believe has a minimum FC of 1 ppm. So not a lot of good choices here.