Using liquid Chlorine in the spa yet ORP DROPS??? ...help pls ....

[Maintenance Man]

Hey everyone,
id like to thank u in advance for any help or advice you may have on this issue.

Recently we had a new CAT 2000 chemical control system installed for both our pool and spa
the pool seems to work spot on, and the PH side works for both units as well.
problem im having is when the chlorine feeder kicks in its actually DROPS the ORP value of the
machine and overfeeds to about 30 - 50 ppm before the ORP starts rising again..

ive done everything from installing new probes, eliminating any possibility of electrical interference in the water,
checking for chemical flaws in the water, draining and starting new, even diluting the ch... nothing works..

we are Using sodium hypochlorite 12% ordered from swish.. I recently read an article explaining that initially it will drop the orp value when introduced as hypochlorite ions will do that until they convert..

is there ANY way I can make this work??? is there an alternative that I can use that wont have this effect??

 

[chem geek]

Welcome to TFP!

This is yet another reason why we don't normally recommend ORP systems.

Is your chlorine feeder coming AFTER the ORP sensor in the circulation flow? If not, then that is what you need to do though even the hypochlorous acid level in chlorinating liquid should be high enough to significantly raise ORP. As for hypochlorite converting to hypochlorous acid, that happens almost instantaneously as the chlorinating liquid gets diluted into the bulk pool water.

Some threads describing various problems include the following (this is NOT an exhaustive list and there are many more such reports on this forum):

pH & ORP meter readings fluctuating wildly, & other problems
ORP drifts high regardless of chlorine concentration
ORP and pH Rise Over Hours in Sample
ORP Probe Failing; Current in the Water; Bonding Issue?
ORP and chlorine
ORP Control
ORP probe has some issues, it reports 770mV regardless of FC.

 

[Maintenance Man]

No the feeders are located on the return line after the testing lines ....
every time the ch pump goes on the orp drops immediately (after a few mins)
and continues to drop until I shut the feeder off... then the orp rises as it should...(again after about 10 mins it starts)
is it possible that our spa could just be a "perfect storm" scenario with the return jet being aimed just the right way that hypochlorite will have this effect?
....... im going to try using ch pucks for a test and possibly bromine after if that does not work..

 

[chem geek]

I think this is something you'll have to bring up with the manufacturer of your ORP control/automation system.

 

[Maintenance Man]

Actually, I made a shock / water mix and the machine works EXACTLY as it is intended to.
turns out that the problem was in fact the Sodium hypoclorite

keep this in mind if anyone else ever experiences this same issue

 

[chem geek]

What did you use for "shock"? The only difference there is that depending on the shock product it has CYA (Trichlor, Dichlor) or calcium (Cal-Hypo) in it. That shouldn't make a difference unless the problem is pH related in which case Cal-Hypo would likely have the same problem. However, if the chemical is well mixed into the water, you shouldn't see any effects so maybe your return into the pool is too close to the skimmer or other input back to the pump so you are getting a feedback effect.

There are plenty of control systems using ORP with sodium hypochlorite using peristaltic pumps so there is something different about your setup causing the problem. If it's pH related, then if you add a pH raising solution such as a solution of lye you may find the same problem. The high pH will lower the ORP even with the same hypochlorous acid concentration.

 

[Maintenance Man]

Used Super Shock, Calcium Hypo, 500 grams to 5 gal of water mix.
And yes... I believe the way our spa was built is actually the issue, the chems are feeding back to quick rather then mixing ....
however, the shock mix does not have the same drop of ORP as the sodium hypo.

- - - Updated - - -

and no, its not PH issue, the PH side of the controller works perfect from the start, always maintained and confirmed with taylor drip tests at 7.4-7.5 ( on occasion when jets are on.. can go as high as 7.7 and after overfeeding as low as 7.3 after jets turn off, but always balances back to 7.4 withen 30 min or so )

 

[chem geek]

Well that's very strange since Cal-Hypo should be very similar to sodium hypochlorite except it adds calcium. You might try an experiment where you mix 11 cups of 12% chlorinating liquid in 5 gallons of water to simulate what you did with the Cal-Hypo (I assumed 65% Cal-Hypo) which is roughly a 17,000 ppm FC solution compared to 120,000 ppm FC with the undiluted chlorinating liquid. If that is OK, then it's the concentrated chlorine that is the problem and it not getting diluted quickly enough. If you have control over the feed rate, perhaps you could turn that down so it doses more slowly over a longer period of time.