DEchlorination

[Davidtja]

I have had a pool guy look after my pool for past year, just brought out of winterization and when weather turns here in UK (month or so hopefully) and we get the heater on it will start being used. THe free chlorine level was high at end of last season as the auto chlorinator stopped working, he then, without my knowledge and without checking levels (I assume) winterized it and pouring in a load of sod-hypo so made it even higher.
My question however is it is: With the chlorine on the high side reading 10 free - On opening it up he advised to just let it use itself up over time. Is that correct advice or should I use a dechlorinator. Total Chlorine, PH and Alk are within tolerance.

 

[mgtfp]

Welcome to TFP!

Your PoolMath logs say you have 50ppm of CYA. For that level your FC is a tad high but nothing to worry about. SLAM level would be 20ppm for CYA 50. Up to SLAM-FC the pool is safe to swim in.

Have a look through our Pool School:

Pool School

www.troublefreepool.com

Particularly the FC/CYA chart:

The Free Chlorine and Cyanuric Acid Relationship

Managing your cyanuric acid and Free Chlorine levels is key to stabilizing your pool. Learn how these two items affect each other.

www.troublefreepool.com

With CYA, most of the chlorine is bound to CYA, where it doesn't have any sanitation capabilities - we call that reservoir chlorine. Only a fraction of the chlorine is actual active chlorine. Both show due to how the tests work as free chlorine (FC). That's why FC needs to be increased as CYA levels rise. A constant FC/CYA ratio ensures a constant active chlorine level. As the active chlorine gets used up, more reservoir chlorine gets released to become active.

It is necessary - and safe - to maintain FC levels according to TFP's FC/CYA chart.

Just let your FC slowly come down until you are in the target range for SWG-pools (I assume with auto chlorinator you mean Salt Water Chlorine Generator?). No need to force it down. Make sure to never drop below the min-FC for your CYA, otherwise algae can quickly take over.

What kind of test kit are you using?

 

[Davidtja]

Thanks so much for that detailed response. No not salt Its a basic Chlorine tablet hopper (I believe the basic valve is faulty - so it appears to be open or closed) , that's how I believe I over-chlorinated. I'm using test strips but have ordered a poollab 1.0.

 

[mgtfp]

OK. Watch out that you don't get the CYA too high with the tabs. We usually recommend liquid chlorine over tabs to keep a lid on CYA intake. But you might be "lucky" enough to get sufficient rain to flush the excess out, your current CYA level looks OK - but that's after winter.

The test strips are more guess strips. Best test kits are those that allow titration tests for TA, CH and FC (FAS-DPD test, accurate and reliable up to and beyond SLAM level), like the Taylor K-2006. But I've read a few posts recently that it seems to be quite difficult to get decent test kits in the UK.

The Poollab seems to be a viable option should better kits not be available. I actually have one myself, but don't really use it. Bought it more out of curiosity. My experience with it:

pH test seems to be pretty good, but will be affected by higher chlorine levels sooner than the Taylor pH-test would.

FC seems to be OKish, but limited to 8ppm (dilution with distilled water allows testing of higher levels, but with reduced accuracy and more effort). It seems to take a while until the tablets are fully dissolved and the reading stabilises.

CYA seems to be in the ball park (at least more than test strips), I'd say within about 30%. But I haven't tried that much.

The test that seemed to be completely off was TA (showed 40ppm when my titration showed 80ppm). But you could make an aquarium test work there (there is one from Red Sea for example) if you can source thiosulfate dechlorination drops (aquarium tests don't have to worry about chlorine). If you can't get thiosulfate, then hydrogen peroxide might have to do, but I haven't tried if that creates any interferences with the TA-test.

I haven't tried the CH-test. But there are also aquarium tests that could be an alternative (e.g. Red Sea again). Aquarium tests usually use different units (like ppm Ca instead of ppm CaCO₃), but they can be calculated into pool-units.

I found the Poollab less accurate than the Taylor titration tests and more fiddly to get those tablets dissolved without scratching the test vial. But an option, should nothing Taylor like be available. Best option would be to source a Taylor K-2006 from the US (not the K-2005, that doesn't have the FAS-DPD test, only DPD). But that could be pricey or even impossible. Or another brand that offers FAS-DPD testing for FC.

 

[Davidtja]

Thanks, i've been reading up all day. The pool had been closed/unmaintained for about 8 years and I revived it late last year due to Covid and being stuck at home. I recently got a hot tub as well so am investing more time in getting it all right rather than what was done historically by "pool man" in just throwing in lots of chemical.
I now realise that the poollab may not have been a wise purchase but its reviews here are pretty good. Having done the research today incredibly there seems to be no suppliers of "proper" kits (as referred to on here) available in the UK. Indeed on another forum an on an old entry a company was mentioned but they no longer seem to do these kits. I may have to order from the US.