Chlorine Lock Questions?

[Scollops]

Hi Everyone - have been following TFP for a while now and pool is looking great. Testing weekly - noticed with all the rain my CYA has dropped to 45 and pH consistently above 8 each week, TA hovering around 70.

Normally I had my CYA about 70 ish - kept my chlorine levels fairly consistent after that. Popped to my local swim store just for chemicals . Idle chit chat led to him talking Chlorine Lock, as I mentioned my 40k pool and being 40cya aiming to take back to 70. He was adamant that was a no no and 30-40 max talked about chlorine lock. First I’ve heard so had to check in as I started doubting myself - I recall early on my CYA is 30 and FC was shockingly low.

 

[Oly]

The term "Chlorine Lock" is a misunderstading of the CYA/FC relationship and how CYA can protect FC from rapid degredation while also making it available for needed sanitation. Yes when CYA becomes very high, most often due to mismanaged chemistry, FC must also rise. Those who do not follow their water chemistry begin to see algae, increased FC loss and are now faced with zero FC test readings and a swampy mess. Pool stores sell expensive potions to cure "chlorine lock" and then off and down you go. Eventually draining the pool, maybe finding TFP then investing in a good test kit.

With a SWG you will want to run your CYA higher to lower FC loss, with the goal of making your SWG work less, while keeping your water sanitized and making it last longer. There are TFP observations that SWG pool systems are unique in FC loss, ideal CYA levels and effective FC sanitation levels when compared to liquid chlorine pools.
TFP recommends a CYA of 70- 80ppm for efficient SWG systems. At this level the FC SLAM level is high but due to the way SWG system operates a SLAM should not be needed, (yes never say never). Now I am a new SWG owner/operator and am on a quest to find the proper CYA levels and optimum run times for my unique pool sanitation system. I will detail my experience so far and suggest you explore your unique system to find your ideal levels.

I calculated how much CYA was needed to raise my CYA 10ppm and I began at 40ppm ( which was ideal for my liquid chlorine system) and have worked my way to 60ppm as a starting point. While doing this I have monitored my FC levels and have observed slightly less daily FC loss as CYA rises. I anticipate my solar radiation to increase as the summer progresses so keeping my FC level on the high side of my target is key as this is a moving target now. My goal is to ease my CYA up toward 70ppm and find my optimum SWG runtime % when utilizing a ten hour daytime run time as I reach peak FC demand in the next 60 days. I suspect I will confirm what TFP recommends after years of observation with many different pools but this is my pool and my experience so far this SWG has several benefits I appreciate. Have fun.

 

[Exlonghorn]

Oly,
I think you'll find 70 to be right on target. Scrolling through my PoolMath logs it's been right on or near 70 for quite a while and my SWCG has no trouble keeping FC at 3-5 running 3-5 hours per day.

Scollops,
I think the local store guy might not be a match for the combined wisdom/experience of TFP, particularly when you're not using their pucks and chems.

 

[Scollops]

Cheers guys need to shop around for my chemicals ideally day nothing - or I’ll get the war and peace of what my pool should be.

 

[mgtfp]

Just adding to @Oly's great post:

@mas985 did a few years ago some experiments that confirmed that the chlorine loss to UV at higher CYA is lower (i.e. the SWG needs to generate less chlorine per day) even though this requires a higher chlorine level to avoid what gets mystified as "chlorine lock" by the "old school".

Just yesterday he posted a refresher on that in another thread:

I did a few experiments many years ago and found that I used LESS FC (lower SWG %) with higher CYA even though the FC level needed to be higher. As it turned out, the UV extinction of the FC is not exactly linear with CYA.

 

[needsajet]

Sadly "chlorine lock" is a commonly used term, and confusing and ominous enough to sell plenty of extra stuff at pool stores, especially copper-based algaecides which should never be put in a swimming pool, and powdered "pool shock" that raises CYA almost as much as FC.

The correct term is "over-stabilised" and only relates to pool operators who are obliged to stay within an artificially imposed range of FC. Say, for example, a commercial operation with a stipulation of 2-4 ppm chlorine. In that case, CYA could not be kept above ~50 ppm without starting to be over-stabilised, and by 80 ppm CYA would very likely start getting algae. This led to problems in some commercial pools and even anti-CYA rules in some places.

TFPC overcomes all of that, with a scientifically-backed progressive chart of CYA and FC levels, found here: FC/CYA Levels

In a nutshell, keep FC at or above 5% of CYA for SWCG pools, and at or above 7.5% for liquid chlorinated pools. For your preferred 70 ppm CYA, that's FC at 3.5 ppm and up for SWCG, and 5 ppm and up for non-SWCG. With the buffering effect of stabiliser, you would be safe for swimming up to 28 ppm FC at 70 ppm CYA, or up to 16 ppm FC at 40 ppm CYA, so the 8 ppm FC you mentioned is simply no concern whatsoever.