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Where Is My Chlorine Going?


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My pool is not being used but the chlorine is dropping, why?

The pool has been open all winter with the pump running 1 hour a day. Liquid bleach is delivered using a solenoid metering pump. The pool is crystal clear and the water readings, with my 'new' Lamotte 7022 FAS DPD kit are as follows:

FC 7ppm (my CYA is high so I am trying to keep FC at 10ppm)

TC 7ppm

pH 7.6

ALK 100 (adjusted for CYA 73)

CH 500

CYA 80 (due to water restrictions in Spain it is not possible to drain & refill)

The pool is at my holiday home and when I arrived on the 25th April the FC was 1.5ppm, since them I have been adding 5ppm per day but the rise in FC has been slow:

25th April 1.5ppm

Added 5ppm per day

28th April FC 2.5

Added 5ppm per day

3rd May FC 4.0

Added 5ppm

4th May FC 7.5

At this point I decided to conduct a controlled trial

7th May 11:00h FC 8ppm

Added 2ppm by hand

7th May 17:00h FC 9ppm

8th May 10:30h FC 7ppm

now adding 2ppm over 2 hours and will test again.

I go home on Thursday so I really want to get this under control before I leave so any advice would be great.

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My pool is not being used but the chlorine is dropping, why?

The pool has been open all winter with the pump running 1 hour a day. Liquid bleach is delivered using a solenoid metering pump. The pool is crystal clear and the water readings, with my 'new' Lamotte 7022 FAS DPD kit are as follows:

FC 7ppm (my CYA is high so I am trying to keep FC at 10ppm)

TC 7ppm

pH 7.6

ALK 100 (adjusted for CYA 73)

CH 500

CYA 80 (due to water restrictions in Spain it is not possible to drain & refill)

The pool is at my holiday home and when I arrived on the 25th April the FC was 1.5ppm, since them I have been adding 5ppm but the rise in FC has been slow:

25th April 1.5ppm

Added 5ppm per day

28th April FC 2.5

Added 5ppm per day

3rd May FC 4.0

Added 5ppm

4th May FC 7.5

At this point I decided to conduct a controlled trial

7th May 11:00h FC 8ppm

Added 2ppm by hand

7th May 17:00h FC 9ppm

8th May 10:30h FC 7ppm

added 1ppm over 2 hours

8th May 12:30h FC 7.5ppm

added 2.5ppm over 1 hour

8th May 13:40h FC 8ppm

Thought I had found the problem at this point, the entry to the pool plumbing where the chlorine is injected was almost completely corroded up, cleaned it all, checked that the injector was working OK and continued pumping chlorine in:

added 2.5ppm over 0.5 hours

8th May 18:20h FC 8ppm

added 2.5ppm over 0.5 hours

8th May 19:00h FC 9.5ppm

8th May 20:30h FC 9.5ppm

.

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If the pool is exposed to sunlight all day, then at 80 ppm you would typically lose around 20% of the FC each day, so at 8 ppm that would be around 1.6 ppm. Typical daily FC loss for many pools exposed to sunlight is around 2 ppm. I suspect you had a much higher chlorine loss rate because the low FC/CYA ratio you started out with (1.5 ppm FC with 80 ppm CYA) allowed algae to grow even though it wasn't yet visible. The chlorine you were adding was just barely able to kill the algae only a little faster than it could reproduce. When the FC gets too low and the chlorine demand rises, even if you don't see algae, then you need to shock the pool (i.e. raise the FC level to 20-30 ppm given the 80 ppm CYA level). You can confirm that the chlorine demand isn't just from sunlight by doing an overnight chlorine loss test (i.e. when the sun is not on the pool, see how much chlorine is lost from roughly sundown to sunrise or some overnight period of time).

Once you shock the pool to kill all the algae and have preferably < 1 ppm FC loss overnight (and <= 0.5 ppm CC and a clear pool), then you should be able to maintain it with an FC that is targeted at 7.5% of the CYA level or 6 ppm.

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Richard, Many thanks for such a comprehensive explanation. Just by chance I had decided to check the chlorine loss overnight with a pool cover on and found that FC dropped from 9.5 at 20:30h to 8.0 at 08:30h, so a loss of 1.5. However, what has happened today has me completely stumped!

I decided to shock the pool as you suggested so at 10:30 I added 10ppm over 2 hours, the result was 13ppm, an increase of 5. As I was getting increasingly frustrated I added another 5ppm by hand, ran the pump for half an hour to circulate the chlorine and retested only to find that the FC had dropped to 11.5!!! Just to confirm there are no CC as when I add DPD#3 the colour does not change so I assume that the chlorine is not being used to kill off algae.

I then left the pool till 18:40 and re-tested to find that FC was 8.5ppm, pH had risen to 8.0 (but no water has been added to the pool) and ALK has also gone up from 100 to 120.

As I was convinced that the chlorine level should be somewhere around 30ppm by now and as I was now doubting my shiny new Lamotte 7022 test kit I tested the chlorine with an old OTO kit and by trial and error diluted the pool water until I got a good reading, this gave me 32ppm or there abouts. As OTO is a reliable test where does this leave me..................HELP!!

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Don't panic. Just keep it at shock level until things stabilize. It might take a day or two (if you have to leave, then have someone else take over for you). In a large body of water, there can be a lot of algae in it and still not be visible so takes chlorine demand. Also, you might have some biofilms that started to form and those take longer to get rid of (if you had the chlorine be too low or zero for quite a while, then you could have more serious problems, but it doesn't sounds like that is the case). I would brush the sides and bottom of the pool just in case there is nascent algae there -- brushing helps to let the chlorine reach what might otherwise be stuck on surfaces.

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I'm a little calmer today but still very confused by what has been going on. On the plane home today I looked again at my notes, fortunately I took some, and I am beginning to wonder whether the test kit I have is giving false readings, here is why.

Historically my pool has lost 1.5ppm FC per day. Over the winter the FC has been kept around 8ppm and 1ppm was added daily, however, the temperature of the pool started to rise above 18C in mid April and when the pool was tested (with a DPD kit) on the 23rd April the FC was 4.5.

When I first tested the pool on the 25th April the water temp was 20C & FC & TC were 1.5ppm. I increased the chlorine injection to 2ppm and on the 28th (3 days later) the FC was 2.5. This makes sense as 1.5 + 6 (daily injected) - 4.5 (daily usage) = 3 so 2.5 is there or there abouts. 5 days later the FC was 4.0 which also seems about right as the start point was 2.5 + 10 (daily injected) - 7.5 (daily usage) = 5 so again 4 is not that far out. I then added 5ppm per day and the next day the FC was 7.5, again about right as 4.0 + 5 (daily injected) – 1.5 (daily usage) = 7.5

It was after this that things go a little wonky, I had been adding 5ppm per day (sadly the chlorine level in my drum was low so it is possible that not as much was injected as expected but I don’t think that this was the case)

On the 7th May (3 days on) the FC should have been 7.5 + 15 (daily injected) – 4.5 (daily usage) = 18 but instead it was 8ppm and after adding 2ppm it only rose by 1ppm. Then on the 8th the FC started at 7 and after adding 8.5ppm during the day the FC had only risen to 9.5. There was a similar story on the 9th when the FC started at 8ppm and after adding 15ppm over the day the FC only rose to 8.5!

I am beginning to wonder whether there is some form of beaching going on here, I know that FAS DPD is not supposed to suffer from this but it is odd that the apparent errors seem to start as the FC may have exceeded 10ppm. I think I will give the supplier a call to see what they say.

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Have now spoken to tech support at Lamotte and have been told that because the FAS DPD test still uses DPD it suffers from the same bleaching issues that all DPD test do. He told me that in their experience FAS DPD is accurate to around 7.5 or 8.0ppm but not above that. This is a real blow as one of my main reasons for buying this test kit was because of its ability to accurately test high FC concentrations. However, on the up side it explains the strange reading that I have been getting, now I just need to bring the chlorine back down to a readable level!

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