Chlorine is disappearing!

[Youfah Mizzum]

Firstly some background information:

Location: Sydney, Australia (well, just an hour north near Gosford)
Pool volume: 22,000 L (4850 gallons)
Salt water chlorinator
Pool cover mostly kept on except when swimming
No opening/closing routine for winter/summer. Coldest it gets here is around 5 deg C (40 deg F)
Age of pool: 2 years, so pretty new.
Surface: plaster - beadcrete.
Chem test kit from Clear Choice Labs (very similar to Taylor I believe). I also take a sample to my local pool shop.

I don't know which forum to post this in as it falls under multiple headings, but I figure Water Chemistry might be the most appropriate.

So onto the query!
Last November we had a big windstorm which deposited a fair bit of leaves and debris (dust, twigs etc) into the pool.
I hadn't tested or checked the water for about a week (it's under my pool cover) but when I uncovered the water, it was a bit green & chlorine level was basically 0 as expected with all that Crud!
So I increased the chlorinator level & added extra liquid chlorine to clear up the outbreak. I basically SLAMmed with a big dose of liquid chlorine for a few days.
Vacuumed, backwashed a few times.
A few days later & we're all good. Nice clear water.
I mention the above as that seemed to be the start of my issues - whether it has any bearing or not I don't know, maybe just coincidence.

But I was never able to keep the chlorine level at appropriate levels, around the 4 to 5 ppm.

Since I have had the pool, my chlorinator would run for about 9 to 10 hours a day split between day and night, so 5 hours in the day and 4 hours at night. And it's chlorination level was about 35%. That is summertime. In the winter, it's about 7 hours at 5%.

But since the November incident I cannot get a chlorine reading above 1 (usually 0 to 0.5) & I have increased the running to 11 hours a day, running at 95%.
I just don't know what is causing the low levels - either something is consuming the chlorine e.g. algae. Or the chlorinator isn't producing chlorine to it's pre November levels. By the way, I can visually see the chlorine getting produced by the cell blades so I know it IS working.

All other chem levels (CYA, salt, TA, CH, phosphates et al) are well within correct ranges although I have always found it hard to maintain the recommended CYA level (70 to 80) so the CYA is typically around 50 but it drops quickly so each week I find I need to add more CYA to keep it at the 50 mark or thereabouts. That has always been the case but just mentioning it in case it has any relevance.

I did have low salt thanks to flooding rains over 2022 but I have since thrown in a few bags of salt so all good there. It's currently sitting around 5000 ppm.
I vacuum every 4 to 5 days using a robot (this one Dolphin S 200)

Over the following weeks, I have kept a close eye on chem levels, and have added liquid chlorine just to boost the chlorine level.
Nevertheless, just before Xmas I submitted a warranty claim on the chlorinator as I had eliminated just about every other option.

Last Friday (today is Tuesday) the technician came out and replaced the chlorinator with a new controller and cell, so the whole kit was replaced with new (or what I assume is new).
So it's 4 days later & was hoping to see chlorine levels back up to 5 ppm or even higher as it's running 11 hours a day at 95%.
But alas, the chlorine levels are still in the small 0.nn range.

I am completely baffled why I can't get good chlorine levels.

 

[wireform]

Youfah
So after reading all I must say we need you to do a complete test in following manner:
FC
CC
PH
CH
TA
CYA
SALINITY
It's hard to understand how your CYA keeps changing as to lower it you either need to drain water or have a leak. You also have a discrepancy in your pool capacity. You stated 6076 gallons but your signature has 4850.
The slam you speak of probably wasn't completed as per SLAM Process and that may be the issue with the FC you're experiencing. I'd recommend you do some reading Pool Care Basics
Overnight Chlorine Loss Test
FC/CYA Levels

 

[AlexPowell]

You should definitely SLAM the pool. The thing I see many people forget here is they bring FC up to shock levels but it is not maintained until combined chloramines is below 0.5ppm, the pool is clear and overnight chlorine loss is less than 1ppm.

It seems that this is the case with you as well, you increased chlorine to shock levels, however it was not maintained. The best time to shock in Australia this time of year is around 8pm at night because then the sun won't just burn it all off.

It's also possible that CYA levels are off the charts and you are somehow testing wrong. I would get it tested by a 3rd party. The pool shops will be accurate enough because you are just checking that CYA is not something crazy like +100ppm

Please note that I am a beginner and am just mirroring what I have seen here, and it might be completely wrong in this context.

 

[aussieta]

we need recent test numbers
your pool math logs are very lacking
without recent test numbers we are all just guessing

 

[Youfah Mizzum]

It's hard to understand how your CYA keeps changing as to lower it you either need to drain water or have a leak. You also have a discrepancy in your pool capacity. You stated 6076 gallons but your signature has 4850.

Re CYA dropping. It's always been like that since getting the pool installed 2 years ago. It regularly needs a top up of CYA. I have heard from other pool owners that once a CYA level is reached it generally stays constant but not for me. Definitely no leak - the water level isn't dropping. So for me, it's quite normal to add maybe 1/4 to half cup of CYA (it comes in a powder) weekly or fortnightly.

My testing & pool shop testing both align with pretty much the same CYA levels so I am confident in the tests. 4850 gallons is correct. Basically 22000 litres.

You should definitely SLAM the pool. The thing I see many people forget here is they bring FC up to shock levels but it is not maintained until combined chloramines is below 0.5ppm, the pool is clear and overnight chlorine loss is less than 1ppm.

Yeah, done that. SLAM done early December following TFP protocol.

I should add, the water is crystal clear and looks lovely. Its just that my chlorine is getting lost somehow.
I could do another SLAM or at least saturate with liquid chlorine for a few days I guess.

FC: 0.17 My test & shop test very similar result
CC: 0.2
PH: 7.4 My test & shop test very similar result
CH: 310 My test & shop test very similar result
TA: 85 (my test) Pool shop shows 115
CYA: 45 My test & shop test very similar result. Their recommendations is between 25 - 50. TFP recommendations between 70 - 80.
SALINITY: 4720 (that's from the pool shop). My test kit shows it around 6000.
Phosphates: 0

I've done some googling and I can confidently guess my pool shop uses LaMotte WaterLink Spin Touch for their testing. Professional gear so I trust their figures.

 

[Mdragger88]

The best time to shock in Australia this time of year is around 8pm at night because then the sun won't just burn it all off.

To clear this up - during the SLAM Process you Maintain slam level for your cya as often as possible, multiple times per day, not just at night.

 

[Texas Splash]

CYA can/does degrade and disappear. It may be slowly, anywhere from 3-10 ppm per month depending on the pool and amount of use, but it does.

Free chlorine disappears from one of two things - lack of protection from the sun (low CYA) or algae. So you have to be 100% there is no algae causing a heavy organic load on your chlorine. Your FC level of 1.7 is extremely low and a warning sign to us already. If you refer to our FC/CYA Levels, the FC should never be that low. If I were you, my first step would be to increase the FC to about 7-8 ppm and do an Overnight Chlorine Loss Test to rule out algae. If you pass the OCLT, then increase the CYA to help preserve the FC generated by your SWG. But if you fail the OCLT, then you know for sure it's organics and time for another SLAM Process.

 

[Mdragger88]

Re CYA dropping. It's always been like that since getting the pool installed 2 years ago. It regularly needs a top up of CYA. I have heard from other pool owners that once a CYA level is reached it generally stays constant but not for me. Definitely no leak - the water level isn't dropping. So for me, it's quite normal to add maybe 1/4 to half cup of CYA (it comes in a powder) weekly or fortnightly.

My testing & pool shop testing both align with pretty much the same CYA levels so I am confident in the tests. 4850 gallons is correct. Basically 22000 litres.


Yeah, done that. SLAM done early December following TFP protocol.

I should add, the water is crystal clear and looks lovely. Its just that my chlorine is getting lost somehow.
I could do another SLAM or at least saturate with liquid chlorine for a few days I guess.

FC: 0.17 My test & shop test very similar result
CC: 0.2
PH: 7.4 My test & shop test very similar result
CH: 310 My test & shop test very similar result
TA: 85 (my test) Pool shop shows 115
CYA: 45 My test & shop test very similar result. Their recommendations is between 25 - 50. TFP recommendations between 70 - 80.
SALINITY: 4720 (that's from the pool shop). My test kit shows it around 6000.
Phosphates: 0

I've done some googling and I can confidently guess my pool shop uses LaMotte WaterLink Spin Touch for their testing. Professional gear so I trust their figures.

Regardless of your previous slam, your fc has regularly been below min for your cya & it is very likely you now have an algae problem.
Do the Overnight Chlorine Loss Test @ target fc levels FC/CYA Levels as recommended above to confirm this suspicion.
Raising fc haphazardly for a couple days may hide the problem & kick the can down the road but won’t solve it.
If you wish to eradicate algae you must do the SLAM Process completely.

 

[Youfah Mizzum]

Your FC level of 1.7 is extremely low and a warning sign to us already. If you refer to our FC/CYA Chart, the FC should never be that low.

Not 1.7. 0.17! Ten times lower!
I am well aware that it's low - that's the whole reason for this thread. I just can't work out why.
My chlorinator was replaced by the manufacturer (or their rep) 4 days ago in case it was faulty. I have sun protection - not only with my CYA (see levels previously posted, about 45) but I also have a pool cover.
It's running at 95% (waaaaay higher than it should need to be) for 11 hours split between day & night.

I guess my next step as you say is to do a OCLT. I'll turn off my nighttime timer for the chlorinator and I'll raise the chlorine around dusk to 5 to 7 ppm and test as soon as I wake the next morning. BUT, my CC is already < 0.5.
Maybe there is hidden algae? It's a simple rectangle pool, no odd nooks or crannies for algae to hide.

 

[Texas Splash]

Maybe there is hidden algae?

That's always a possibility, but the Overnight Chlorine Loss Test should confirm. With those results you can feel confident on the next step)s) to take. Also be sure to look under that cover for algae. Covers are great for protection from the sun and to help keep the pool clean, but over time they can hide some junk underneath.

 

[Bperry]

I think part of the issue is that no one here has any confidence at all in a pool store test. Maybe all the stores in AUS are much better than the rest of the world, but it’s not likely. That’s why it’s hard to believe the posted test results if you say the pool store test for FC matches your clear choice kit results when the result is 0.17ppm. The kit would call that “zero”. Do yourself a favor and never have the pool store test the water, it’s just not reliable.

If we’re betting here, put me down for algae. Plenty of times algae is in the water and not visible. It only becomes visible when it’s super bad.

 

[Youfah Mizzum]

4850 gallons is correct. Basically 22000 litres.

Oops wrong. 22000 litres is correct (that's how we roll in Oz) but gallons is wrong. Converts to 5812 gallons. I've fixed my sig.

the result is 0.17ppm. The kit would call that “zero”.

My kit does call it zero. Like you say, "Maybe all the stores in AUS are much better than the rest of the world". Well, after researching what gear they use, and their similarity with the results I get (note similar results - not identical results) I trust their tests as much as I trust my own kit.

I'll be raising chlorine levels today & running a OCLT overnight to see what it says.

 

[mgtfp]

Best way forward would be to turn off the chlorinator, use liquid chlorine to raise FC to maybe 7-8ppm and do an OCLT. Then we'll know.

Add 8ppm worth of liquid chlorine (use PoolMath to calculate the required amount) after sunset. Confirm FC after about 30min with the pump running (but SWG off). Test FC again before sunrise. Make sure to run the pump for about 30min before the morning test. To avoid having to get up even earlier, you can just run the pump through the night.

Definitely no leak - the water level isn't dropping. So for me, it's quite normal to add maybe 1/4 to half cup of CYA (it comes in a powder) weekly or fortnightly.

That sounds like at least about a cup per month, maybe 200g, so a bit less than 10ppm per month in 22000l. There are folks in hot desert climates that report CYA losses of 5-10ppm per month. 10ppm (or even 20ppm if you added actually 1/2 cup weekly) still sounds a lot, especially with virtually no chlorine. My understanding is that CYA mainly disappears by oxidation from chlorine. This is also temperature dependent, so what's your water temperature? It could help to quantify the loss a bit better by weighing the added CYA for a while and logging the additions. You can use PoolMath for that.

Do you have an autofill that might hide a leak?

How about your salt levels? If you lost 20% of your CYA per month through a leak, you would also lose 20% of your salt or, assuming 6000ppm is correct, about 1000ppm per month. Let's say about one 20kg bag of salt per month in your 22000l pool. Just to make that clear: You only lose salt via water losses. SWG turns salt into chlorine, which gets turned back into salt by UV or sanitising/oxidising things. In my case, I lose salt mainly over winter when I often have to drain water after heavy rain.

 

[mgtfp]

Oops wrong. 22000 litres is correct (that's how we roll in Oz) but gallons is wrong. Converts to 5812 gallons. I've fixed my sig.

Yup, about 5800 US gallons (or about 4850 imperial gallons).

 

[mgtfp]

Like you say, "Maybe all the stores in AUS are much better than the rest of the world". Well, after researching what gear they use, and their similarity with the results I get (note similar results - not identical results) I trust their tests as much as I trust my own kit.

I don't. Their gear might be good. But it still has to be calibrated properly and used properly. And most importantly, your sample should not get contaminated with the previous customer's water. I believe that there are experienced shop owners that get all of these things right. But the teenaged casual? Really?

While there are often 1 or 2 test results that look about right, there usually also is at least 1 test result that doesn't.

A couple of weeks ago, I got pool store results to have something "official" to support an SWG warranty claim (didn't need it in the end). One pool store tested my salt as 3800 ppm and wanted me to add 2 bags of salt. Another pool store tested my salt as 4900ppm. And my own testing came up as 6000ppm.

 

[Youfah Mizzum]

10ppm (or even 20ppm if you added actually 1/2 cup weekly) still sounds a lot, especially with virtually no chlorine. My understanding is that CYA mainly disappears by oxidation from chlorine. This is also temperature dependent, so what's your water temperature?

Your phrase "especially with virtually no chlorine"... My CYA loss has been happening since day dot with this pool, not just this recent episode. A loss around 10 - 20 ppm per month sounds about right from my experience. To me it's normal.

My water temp in winter is around 10⁰ C (50⁰ F) and in summer, i.e now it hovers around 25⁰ to 30⁰ (77⁰ F to 85⁰ F).
I don't really check the CYA levels in winter as no one is swimming. Maybe every 6 to 8 weeks. As long as the water looks sparkling & Ph is within range I'm happy. I get the pool out of winter mode just by raising the chlorinator production from 5% to 35%-ish maybe in 5% to 10% increments over a week or two from mid/late Sept to Oct. And check all other chems for recommended ranges.

How about your salt levels?

Salt is great. Well within range. I'm not adding salt constantly. During 2022 up until August or so we had really heavy rains. Several areas were in flood and I was needing to drain the pool after each rainstorm. So I certainly had dilution not only with salt, but also alkalinity i.e. TA and hardness i.e. CH. Those levels are spot on now (see results earlier in this thread) and have been since Sept or so.

If I had some kind of leak I would see my water level drop. I don't. No autofill.

 

[mgtfp]

Very interesting case. You are right, doesn't look like a leak, good to rule that out. 10-20ppm loss per month is a lot. If you can find out what's behind that, you can make a lot of money selling your water to those who are desperately trying to reduce their CYA

 

[AlexPowell]

I think part of the issue is that no one here has any confidence at all in a pool store test. Maybe all the stores in AUS are much better than the rest of the world, but it’s not likely. That’s why it’s hard to believe the posted test results if you say the pool store test for FC matches your clear choice kit results when the result is 0.17ppm. The kit would call that “zero”. Do yourself a favor and never have the pool store test the water, it’s just not reliable.

If we’re betting here, put me down for algae. Plenty of times algae is in the water and not visible. It only becomes visible when it’s super bad.

I don't. Their gear might be good. But it still has to be calibrated properly and used properly. And most importantly, your sample should not get contaminated with the previous customer's water. I believe that there are experienced shop owners that get all of these things right. But the teenaged casual? Really?

While there are often 1 or 2 test results that look about right, there usually also is at least 1 test result that doesn't.

A couple of weeks ago, I got pool store results to have something "official" to support an SWG warranty claim (didn't need it in the end). One pool store tested my salt as 3800 ppm and wanted me to add 2 bags of salt. Another pool store tested my salt as 4900ppm. And my own testing came up as 6000ppm.

My first job was calibration and repair of measuring instruments. The equipment that pool shops have are very accurate, however I have personally seen the pool store staff use them incorrectly. The stores are quite often busy and nothing is rinsed. They will pour the sample in a glass beaker but then do not rinse it with the sample before use. Then a plastic pipette is used which is not fresh or rinsed. This is not so bad at first but this is repeated hundreds of times.

I can personally say from professional experience that if you take an electronic pH meter and test a sample with pH of 8, then shake it off until it's dry and test another sample of pH 7, the measuring instrument will give an incorrect result. It will actually read one result and start climbing up. In fact they should be rinsed with a medium of known pH prior to every single use, reset to that level, then rinsed with the test medium before use - every single time.

 

[mgtfp]

My first job was calibration and repair of measuring instruments. The equipment that pool shops have are very accurate, however I have personally seen the pool store staff use them incorrectly. The stores are quite often busy and nothing is rinsed. They will pour the sample in a glass beaker but then do not rinse it with the sample before use. Then a plastic pipette is used which is not fresh or rinsed. This is not so bad at first but this is repeated hundreds of times.

I can personally say from professional experience that if you take an electronic pH meter and test a sample with pH of 8, then shake it off until it's dry and test another sample of pH 7, the measuring instrument will give an incorrect result. It will actually read one result and start climbing up. In fact they should be rinsed with a medium of known pH prior to every single use, reset to that level, then rinsed with the test medium before use - every single time.

100% agree.

 

[Youfah Mizzum]

Interesting anecdotes fellas.
I watch the testing getting done & whilst I can't verify their calibration, they definitely rinse with tap water then rinse with my sample after that.

Edit:
So today I have commenced another SLAM (despite the water being nice & clear & no obvious sign of algae) and funnily enough, I was struggling to maintain FC levels even during the day. I added chlorine and tested every 2 to 3 hours or so and found I was needing to add chlorine each time.
CYA around 45. Pool cover on. Cloudy day too so no great source of chlorine loss via "normal" losses (i.e sunlight).

During my final 2 tests (7pm and 7:45 pm) I didn't add chlorine since my test at 5:30pm when I added a decent 500ml which should've brought up to about 7ppm according to PoolMath.
So at 7pm I tested and it showed around 6.5ppm.
At 7:45 it was somewhere between 5.5 and 6. It took 11 to 12 drops of the titrating agent, that is at 11 drops it was a barely pale pink (hardly noticeable) but the 12th drop & it was definitely clear.

I am suspecting the OCLT will show significant losses when I test tomorrow morning. I guess that indicates algae even though its invisible. In which case I continue to SLAM tomorrow!
And despite the consensus of the lack of faith in shop testing, I took a sample at 7:45pm and I will take another tomorrow morning & take both samples to be tested. Just to compare.

EDIT: So it's the morning and yes, there is chlorine loss. It was 1 ppm possibly less as the first drop of titrating reagent immediately turned the vial clear.
So it's SLAM ahead.
But it still beggars the question, where the heck is the algae? It's not in the pool as the water is crystal clear, walls & floor scrubbed and clean.
Which leaves the pipes, pump, filter and filter media.
Anyway, onwards with SLAMming.