Low Free Chlorine and Low Cyanuric Acid

[JulesInFrance]

Hi,

I am a new member desperately trying to learn how to maintain my pool, but I have a problem with Low Free Chlorine and whatever I do, I just cannot fix it!

The pool is outdoor (in France, so lots of sun), water is currently 26 degrees, 90,000 litres (20,000 gallons), vinyl lined with a hard cover that is put over each night. Testing is always done in the morning and all chemicals are added in the morning except pool shock, which is added in the evening just before putting the cover over (and pump then run overnight).

Currently my "OK" readings are as follows:
- Total Alkalinity is stable at 80 ppm
- PH is stable at 7.5
- Calcium hardness is low at 80 ppm but it's under control and I'm less worried about that, for now at least.

But that's where the good news stops.
- Problem 1: Free Chlorine is always very low. 0.16 ppm is the highest I have got it this year, the day after a chlorine shock was added. But at the same time, Combined Chlorine jumped to 2.38. I always add the amount of chlorine shock suggested on the product for the size of the pool. This is the 4th year I have had this pool and I have never been able to get Free Chlorine higher than Combined Chlorine. That said, we've never had any noticeable problems as a result.
- Problem 2: Having researched problem 1, I came to the conclusion I must have low Cyanuric Acid levels (although the regular tri-chlorine blocks I use do have CY included in them). So I tested the CY levels and indeed found them very low (does not even register a reading on my tester, just get "LO" result). So I have added 2.5Kg of Cyanuric Acid to the pool, which should have raised CY to about 25 ppm (assuming the start point was 0 ppm), but I am still get a reading of "LO" and am now nervous about adding more. I did have a lot of trouble getting the stuff to dissolve and had to sweep it around for a long, long time to prevent damage to the liner. I have since received a tip that it should be diluted in warm water first, something I will try next time.

So my questions are as follows:
Question 1: Am I simply not adding enough Chlorine to shock the pool so that it never reaches its breakpoint? Should I add more at a time to try to correct this?
Question 2: Should I be worried about the CY levels? Having already added 2.5Kg which should have at least made the levels register, should I add any more or do a pool shock first? I seem to be completely wasting my chlorine when shocking though so do I try to correct the CY first?

If it's of interest, all my readings this year so far are below (#N/A means not measured that day). All tests done using Exact iDip so should be fairly accurate.

15/05/2020. Added 5L liquid bleach for initial shock (opening)
16/05/2020, Added 5L liquid bleach for initial shock (opening)
17/05/2020, AL: 48, PH: 7, CL1: 0.04, CL3: 0.97 , CL4: 1.02, CA: 62, CY: LO;
18/05/2020, AL: 40, PH: 7.2, CL1: 0.05, CL3: 1.41, CL4: 1.46, CA: #N/A, CY: #N/A
20/05/2020, AL: 40, PH: 7.5, CL1: 0.05, CL3: 1.51, CL4: 1.56, CA: 66, CY: LO
21/05/2020, AL: 54, PH: 7.8, CL1: 0.11, CL3: 1.68, CL4: 1.79, CA: 68, CY: LO; Added Sodium Bicarbonate am. Shocked pool pm.
22/05/2020, AL: 41, PH: 7.1, CL1: 0.16, CL3: 2.38, CL4: 2.54, CA: 66, CY: LO; Added 1800g sodium bicarbonate to increase TA
23/05/2020, AL: 63, PH: 7.2, CL1: 0.11, CL3: 1.79, CL4: 1.9, CA: 62, CY: #N/A; Added 1800g sodium bicarbonate to increase TA
24/05/2020, AL: 77, PH: 7.3, CL1: 0.11, CL3: 1.68, CL4: 1.79, CA: 62, CY: LO; Added 900g sodium bicarbonate to increase TA
25/05/2020, AL: 84, PH: 7.7, CL1: 0.05, CL3: 1.41, CL4: 1.46, CA: 58, CY: #N/A; Added 1200g calcium chloride to increase CA
26/05/2020, AL: 82, PH: 7.4, CL1: 0.05, CL3: #N/A, CL4: #N/A, CA: 68, CY: N/A; Added 1800g calcium chloride to increase CA, Added 900g cyanuric acid to increase CY
27/05/2020, AL: 81, PH: 7.8, CL1: 0.05, CL3: 1.14, CL4: 1.19, CA: 70, CY: LO
28/05/2020, AL: 77, PH: 7.5, CL1: 0.16, CL3: 0.92, CL4: .08, CA: 72, CY: LO; Added 600g cyanuric acid to increase CY
29/05/2020, AL: 78, PH: 7.9, CL1: 0.11, CL3: 0.87, CL4: 0.98, CA: 70, CY: LO; Added 900g cyanuric acid to increase CY
30/05/2020, AL: 80, PH: 7.5, CL1: 0.05, CL3: 0.65, CL4: 0.7, CA: 70, CY: LO
31/05/2020, #N/A
01/06/2020, #N/A
02/06/2020, AL: 80, PH: 7.5, CL1: 0.03, CL3: 0.32, CL4: 0.35, CA: 76, CY: LO

Any advice would be very gratefully received!

Thanks,
Jules

[Denniswiseman]

First of all you really need to get yourself a decent FAS/DPD test kit or at least a CYA test not one that says Low, OK, High
I don't believe your CYA is low because of the products you have been putting in and adding
Excessive CYA renders your chlorine ineffective and you have to use more to get the same sanitation
For every 10 ppm Free Chlorine (FC) added by Trichlor, it also increases Cyanuric Acid (CYA) by 6 ppm.
For every 10 ppm FC added by Dichlor, it also increases CYA by 9 ppm.
For every 10 ppm FC added by Cal-Hypo, it also increases Calcium Hardness (CH) by at least 7 ppm
Forget about calcium with a vinyl or fibreglass pool
The fact that you have CC means you need to Slam (Shock Level and Maintain) with relation to Chlorine / CYA Chart and Recommended Pool Levels

[JulesInFrance]

Hi Dennis. Thanks for your quick response.

My tester is in fact quite sophisticated (it costs $200!) and it does give exact readings but it says "LO" when the reading is 'off the scale' low. However, your response inspired my to look again at the instructions (you know what's coming don't you) and I see that there was one step in the CYA test that I missed: shaking the reagent bottle before adding drops to the water sample. I just did a CYA test properly and got a reading of 39ppm. Eureka!! SO, the CYA was low but is now perfect. So I will now Slam the pool following the procedure in the link you sent and will let you know how it goes.

Many thanks so far!
Jules

[JulesInFrance]

Hi again.

So, I had some success yesterday but I don't think I am there yet. I started the pool slam and for the first time ever, got a reading of Free Chlorine which was higher that Combined Chlorine, at 2.2 vs 0.9 respectively. Eureka! I put in more shock and left the pump running overnight with the cover closed. This mornings figures were as follows:

AL: 71, PH: 7.2, CL1: 0.22, CL3: 0.27 , CL4: 0.43, CA: 78, CY: 46

So, whilst CC was less than 0.5 and the water was clear, the FC went down by more than 1ppm overnight, so I failed the Slam Complete test. So I swept and vacuumed, and added more chorine shock. Two hours after, FC went back up to 2.27 and CC was 0.22. This afternoon the reading is FC 0.92 and CC 0.22, so chlorine is still being consumed. I have added another dose of shock.

Then, in the Chlorine / CYA Chart in the link above it says
- for a CY reading of 40, you should have minimum FC of 3ppm, Target FC of 5-7, and SLAM FC of 16
- for a CY reading of 50, you should have minimum FC of 4ppm, Target FC of 8-9, and SLAM FC of 20
As my CY is 46, I guess I should take a minimum FC of 3.5ppm, Target FC of 6-8 and SLAM FC of 18
But what does this mean exactly? That I should target FC 6-8ppm instead of the normal 1-3ppm you see written everywhere? And what is the SLAM FC? I have a shock product where 20 tablets increase FC in a 50,000 litre pool by 4ml/litre (=ppm). My pool is 90,000 litres so the maths tells me I need 36 tablets to increase FC by 4ppm. But as the SLAM FC for my CY reading is 18 (4.5 times the 4ppm increase stated in the dosage instructions), does this mean I should be adding 4.5 times the amount of chlorine required for 4ppm, i.e. 90 tablets? Seems like a lot! Do I keep adding chlorine until I pass the SLAM Complete test? And do I add them all in one go or in batches? Do you think this means my system is "dirty" somehow and the chlorine is being used up somewhere in the plumbing (sand in filter was changed last year)?

Answer to these questions would I think help solve my remaining mysteries!

Thanks in advance again for any help.
Jules

[Denniswiseman]

First of all what are your tabs. If they are Trichlor / Dichlor then you are increasing your CYA. You should be using liquid chlorine (sodium hypochlorite or plain bleach)
Yes you should target 6-8 because of your CYA. There is no normal, one size doesn't fit all
Slam stands for shock level and maintain, so you should put sufficient chlorine to reach your shock level and test often to keep it there. Slam is not a single process
Reach shock level with as much chlorine as necessary. With liquid chlorine you can test after an hour and top up if necessary to keep your shock, level with tabs you have to wait for them to dissolve
Don't worry about the system being dirty, backwashing takes care of that. you just have either algae or CC (used up chlorine)
Pool Maths will help you work out what's needed
Use these common products to balance your pool
Liquid chlorine (sodium hypochlorite or plain bleach)
Muriatic acid (hydrochloric acid) to lower pH and TA
Bicarbonate of soda to raise TA
Aeration will raise pH only
Soda ash will raise pH and TA

[JulesInFrance]

Hi Dennis,

Thanks again for this. This morning's figures:

AL: 73, PH: 7.0, CL1: 0.32, CL3: 0.16 , CL4: 0.48: CA: 74, CY: 54

So I am still doing it wrong. Unfortunately I bought different shock tabs this time which do have CYA in them (the normal ones I use do not) so my mistake. As a result the CYA has gone up to 54 so, I will stop using them anyway as per your advice and use liquid bleach instead at 9.6% chlorine. I need to back wash regularly and top the pool up anyway so the CYA is not a worry at the moment.

I have done the maths in Pool Math (nice app, thanks) and it says my target SLAM FC is 21 and that I need to add 19 litres of bleach (9.6%, called Eau de Javel in French) to reach the target! So I will do this and try to keep it there, which I guess will need 5 or 6 5 litre bottles, (4 to get to the target and another couple to make sure I can keep it there). The shop will think I have gone mad as I can tell you that nobody here in France seems to do this properly!

Will keep you informed. Thanks again.
Jules

[Denniswiseman]

I assume CL1: 0.32, CL3: 0.16 , CL4: 0.48 are FC CC TC all still very low
Any tab or powder has CYA or calcium
You really should chlorinate with bleach and not tabs or powder as it will push your CYA up high. The only advantage is when you are away you can for a time (holiday) it will stop you getting an algae bloom

[JulesInFrance]

Hi.

Yes, CL1 = FC, CL3 = CC and CL4 = TC.

So yesterday I added 20 litres of liquid chlorine (9.6%) as recommended by Pool Math (the target was 22 ppm). The FC went up to 11.7 ppm after 1 hour at which point I had to leave it overnight. This morning it is at 10.9. So, although I did not reach the target of 22 ppm, does the fact that it went down less than 1ppm overnight mean I have finished my SLAM?
Also, do I now simply wait for FC to reduce to the target 6-8 ppm before adding my regular (250g) chlorine tabs, and also before swimming?

FYI, today's full set of readings were:
AL: 80, PH: 7.8, FC: 10.9, CC: 0.22, TC: 11.12, CA: 70, CY: 56

Thanks again. Appreciate all this valuable advice.
Regards, Jules

[Denniswiseman]

I would give it another day just to be sure before finishing the slam
You can swim up to your shock level so that is OK
Don't use the chlorine pucks as they introduce more CYA which renders your chlorine ineffective, continue with the bleach
Bear in mind that bleach doesn't hold it's strength indefinately and reduces over time
You have at last got sufficient FC in your pool now

[JulesInFrance]

Right. At 5pm this evening FC was 11. Pool Math said add 10 litres of my 9.6% chlorine, which I did. Two hours later, FC is 11.9, whereas it should be 22, or at least on the way there!
I put the cover over after adding the chlorine as the weather today is rubbish but I am starting to wonder whether that is the problem. I pressure cleaned the cover when I first opened the pool, but maybe it's dirty underneath (no easy way of cleaning that)?
I will try again tomorrow leaving the cover off. Un petit mystere as they say here.

[Denniswiseman]

Strange no way it should be that low unless the bleach is old and has lost it's potency. Is there a date stamp on it
See what it's like in the morning

[JulesInFrance]

HI Dennis, Sorry for the radio silence, just had a birthday weekend and was otherwise busy.

I found out on Friday evening during some research that my tester only tests up to 12ppm for chlorine! So I hope the 11.9 measurements I took weren't in fact way beyond that but just registering at the limit! Anyway, I decided not to add any more chlorine as a result and to see what happens. I think this was the right decision; here are today's figures having had the cover closed for a couple of days owing to bad weather:

AL: 79, PH: 8, FC: 10.5, CC: 0.22, TC: 10.72, CA: 72, CY: 55

I think I will just wait for the chlorine levels to come down naturally. The pool looks amazing and doesn't smell, and I am very pleased FC is not disappearing and CC is much less than FC, so I am of the impression the SLAM has worked and the pool in clean. Shame my tester doesn't test beyond 12 so I know what's really going on, but I have put in enough chlorine I think without going over the top and the other levels seem OK. With time I will bring the CYA back down slightly (evaporation plus refill).

With the exception of perhaps a bit of PH-, would you agree I am finished for now?
Thanks and kind regards,
Jules

[Denniswiseman]

That explains everything
Yes some acid to bring the pH down to about 7.4 and all seems good. It will reduce your TA but you can bring that up again if you need to with bicarbonate of soda
To get a better idea of your FC mix a sample of pool water with an equal amount of fresh water, test and double the result
Evaporation doesn't reduce CYA only fresh water change including rain

[JulesInFrance]

Yes, I think I'm nearly there.
Good idea about the "double" test. I will do that and see how it comes out.
Will give you a final update once these things are done.
Thanks

[JulesInFrance]

Hi Dennis.

The "double" test worked a treat for the chlorine and I am pretty much sorted now. Today's measurements are:
AL: 72, PH: 7.1, FC: 7.5, CC: 0.16, TC: 7.66, CA: 68, CY: 54

So I am very happy with the Free Chlorine performance now the Cyanuric Acid levels are much better. All I need to do now is bring the total alkalinity up a bit and I am done.

I would like to thank you very much again for your kind help. Keep up the good work; it's a savour to the likes of me!

Thanks,
Jules