Salt water pool, couple of questions

[IanD82]

Hi everyone,

So last month we had a salt water gunite pool built and filled with fresh water, my Taylor kit arrived recently, so I've done some tests :

FC 1.6
CC 0
PH 7.6
TA 170
CA 60
CYA 0 (or < 30, mixture not cloudy at all)

I did a few retests just to try and make sure I was doing it properly.

Couple of things stand out to me, FC is low - is this because the SWG hasn't been running for a very long period of time? Not sure how fast the FC should rise with salt.

PH seems to be under control with an automated system, it's set to 7.6 but rises to 7.75 every night and every morning the machine adds some pH minus - without this I'm sure the pH would rocket up.. not sure it's normal to be adding pH minus on a daily basis. Is this because the TA is too high?

The CYA mixture in the Taylor did not go cloudy at all, so I didn't get a registered number. I'm presuming this is not a good thing reading the guides (60-90).. I will ask the pool company if they added any pool stabilizer but it appears not - could there be a legitimate reason?

Would appreciate any thoughts and which values to try and fix first, I'm not sure of the order of importance.

Thanks,

Ian

 

[cowboycasey]

Welcome to TFP

There are a couple of things different with a saltwater pool

1. always raise your FC level with liquid chlorine and then keep it where you want with the SWG..
2. Your PH looks good, I take it you have some kind of acid system
3. Yes, the TA is high but as you drop the PH it will come down.. What is your fill water PH and TA? If your fill water has a high TA it will raise your pool TA every time you add water...
4. They will probably have no idea about CYA and why they would put it in a pool.. Get dry CYA and add it in a sock to bring it to 70 or 80 and see how your pool likes it... Your SWG will be way more happy with CYA at that level and you will probably be able to turn it down..

Hope this helps

 

[mgtfp]

Whether your FC is too low really depends on CYA being in the water or not. If you can't find out if the pool company added any, then I would add 30ppm worth first. That should get you into the measurable range, but shouldn't get you above 60ppm if there are, as a worst case, already nearly 30ppm in the water.

As mentioned by cowboycasey, best is to reach your target FC by adding liquid chlorine, and let the SWG take over from there.

How much chlorine (in ppm) your SWG produces depends on the size of your SWG and the pool volume. My SWG is for example rated to produce 25 g Cl₂ per hour, and my pool has 66000 L. That means that my SWG adds about 0.38ppm of FC per hour when running on 100%.

What is the "pH minus" that your system is adding? Is this Hydrochloric Acid (or Muriatic Acid as it is usually called in the US) or is this dry acid powder (Sodium Bisulfate)?

It would be great if you could add your pool equipment and pool volume to your signature, that helps people supporting you and avoids repeating the same questions over and over again.

 

[IanD82]

Thanks for the replies.

What is your fill water PH and TA? If your fill water has a high TA it will raise your pool TA every time you add water...

I haven't added any additional water yet and unfortunately I wasn't able to test the fill water (from a tanker) at the time because I didn't have the kit. I will test the tap water though for future reference when I do top up.

I will get some dry CYA and read up on the 'sock' method.

Should get the CYA level up before adding the liquid chlorine? I guess so otherwise I won't know what FC the pool should have.

What is the "pH minus" that your system is adding? Is this Hydrochloric Acid (or Muriatic Acid as it is usually called in the US) or is this dry acid powder (Sodium Bisulfate)?

It's the liquid Hydrochloric Acid not powder and it seems to be going down very fast but I've been told that's normal, at least in the beginning. That's when they recommend to set the pH to 7.6 instead of 7.4 because it's easier to maintain 7.6 - without the automated pump doing it, I would be adding pH minus every single day which doesn't seem right.

I've attached a graph of the pH levels in the Astralpool app, looks like it's adding small amounts 2 or 3 times a day but the most at 08:00 when the system starts.

Thanks for the signature tip, I've updated it now.

 

[mgtfp]

Yes, add the CYA and then add chlorine to match that CYA-level. Once there is some CYA dissolved, you can add the chlorine, you don't have to wait until it's all dissolved.

Over time, with more and more acid added, TA should get lower and reduce the pH-drift - don't increase TA until it gets below 50. It will also depend on the TA of your fill water. If that is high in TA and you have to replace a lot of evaporation, than you keep adding, and you might just be able to to maintain TA with your regular acid addition. Then, a cycle of manual acid additions and aeration could make sense to bring TA down a bit. Test the TA of your fill water to know where you're at.

Once your TA is lower, you might even set your pH to 7.8, if the system allows that. Keeping pH around 7.8 and adding acid only once pH reaches 8 helps to reduce acid additions. Also check your CH and calculate the CSI with PoolMath to make sure that you don't run into scaling problems when maintaining higher pH.

 

[IanD82]

Thank you so much for the help, I have a plan of action.

PoolMath is saying I should 1.4kg of dry stabilizer to get a target of 30 (presume 30ppm?) - before I blindly trust the figures - does that sound about normal for 48,000L pool?

How much chlorine (in ppm) your SWG produces depends on the size of your SWG and the pool volume. My SWG is for example rated to produce 25 g Cl2 per hour, and my pool has 66000 L. That means that my SWG adds about 0.38ppm of FC per hour when running on 100%.

Just looked it up, 30g Cl2 and my pool has 48,000L.. according to the tech sheet this machine is suitable for a pool up to 140,000L.

Hoping that they had good reason to select this one for me and I'm guessing it can just be run at a lower setting, compared to a lower output model.

Maybe then the cell lasts longer and in the long run saves money?

 

[cowboycasey]

After all the conversions that is the correct amount of CYA

that is exactly why we recommend bigger cells than what the pool size is.. you run them less and they last longer and save money from not running them so long

 

[mgtfp]

Yep, as cowboycasey said, looks properly dimensioned. That will have an output of about 0.6ppm per hour on 100%

CYA can take a while to get fully dissolved, don't get impatient and add more too early, you don't want to overshoot.

And a question: Where did you buy your Taylor kit? Many in Europe seem to be struggling to source one. Looks like you might be able to help them out

 

[IanD82]

And a question: Where did you buy your Taylor kit? Many in Europe seem to be struggling to source one. Looks like you might be able to help them out

Yes, it wasn't easy to find and I'm sure I paid a hefty premium, I got it from:

Analizar Agua: Tienda online de analtítica

Encuentra material e instrumentación de laboratorio, analítica, piscina, alimentación... Los mejores productos al mejor precio.

analizaragua.es

170 EUR but looks like they are down for maintenance at the moment.

 

[IanD82]

OK I've added CYA using the sock method, completely dissolved in about 30 minutes in front of a return jet.

I added enough to raise 30ppm and I've just measured a week later and it's about 50ppm, so that's good. Need to a little more but no problem.

What does concern me... Previously my FC was low, 1.6 now I've just measured it, adding nothing but CYA and the SWG running at 70% - FC is now 21

Is that even possible or have I seriously messed up my measurements? It just took 42 drops in a 10ml sample to go clear. I've just put the SWG down to 10% to try and lower it.

Can adding CYA make that drastic change in a week?

 

[cowboycasey]

nope, but an oversized cell can make chlorine fast in a cold pool.. just turn the SWG off until your FC gets down to 7 or 8..

 

[mgtfp]

With enough CYA, you are not loosing most of your FC to UV anymore. So, rather than just replacing UV-losses and just being able to maintain FC, you are now generating a daily FC surplus. You will have to turn the SWG down as cowboycasey already mentioned.

 

[mgtfp]

I assume your "Smart Next 30 SWG" produces 30g Cl2 per hour on 100%. In 46000L, that gives you about 0.65ppm per hour or 7.8ppm per day. On 70% that's still 5.5ppm per day. How many hours per day is your SWG running?

 

[mguzzy]

@mgtfp nailed it. You are not having to make up for UV losses now. In fact as you increases your CYA you should find a sweet spot where you get the best FC retention... in my pool it happens at CYA of 70 ppm

 

[IanD82]

I assume your "Smart Next 30 SWG" produces 30g Cl2 per hour on 100%. In 46000L, that gives you about 0.65ppm per hour or 7.8ppm per day. On 70% that's still 5.5ppm per day. How many hours per day is your SWG running?

It's running 8 hours a day at the moment, though I think I probably could reduce that until summer arrives.

So it's producing about 3.64ppm in 8 hours and it's been about a week since I added the CYA, 25.5ppm and losing a little to UV, now is 21ppm - I guess that makes sense!

And if I'm losing about 0.6ppm/day now due to UV, I can put the SWG on about 15% and should be OK maintaining.

Thanks again everyone.

 

[mgtfp]

Just watch your FC in the transition period to summer. I have about 5% FC loss per day (i.e. if my FC is 10ppm, I loose about 0.5ppm) in winter. Once the sun gets high enough above the neighbours' house, fence and trees, my pool is hardly shaded anymore, and FC loss get up to 10% (on a cloudy day) or 20% (sunny day in the middle of summer). UV-index and shade on pool can change massively between winter and summer. You will have to adjust your settings as summer is approaching.