What do you reckon on these results?

[Nuscw]

Hi all,

Just new to the site. We have a 35,000L (just over 9,000USG) in ground pebblecrete pool which is about 6-7yrs old. Equipment is an Autochlor S30 SWG with chlorinator set to 'LOW' (can't find out how much the cell actually generates per hr), 18,000Lp/h pump & cartridge filter. At the moment it's winter up here, so the system is on 3hrs am and 3hrs pm. Pool is in full sun and still gets used, (daytime air temps around 28, night around 15deg, pool is a steady 20-23deg) but nothing like the bather load in a usual summer where the system is set for 4hrs + 4hrs. Returns are in-pool surface directed, so no other aeration than that.

We've just bought this place a month or so ago - so just getting used to the pool etc. Testing was done by a pool guy up until now, so my first lot of test results showed pH off the scale past 8.2, TA 190, Cl off the scale past 5. We did have a fair bit of rain recently which added a good 3 inches to the level, so the pH rise wasn't unexpected.

Sampling at the local pool shop today shows:

Total Cl 8.0
(Free Cl 5.0, Comb Cl 3.0) There is that definite chloramine smell about the water...
pH 7.9 (I added a litre of HCl after the initial test result of >8.2)
TA 180
NaCl 4000
Stabiliser 0.0
PO4 0.0
Ca Hardness 100

Clearly the pH/TA needs addressing (pool shop reckons almost 3L HCl). But what about the Cl? The suggestions from the shop were to switch off the SWG for a few days and retest, but I'm not so sure about that. I can't seem to get my head around the chemistry, is this overchlorination by the cell, or too much combined Cl needing shocking? Do I need to stop the SWG for a while and turn down the running times?

There's obviously been no CYA for a while either, so I'll stick a few kg of that in soon, but what difference will that make to the results? I've seen some posts on here about borate - would that be useful?

Basically it would be great to get your ideas about what's actually going on chemistry wise, as well as what to actually do about it.

Regards from the Tropics, Stuart.

 

[JasonLion]

That is a puzzler. I can't think of a way to get that combination of results. My first guess would be that the test results are wrong in addition to whatever is going on.

Your TA is high and combined with a SWG that means PH will tend to rise. You need to keep PH between 7.2 and 7.8 by lowering it with acid as often as required. If you get the TA down the PH will be much easier to manage.

If stabilizer/CYA is zero, or very low, all of your FC should vanish fairly quickly any time it is sunny. That doesn't fit with the high FC level.

High CC would give the smell you noticed but high FC levels should clear up the CC fairly quickly unless there is a significant source of organic contamination, especially at low CYA levels.

Your calcium level is very low for a plaster/pebble pool. You should bring the calcium level up to 200 fairly soon and perhaps 300 when you get the TA level down.

Most of which assumes that the test results are right, which I am not at all sure of. The best possible thing you can do is to get a top quality test kit of your own. A good test kit is the best investment in your pool you can make. I recommend the TF Test Kit. The Taylor K-2006 is also a good choice.

 

[Nuscw]

Cheers for that Jason,

Has me puzzled too. My test kit shows the same as the pool shop, but I'll get it done somewhere else to be sure anyway.

The automated results thing from the shop says:

1. Add 3L HCl in total
2. High CC needs oxidiser treatment
3. Add hardness increaser
4. Add stabiliser

It also gives ideals (which seem different here in Oz - they don't give an a measurement unit just a number) TC 2, (FC 0.0, CC 0.5) TA 110, Ca hardness 120. Interesting that the Ca hardness figures are so different, I do have a Ca scale problem especially on the steps, which I spent 2hrs removing yesterday armed with snorkel, mask and syringe full of acid - made the kids smile anyway...

As for the pool itself, it's lovely and clear. There's certainly no sign of any contaminants. The Cl smell is less today (maybe 'cos the sun's been out). My plan for now is add another L of HCl today, get some Ca hardener in as well as some stabiliser. I'll run with the SWG off for a day or so and see what happens - Is it possible that it could be overchlorinated?

Thanks again, Stuart.

 

[JasonLion]

Try reading the sticky on lowering TA. You don't want the PH getting too low while you are in the process of lowering TA.

Yes, the pool could be over chlorinated, but that would not cause CC to be above zero, it would drive CC down towards zero. If your CYA level is really that low you could lose FC very quickly on a sunny day.

High PH can cause scaling. With TA as high as it is and if your CH level was higher before and given the periods of high PH you have had, there could easily have been some calcium scaling. Also, the scaling would have lowered the CH level to where it is now. It is very important to keep PH under 8.0 at all times.

If the CC doesn't go away fairly quickly you will need to shock the pool to get rid of it.

 

[Nuscw]

Will follow the guide on the TA sticky and see where it gets me.

I can't think of a reason the CC is up - it's a clean looking pool with no organic matter and not much bather load at this time of year so still a little unsure whether or not to run the SWG? Could the filter be the issue?

Also, at what point would you consider shocking it? When I've sorted TA/pH? Like you say the FC's high enough already so that should be knocking the CC down. I think serial tests to give me an idea of the trend is the way to go.

 

[Nuscw]

Have been following the TA advice religiously over the past week. Took it easy through lack of aeration. TA testing today is 80, CYA 50. My high FC issue seems to be simply running the cell for too long, it's coming down slowly, around 5 today - was 8 last week.

I'm still having rapid pH rises though, from ideal 2 days ago to 8.0 this morning. And this is with the SWG turned off! I realise the water will always try for >8.0 but this quickly? Should I just keep on adding the acid?
Did mention borates in all the pool shops but no-one seems to use it here - got blank looks in every store.

On a slightly different track and just out of interest (I work in ICU so have been amazed by how similar pool acid/base balance is to what I usually do every day in managing human A/B issues). Given that the acid is a proton donor, what happens to the residual chloride ion when adding HCl to the pool?

Thanks, Stuart

 

[Matt]

My pool also trends up in pH and I often have to add acid. However, that is with the SWG going. All the outgassing of carbonate may be increasing your pH, although you shouldn't see much outgassing at a pH of 8.0 with a TA of 80.

Given that the acid is a proton donor, what happens to the residual chloride ion when adding HCl to the pool?

The chloride ions generally accumulate in the pool (like when you add salt, the Na+ and the Cl- remain in the pool until you drain it or splash it out). The amount added by the acid is pretty small and would probably be lost just to splashout. The chloride ion is pretty happy with the negative charge and not that reactive because of its high electronegativity. Your SWG can use the chloride ions to make more hypochlorite too (it doesn't actually need Na+, that is just how the Cl- is delivered).