New TFP user, need some clarification on chemistry.

[duurmeehr]

I have been reading a LOT on TFP the last few weeks and would like some more info on some things I don't understand.

I am running a SWG in a vinyl inground.

I am at 6.5 FC, 0 CC, 70 TA, 7.5 PH, 45 CYA, 50 CH.

My water is clear.

1) I don't understand why they want me to run such a high CYA just because I am running a SWG. (70-80 ppm with a shock level of 28-31)

2) I don't understand where the recommended shock levels come from. I have always done breakpoint superchlorination. Whenever my CC gets over a .5 I shock to ten times whatever it is including my FC level. So now what are the signs that I need to do a SLAM? Is it saying that a SLAM with the recommended CYA levels is 28-31 ppm in a SWG pool?

It's obvious the method works, I am just having some trouble getting my head around some of it.

Two days ago I got 2 inches of rain in one hour. Yesterday I had seven 12 year old girls in the pool for 6 hours covered in sun tan oil and playing like crazy. In the old days I would have shocked the pool just in case. I didn't and today all appears well.

Any help bringing me up to speed will be greatly appreciated.

 

[Donldson]

A slightly belated Welcome to TFP!

1. With CYA you always want to balance between the protection it offers from the sun vs the buffering it has on chlorine. With an SWG you want better protection from the sun. With a manually chlorinated pool there is more chance of missing a day or two and you are more likely to need to SLAM, so a lower CYA makes the SLAM easier. With an SWG you will probably always have enough FC to ward off any problems. There is also some evidence that with water getting a higher dose of chlorine as it goes through the SWG cell it allows the FC to be run lower than with a manually dosed pool.

2. Breakpoint superchlorination is an older technique that completely ignores the FC/CYA relationship. TFPC shock level is 40% your CYA level. This gives enough unbuffered FC to kill algae faster than it can reproduce. It is a process that assures that EVERYTHING is eliminated from the pool that can cause problems, while keeping active FC levels low enough that it doesn't cause damage to the liner or equipment. As you have noticed though, SLAM is not typically needed. Normal signs of needing to SLAM in an SWG pool include the inability to maintain your FC, an increased CC reading, or cloudy water.

Chem Geek has several posts in this thread describing how Ben Powell came up with the FC/CYA chart and how Chem Geek, with a background in chemistry (thus the name), and several members of The Pool Forum and TFP polished it in to the very specific model it is today: Chlorine/CYA Chart

 

[chem geek]

SWG pools tend to rise in pH from a combination of the increased aeration from the hydrogen gas bubbles and possibly some undissolved chlorine gas outgassing. Having a higher CYA level even with a proportionally higher FC levels uses less chlorine since the CYA non-linearly protects the chlorine from breakdown from sunlight. So losing less to sunlight means you can turn down the % ontime on the SWCG and that reduces the effects that would have the pH rise. It also has the cell last longer.

The recommended SLAM levels are those needed to kill off algae quickly or oxidize organics faster. You should not normally have to SLAM your pool ever if you properly maintain it. The actual level is somewhat arbitrary since anything above the minimum FC level (for a given CYA level, so the FC/CYA ratio) should kill green and black algae faster than it can reproduce, but if there is an algae bloom then the clumped algae is slower to kill so requires a higher chlorine level to be able to kill faster and to penetrate the clumps faster to kill the algae inside. The FC also gets depleted faster in that situation and the higher SLAM level helps prevent the FC dropping too much.

Breakpoint chlorination for ammonia is continuous and in a residential pool there is no need to elevate the chlorine level since the bather-load is very low. When you measure any CC, it is more likely to be a slower-to-oxidize chemical such as monochlorourea which is relatively innocuous, isn't volatile, doesn't smell, is not irritating, so can be left alone. However, in an outdoor residential pool exposed to sunlight, CC is rare because the UV in sunlight breaks down chlorine into very powerful but short-lived oxidizers called hydroxyl radicals. They usually take care of any organic precursors in the water and the UV breaks down dichloramine directly as well.

Furthermore, the 10x rule for breakpoint chlorination in pools is wrong because the rule comes from ammonia measured in ppm Nitrogen units whereas Free Chlorine (FC) and Combined Chlorine (CC) are measured in ppm Cl₂ units that are 5 times larger. Also, CC already has a chlorine attached to it so in reality it only takes between 1/2 and 1 times the CC to oxidize it if it were monochloramine. Of course, a higher FC/CYA ratio will have any such oxidation go faster, but you can just forget the 10x rule and shouldn't need to worry about CC anyway. If you have CC and are using a pool cover, just uncover the pool to expose it to sunlight and to let it air out and you should be fine. Finally, if you were to use a 25 ml water sample in the FAS-DPD FC and CC test, I bet you'd find that you really have <= 0.2 ppm CC which is nothing to worry about.

To give you an idea of how bather-load affects the chlorine demand in your pool, one person-hour in a pool typically requires around 4 grams of chlorine (as ppm Cl₂) so in your 24,000 gallon pool that is 90,850 liters so (1000 mg/g)*(4 g/person-hour)/(90,850 liters) = 0.044 mg/L so 0.044 ppm FC. So even 10 people for an hour in the pool would only be 0.44 ppm FC of chlorine demand. Of course, urinating kids would be another matter since one cup of urine is equivalent to roughly 6 person-hours.

 

[poolnoob.ca]

Regarding shocking after rains, i have a feeling that may also be a pool store urban legend, as everyone here says that rain has little effect on chemical levels. after all the pool volume is only increasing 1 or 2% if that

 

[duurmeehr]

Thank you all for the answers.

The breakdown from chem geek was awesome and clears everything up for me.

As a technical person I need to know how things work to understand them.

I was afraid to bump the CYA up remembering my old pool store days but now that I understand I will run it up a bit.