No problems yet with lower CYA and FC levels

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C

cgarai

0
May 5, 2014
9
California
New to pool chemistry, so I wanted to just post and see if anyone sees problems headed my way.
At the moment the pool is just fine.

As mentioned on another post i'm trying to reduce my TA, which explains the low pH

Current Chemistry:
pH 7.0 - 7.2, before this I have been keeping it below 7.6
TA 90-100 was 130ppm at the start of aeration
FC 1.5-2ppm
CC 0 FAS/DPD
CYA 25-30ppm
CH 410 Pool was filled with well 220ppm well water, now maintained w softened water
Salt 3400ppm
Temp 73F

According to Pool Math and the The Chlorine/CYA Chart http://www.troublefreepool.com/threads/2177-Chlorine-CYA-Chart
it seems that I can run my CYA and FC lower than the recommended levels for SWG pools. I assume that there must be some risk or tradeoff in running at the lower levels, but I don't understand why.

Keep in mind this pool doesn't get lots of use. The temp (no solar heat yet) has just reached the point where we are using it. We also keep the cover closed most of the time to reduce evaporation (California).

Thanks for any tips!

Chris
 
The only problem you are likely to run into is shorter life for the SWG cell, vs running at a higher CYA leve and proportionately higher FC since you will have more FC burn off to sunlight. Having said that you are flirting with the low end of the acceptable FC for your CYA level, and I would suggest bumping it by .5 to 1.0 ppm just to be safe.

Ike

p.s. since you have an opaque cover, you may have CC build up issues which could be solved by more daytime UV light exposure, or through a supplemental oxidizer, either chemical like MPS, or mechanical like an Ozonator, or UV system, but I would wait and cross that bridge if you find you have CC issues.
 
If your cover is mostly opaque to the UV light in sunlight, then a higher CYA level is not necessary even for an SWCG pool. The reason it is recommended in outdoor SWCG pools exposed to sunlight is that the higher CYA level reduces the loss of chlorine from sunlight thereby letting you turn down the SWCG on-time and lengthening the life of the cell. In your case, you should already have lower chlorine loss from sunlight if the cover is mostly opaque so a higher CYA level doesn't give you that advantage. Even with a cover, I'd still have the CYA level in the 40-50 ppm range just to give you enough FC at 2-2.5 ppm (or more) to not run out too quickly if conditions change.

If you find that when you use the pool it's open long enough exposed to sunlight for the SWCG to not be able to keep up, then you can certainly have a higher CYA level. Up to you.
 
Thanks to All!

I didn't mention that at the end of last year the FC level was above 5ppm (I didn't have a DPD/FAS kit then) and I turned the SWCG off until the FC level dropped to <1ppm. That caught me off guard so I turned the SWCG up to 100% and in 4 days it was up to 2ppm so I cut it back to 60%. I didn't check it for 8 days and by then it was up to 5.5/0 FC/CC. At that point I turned it down to 15 % and then watched it drop over the next 3 weeks to 3ppm . During the last 5 days I have been opening the cover all day and aerating to reduce TA and the FC quickly dropped to 2ppm so I boosted the SWCG up to 20% and it is holding. I'll boost it a bit more to bring it up over 3ppm.

I have to say I don't understand the relationship between CYA and FC. It seems that CYA not only protects FC from decaying in sunlight, but at the same time reduces the effectiveness of the FC. If a pool with a given CYA level and no swimmer load is left open to the sun, will the FC level decay to a (short term) stable level that is below or equal to the minimum FC level for that CYA level?

And why is the Target FC vs CYA a percentage relationship and not a delta FC ppm? Not a very clear sentence, but here is another attempt.
From the Chlorine/CYA Chart

CYA/Min FC/Target FC
50/3.7/5.7 a delta of 2 ppm
80/5.8/9.1 a delta of 3.3 ppm

Why could the higher CYA of 80 just have a target FC 2ppm higher than the Min of 5.8? I suspect this is a complicated answer and I'm willing to brush up my chemistry to understand it.

Thanks,

Chris
 
The effectiveness of the chlorine is proportional to the FC/CYA ratio. This is technically because of the chemical equilibrium relationship between chlorine bound to CYA and chlorine unbound to CYA where such equilibrium constants are defined by ratios of chemicals species (technical details in this post).

The FC level will drop over time even without sunlight because chlorine will react with the pool cover and anything else in the pool water or filter that chlorine can react with. The breakdown from sunlight is in addition to this base loss. The reaction loss is proportional to the active chlorine level so the FC/CYA level and therefore is a percentage loss so it slows down as the FC gets lower until algae starts to grow at which point the FC drops much faster since it now has algae to react with. The breakdown from sunlight appears to be more related to the FC level since there may be some breakdown from the chlorine bound to CYA (though we are not sure about that).

Because at least some of the chlorine loss seems proportional to the FC level, the Target is a proportion (i.e. based on FC/CYA ratio) rather than an absolute amount above the Minimum. However, the point you are making is that for a given CYA level you should set your target such that if the FC drops (say, when the SWCG is off) it always stays above the minimum. That is in fact the right thing to do. So focus on the Minimum and always staying at or above it so your target during the day will need to be higher so that when the SWCG shuts off you won't go too low by the time it turns on again the next day.
 
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