Understanding the Chlorine/CYA Chart - what do the different levels REALLY mean?

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boringforumname

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Oct 15, 2015
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Kailua, Hawaii
I thought I understood what the target FC levels meant, and I know the specific numbers are related to your cya level, but the more I think about it the more I think I need a clarification.

I'm assuming that you should shoot to keep your FC at the the "target FC" number and never let the FC drop below the "Min FC" number. Does that mean that there is a good chance of algae (or any undesirables) growing between the target number and the min number? Also there is a Yel/Mstrd Min, which is always higher than the Target FC number. Does that mean that all (or most) types of algae would not grow if the level was kept at the target level, but mustard algae could grow at any level below the Yel/Mstrd level? And since none of us want mustard algae, wouldn't that make the Yel/Mstrd level the DEFAULT Target FC level? :confused:

Also, the shock levels....if we keep the correct target levels we shouldn't need to shock regularly correct? If a certain chlorine level kept stable will not allow most algae growth and/or mustard algae growth, why does it take such an incredible FC level to kill it off if it once it blooms. The Shock FC levels are about 3.5-4.0 times the Target FC level on average.

Also, would the answers to these questions relate to the idea of breakpoint chlorination, which is simply raising the FC to a level of 10 times the CC level. So if you had a CC level of 0.2 raising the FC 2 ppm could be considered a shock? (Chemical Shock - Shocking Your Swimming Pool | The Swimming Pool Library I also read about breakpoint chlorination in the Taylor 2006 instructions) And why is it that the pool industry recommends shocking once per week anyway? Is it that the average FC levels throughout the week keep most of the algae at bay, but not all, so shocking once per week makes absolutely sure nothing is growing? Or is it based on the fact that most pool services only visit a pool once per week so "shocking" it every or most times they visit is the standard?
 
There will be others that have a lot more insight than i, but here is my attempt. Also, I think you are pulling this from Chem Geek's post from 2007 which was simplified into the currently used CYA/FC Chart.

Breakpoint - forget about it. Mythical like a unicorn, it is an antiquated way to manage a pool I think is the common consensus.
Target vs. Min; Yes, you should shoot for the target and make sure your additions are timely enough to maintain levels above the minimum at all times.
Minimum Mustard/Ylw Algae Target: Mustard algae, although is thought by many to be their problem, is probably much more an exception than the rule. References to a Mustard min were dropped from the simplified/standardized CYA/FC chart. For the unfortunate pools with mustard algae suspected it is suggested now to have a 24 hr MA Slam target at the end of a normal Slam to kill off any stubborn algae.

As for the shock levels, they are not necessarily higher to kill the algae, even the lower FC concentrations kill algae, it is really raised to kill it faster. Algae replicates itself very quickly, by the time it is visible in your pool there is a LOT of it and your kill rate has to be greater than it multiplies. This is why one missed day during a SLAM can extend the overall duration by days or a week.

And correct a maintained pool may never need to be shocked or slammed. There are members that have not done it in many years.
 
You'll see two numbers. Target and Minimum. Minimum is just that: the minimum below which your FC should never drop. Target is a suggested goal to enter into poolmath to calculate the dosage. With time and testing, you'll get a pretty good understanding of what your goal needs to be so that FC doesn't dip below minimum before you next dose the pool. If you lose less than 2 per day and your minimum is 4, you target 6. But if you've been targeting 6 and everyday when you test you come up at 3, you'll need to target 7. Have a bunch of kids with weak bladders swimming in your pool all day every day? You might need to target 8 or 9. The target is just a suggestion, it's not an absolute.
 
I don't know where you are getting your definitions. I use Pool Math to determine my target levels. It gives a NORMAL range, which is where you should normally keep the FC, a lower minimum for those using SWG (I can explain that if needed), a SHOCK level, and a MUSTARD ALGAE SHOCK level. Keeping algae free is easy if you are always in the normal range, drop below and algae can start to grow. Once it begins it takes a lot more chlorine than NORMAL to get rid of it, because it is growing and consuming most of the chlorine you are adding. So you have to get ahead if it and keep the SHOCK level maintained until every bit of algae is dead. That's why the acronym SLAM (shock level and maintain) is used here in place of just shock. Mustard algae is even harder to eradicate and requires even higher levels of chlorine.

As far as "industry recommendations" I haven't shocked my pool in ten years. There is no need, as long as you maintain proper CH levels shocking is unnecessary. Most pool services that only visit a pool weekly or biweekly just raise the CH levels well above the normal range (as long as they don't exceed the SHOCK level the pool is usable) and hope that it doesn't drop below the minimum level before the next visit. If it does...the client will be paying them to clear up the algae bloom that results.
 
There's never really daily discussion here of a minimum FC level to prevent mustard algae specifically. It's a rare bird. People with recurring algae are sometimes told to keep the FC level at a higher %, but that is usually to overcome poor circulation, inaccurate CYA readings, things like that.

Again, it's only when the algae is allowed to grow in the first place that you need to overwhelm it. If your pool is always properly chlorinated, the latent chlorine should easily kill the first algae spores when they land in the pool. You may see an increase in FC loss or a bump in CC, but that's why you test often, to catch things like that.
 
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