Still learning about chems FC/CYA

[Ame]

Can someone explain this FC/CYA ratio diffecences?
Why does CYA have to be higher and FC lower with SWG?
and then,
CYA lower and FC Higher with Bleach?

with Bleach
FC 3-7
CYA 30-50

with SWG
FC 3-5
CYA 70-80

 

[257WbyMag]

Because the SWCG is constantly adding to your FC. Bleach, which is added in stages, must be present in enough reserve to still be effective as it is consumed.

 

[jcowart]

It has to do with how chlorine is added to the pool. A SWG adds during the run time that water is flowing. You get a steady level but a lower level. Thats where the higher CYA comes in. When you dose by hand you get a spike then a steady drop during the day. A lower CYA is called for because if you slip and have a problem it takes less chlorine to get it back right. I run a higher cya level because the amount of sun my pool sees. I also have to run a higher target.

 

[jblizzle]

CYA protects the FC from the sun and buffers the chlorine. The higher the CYA, the more the FC is protected from the sun. Since the SWG adds FC slowly, it needs more protection.

For the same CYA level, the SWG can get away with a lower FC partly because the level is more constant and less prone to user error (forgetting for a day, etc) and partly due to the volatile conditions within the cell itself (like a mini SLAM chamber).

The FC numbers listed for the SWG are a no-kidding, we are serious, absolute minimum, while the minimum numbers for bleach FC levels have a little bit of wiggle room for short periods of time ... since people are human

 

[chem geek]

SWGs are often undersized so wouldn't be able to keep up with the chlorine loss from sunlight that results from the lower CYA level or even if they are sized correctly they are more expensive. Also, SWG output has side effects of raising the pH due to increased aeration (from hydrogen gas bubbles) and possible from some undissolved chlorine gas outgassing so one wants to minimize SWG on-time to minimize these effects and that means minimizing the chlorine loss during the day from sunlight so that means a higher CYA level.

The main reason a higher CYA level isn't also used for manually dosed pools that would also result in a lower daily chlorine usage is that it is riskier since shocking a pool with a high CYA level if/when something goes wrong takes a lot more chlorine. With manual dosing, it is much more likely for someone to forget a dose whereas with the SWG it is more continuously dosing so less likely to miss dosing. Nevertheless, in very sunny hot areas, a higher CYA level is often used in manually dosed pools as well.

The lower FC/CYA ratio for SWG pools is due at least in part to superchlorination of the portion of the water in the SWG cell (high chlorine level at low pH) and due to the more continual and consistent dosing and maintained FC level and due to the higher salt level any or all of which may contribute to somewhat slower net algae growth.

 

[laprjns]

For those of use using a Liquidator, which adds chlorine continuously, should we use the SWG recommend levels or the bleach recommended level,or maybe a hybrid of the two? Seems to me that we should be following the SWG recommendations.

 

[chem geek]

We recommend using the manual dosing recommendations for all non-SWG pools including those using automation such as peristaltic pumps and The Liquidator. The minimum FC for manual dosing doesn't have much play in it -- if algae nutrients are high, you will be on the edge if you get even somewhat below the minimum. So the more continual automated dosing isn't the main reason the SWG numbers are lower. It might be a factor, but not the only one. The superchlorination in the SWG cell, which is high chlorine at low pH at the chlorine generation plate unlike dosing with hypochlorite which is high chlorine with high pH, is likely a factor (ultimately the SWG puts out the same chlorine and pH as hypochlorite after the high pH water from the hydrogen gas plate mixes with the low pH high chlorine water). The other factor is the higher salt level, though that is probably a smaller factor.

We simply don't know the real reason, but I'm guessing it's a little bit of all three in the order of most important to least of superchlorination, salt level and more continuous dosing.

If you wanted to operate at a lower FC/CYA level, you can use products to supplement slowing the growth of algae, but these are extra cost and some have side effects.

 

[Jersey Devil]

Thanks. I had the same question posted elsewhere. I will make appropriate adjustments in order to get back to the manual dosing target. I don't really mind making manual adjustments if they are necessary when the LQ can't quite keep up. As we say trouble free does not mean maintenance free. I was just trying to learn a little more and Chem Geek's posts are always helpful. The Liquidator is fine in that it provides a continuing source of chlorine and makes it easier to keep up with loss due to sun and bather load. Considering an SWG for my next upgrade.