This or That?

[Philo]

Will a pool with 7 FC+60 CYA prevent algae better than a pool with 4 FC + 30 CYA?

 

[YippeeSkippy]

LOL, will you be grading on a curve?

 

[kimkats]

Pool School - Chlorine / CYA Chart

Here is our go to for that.

Kim

- - - Updated - - -

Six of one, half a dozen of another

 

[Texas Splash]

ha ha. I was going to take a stab at this one as follows: Others may have a more scientific reply, but I'll simply start by saying that from an FC/CYA ratio position, they are equally efficient. The driving factor for which one to use is the amount of exposure your pool receives to sunlight. Pools typically found in the hotter parts of the country (Florida, Texas, Arizona, etc) are known for requiring a higher CYA/FC level to combat the sun. Less extreme regions, or locations that receive a good amount of shade, can manage an FC/CYA level much lower. Regardless of which one is used in the applicable situation, they provide equal benefits of sanitation and UV protection. The key is to always ensure the FC and CYA are balanced together as noted on the Chlorine/CYA chart.

 

[kimkats]

Yea but I typed less than you and said about the same thing! LOL

Kim

 

[tim5055]

Will a pool with 7 FC+60 CYA prevent algae better than a pool with 4 FC + 30 CYA?

Talking about preventing algae in a non-SWCG pool both are about the same.

Kim has already linked to the FC/CYA chart and I'll spill the beans about the "secret sauce" behind it - 7.5%

You need t0 have FC 7.5% of the CYA in the water as your minimum FC to prevent algae, again in non-SWCG pool.

4 FC + 30 CYA = 13% +/-
7 FC+60 CYA = 12% +/-

So, both are above the 7.5% minimum

As always Chem Geek says it best:

A reasonable approximation to the above table is the following:
.... "Min FC" is 7.5% of the CYA level
.... "Target FC" is 11.5% of the CYA level
.... "Yel/Mstrd Min" is 15% of the CYA level
.... "Shock FC" is 40% of the CYA level
.... "Yel/MstrdShock" is 60% of the CYA level

 

[JamesW]

In practice, I think that the higher levels are better because there is more reserve. The lower the level, the more difficult it is to maintain a consistent level. I like to keep the cya at 40 to 80.

 

[chem geek]

So the bottom line is that if you are able to consistently maintain the FC level, then they are close to the same at preventing algae growth (8 ppm FC with 60 ppm CYA would be almost exactly the same) and everything else associated with the active chlorine level such as the rate of oxidation of bather waste, swimsuits, skin, and hair. If you are manually dosing the pool and especially if it is outdoors, then the FC will drop from the chlorine breakdown from sunlight so as James wrote the higher FC and CYA levels give you more buffer against having the FC/CYA ratio get too low.