Target FC and Yellow/Mustard FC in SWG Pool

[smallpooldad]

My friend has a 20,000 gal SWG plaster pool with CYA set at 50 ppm. On the Chlorine/CYA chart the target FC is 5.7 and the yellow mustard minimum is 7.5, his current FC is 8, can he lower and if so to what with reasonable margin of safety? I believe he has yellow algae as he does not always have the time to maintain his pool.

He has suction side pool cleaner that runs while the pool pump is on, how many turns of water would you recommend? As he does not wish to run his pump longer than necessary, he is retired and wants to keep his electricty bill as low as possible.

The reason for asking this is that if I am understanding this correctly a SWG pool can be maintained at lower levels, and he wants to lower them as his manual states 1-3 ppm chlorine will make the cell last longer. I will advise him on your thoughts.

As I maintain mine at a CYA of 30 and a FC of 5-6 it is hard for me to tell him less. One point of interest is only his wife swims in it once a day.

Thank you.

 

[chem geek]

That yellow/mustard minimum column was for when you had that sort of algae that you never completely got rid of or that would get regularly reintroduced in some way. For those that have shocked at the high yellow/mustard algae level and gotten behind light niches and removable ladders and thrown in poles and other equipment in the pool and washed swimsuits, etc., then normal chlorine minimums should be OK. So for an SWG, that's roughly an FC that is 5% of the CYA level in the simplified table (it's around 4.5% of the FC in my original table).

However, every pool is different so if you need to adjust to a different minimum to not have any unusual chlorine demand (perhaps from nascent algae growth) then so be it. These tables work well, but there could always be a 1 in 5000 pool that doesn't work with these guidelines.

 

[smallpooldad]

Chem Geek,

I am not sure I am understanding this correctly but this is what I think you meant, please correct me if I am wrong.

Based on a number of 5% for and SWG for minimum free chlorine versus 7.5% for a regular liquid chlorine, etc. of the CYA level, etc. The required chlorine dosage would be 66.66% of what is shown in the "Chlorine/CYA Chart" for a non-SWG pool. Or would the percentage move higher say to 75% at Target FC, and 88% at Yel/Mstrd Min FC ?

For example at a CYA of 50 the settings for a SWG pool might be as follows: minimum FC = 3.7 (regular pool) x 0.6666 or 2.5, Target FC = 5.7 x .6666 or 3.8, and Yel/Mstrd Min FC = 7.5 x 0.666 or 5, for SWG pools.

Could the same calculation be applied to a SWG pool with a CYA of 30 (my pool), for example Target FC = 3.5 x .6666 or 2.3? At lower CYAs would this 66.66% conversion factor work as well? I personally think no but what do believe?

For information's sake: His pool is 20,000 gallons using a brand new (5 days old) Aquapure 1400 cell, set at a power level of 35%, he maintains the FC at 8, the CYA at 50, he is losing 1.09 ppm of chlorine a day, based on the Aquapure manual he produces 1.25 lbs of chlorine in 24 hours, and he runs his pool 10 hours per day. I hope this calculation is correct, but most probably wrong, it is based on a 1 ppm loss of chlorine being equivalent to 2.6704 ozs of chlorine required to replenish a 20,000 gal pool.

One other factor I failed to note is his pool is nearly 20 years old with the top plaster coat missing in many areas some of which have black algae. I told him he needs to drain the pool as it would require Hercules to treat the many spots while the water is in it, we could cover them with towels filled with pure chlorine and treat them together in one night by scrubbing every 30 minutes or so, but he is loath to do this. He is in his late 70's. The pool water is otherwise very clear.

This is why he may be using 1 ppm of chlorine per day instead of 0.5 ppm, but the UV index here, which is very high, may also be a factor.

Am I anywhere close on the numbers or am I hopelessly confused?

But what about turnover, which is also his concern. He could up his power setting to 50%, and reduce his run-time, a major concern of his as the KwH price is two to three times more expensive as the mainland, He and I would like to know what might be a good turnover number for the water, if you help with the turnover rate it would be greatly appreciated.

 

[chem geek]

I'm saying just follow the Chlorine / CYA Chart. If you want to follow the Chlorine/CYA Chart by Chemgeek, use the Min FC column for manually dosed pools or use an FC that is 5% of the CYA level for SWG pools and yes you could use 67% of the Min FC column if you wanted to as a reasonable approximation and yes, you could do the same for the target and Yel/Mstrd columns. However, the Yel/Mstrd Min column was for pools that did not completely get rid of yellow/mustard algae or where such algae was regularly reintroduced.

Don't overthink this. Yellow/mustard algae is far less common so you can't take these chart values as absolute mantras -- they are guidelines that appear to work for nearly all pools for green algae, but for yellow/mustard algae we have a much smaller number of pools that have been treated.

8 ppm FC with 50 ppm CYA is on the high side, but a loss of 1 ppm FC per day is low if the pool is exposed to sunlight. Also, the black algae needs to be scraped with a brush if you want to kill it.

 

[smallpooldad]

Chem Geek,

Very clear.

Thank you