Question about CL levels and salt cell lifespan...

For a given CYA level, the loss of FC due to UV is a % of the residual, not a fixed amount. So the higher the FC residual, the higher the loss during the day. So yes, you need a higher SWG % setting for a higher FC residual. Water temperature seems to have a similar % effect on FC loss as well.

However, the FC demand due to organics could be a fixed amount assuming they are being added to the pool at a fixed rate (i.e. algae bloom). It takes so much FC to kill so much bacteria or algae. But the higher FC levels should kill it faster.

But I am surprised that you only lost 1.5 ppm with a 20 ppm residual. Even at 80 ppm CYA, normal FC loss would be 10-20% due to just UV. Did you have a cover on the pool at the time? Something else might have been going on at the same time.

Your experience of 1.5ppm loss per day was measured with a Taylor K-2006 kit using the 10ml sample size?

If so, the error for each test is at least 0.5ppm (one drop). The measured 1.5ppm loss per day may well have been 1.9, 1.8, 1.7, 1.6, 1.5, 1.4, 1.3, 1.2, 1.1 all within the test's error margin but painting a different picture of chlorine loss.

I'm sorry to sound stubborn, but, higher FC (at a fixed CYA) loses more chlorine to daily UV than lower FC does.

This may be a reasonable tradeoff and cell life may be limited by other factors (in addition to AmpHour ratings).

If you're more comfortable at a slightly higher FC - Great. If you want to squeeze every last penny, then lower FC is probably cheaper but you are exposed to greater risk from CL usage excursions (think all the neighborhood kids pee in your pool one day when usually only 10% of them do).

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Originally Posted by mas985

For a given CYA level, the loss of FC due to UV is a % of the residual, not a fixed amount. So the higher the FC residual, the higher the loss during the day. So yes, you need a higher SWG % setting for a higher FC residual. Water temperature seems to have a similar % effect on FC loss as well.

However, the FC demand due to organics could be a fixed amount assuming they are being added to the pool at a fixed rate (i.e. algae bloom). It takes so much FC to kill so much bacteria or algae. But the higher FC levels should kill it faster.

But I am surprised that you only lost 1.5 ppm with a 20 ppm residual. Even at 80 ppm CYA, normal FC loss would be 10-20% due to just UV. Did you have a cover on the pool at the time? Something else might have been going on at the same time.

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The week I was allowing the CL level to drop, it was very cloudy. Perhaps this slowed the rate of loss? The pool was not covered during this time. Also water temp was around 60.

Pardon my ignorance, but what exactly is "free chlorine residual"?

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Originally Posted by BigDave

Your experience of 1.5ppm loss per day was measured with a Taylor K-2006 kit using the 10ml sample size?

If so, the error for each test is at least 0.5ppm (one drop). The measured 1.5ppm loss per day may well have been 1.9, 1.8, 1.7, 1.6, 1.5, 1.4, 1.3, 1.2, 1.1 all within the test's error margin but painting a different picture of chlorine loss.

I'm sorry to sound stubborn, but, higher FC (at a fixed CYA) loses more chlorine to daily UV than lower FC does.

This may be a reasonable tradeoff and cell life may be limited by other factors (in addition to AmpHour ratings).

If you're more comfortable at a slightly higher FC - Great. If you want to squeeze every last penny, then lower FC is probably cheaper but you are exposed to greater risk from CL usage excursions (think all the neighborhood kids pee in your pool one day when usually only 10% of them do).

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Please, no need for an apology. :) I'm trying to learn, not argue a position. I greatly appreciate the effort y'all are making to help me understand this.

Yes, I use the 10 ml sample with one scoop of powder, so the resolution is .5 ppm. I see what you are saying about the margin of error and it does paint a different picture.

So now I understand, at a fixed CYA level, higher CL levels will suffer higher losses from UV exposure than lower CL levels. So the SWCG will have to work more to maintain a higher CL level. Have I got that right?

I think what I'll do is shoot for a CL level of 5. Perhaps this would be a good compromise between cell life and having some wiggle room. 99% of the time it's just the two of us using the pool. We might have 1 "pool party" a year (may half a dozen adults or so; I know, we sound really boring..lol). I could always give the pool an extra dose of bleach before a higher bather load for a bit more wiggle room.

Thanks. :)

Yes, the cloudy weather cut down the amount of UV so less loss of chlorine. And yes, for a given CYA level a higher FC level will lose more absolute FC because the loss is roughly a percentage loss so the same percentage of a higher number is a larger number. A target of 5 ppm is fine with 80 ppm CYA. 4 ppm would be an absolute minimum with 80 ppm CYA for a pool with a saltwater chlorine generator.

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Originally Posted by chem geek

Yes, the cloudy weather cut down the amount of UV so less loss of chlorine. And yes, for a given CYA level a higher FC level will lose more absolute FC because the loss is roughly a percentage loss so the same percentage of a higher number is a larger number. A target of 5 ppm is fine with 80 ppm CYA. 4 ppm would be an absolute minimum with 80 ppm CYA for a pool with a saltwater chlorine generator.

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Thanks. :)