Can't Get Rid of Algae

[CTIGuy]

Up to what chlorine level is safe for bathers?

[CarlD]

Up to what chlorine level is safe for bathers?

The answer is, again dependent on your CYA level. Usually bathers are safe up to the shock level, but I prefer to let FC come down to at least close to the upper end of the maintenance level specified in the Best Guess table.

So when my CYA is in the 30-50 range, and shock level is 15, I'm personally willing to let anyone in up to 10ppm (Maintenance is 3-6ppm). Just wear old suits you don't mind having fade. At that level, I'm uncomfortable swimming near 15ppm.

But if your CYA is in the 100-200ppm range, your normal maintenance range for chlorine will be 8-15 ppm and it is PERFECTLY safe to swim then--in fact you shouldn't swim in it if it's BELOW 8ppm--it might not be sanitary. Shocking will be at 25 ppm and while it should be OK to swim at 20ppm, my own superstition creeps in and I won't do it. Then again, if my CYA is 100, I'm draining and refilling with new water to dilute it down to between 30 and 60ppm.

[KurtV]

I thought the max in the Best Guess table was the upper limit for safe swimming (e.g. 6 ppm for a CYA level of 30-50 ppm). That certainly seems to be implied by the use of the word max.

[chem geek]

I'm not sure why Ben put in a column called "Max" or what it really means. The "Max" column corresponds roughly to a disinfecting chlorine level of 0.07 ppm which is equivalent to around 0.15 ppm FC with no CYA so is a very low amount of disinfecting chlorine. The "Min" column, for comparison, roughly corresponds to a disinfecting chlorine level of 0.03 ppm. The shock column doesn't correspond to a single disinfecting chlorine level, but is closest in the mid-CYA area to around 0.3 ppm disinfecting chlorine and that's equivalent to around 0.6 ppm FC with no CYA. So even shock level isn't as strong as most indoor pools with 1-2 ppm FC and no CYA.

The above discussion has to do with safety from the reaction rate of chlorine (i.e. concentration or strength) -- i.e. how fast it oxidizes your skin while in the water, for example. This is different from the actual FC level which determines the total amount of chlorine (i.e. capacity) which is more relevant if you were to swallow the chlorine since all of it will react and the rate is less important than the total amount of potential damage (in reality, most of the chlorine should get neutralized by saliva unless you drink an awful lot of it). For swimsuits, the disinfecting chlorine level is relevant while in the water, but when getting out and having it dry out, then the FC level is probably somewhat important though chlorine will also outgas as the water evaporates.

Richard

[CarlD]

Min and Max are the ideal target range Ben came up with for maintenance levels. Min is obvious but I'm thinking Max is the level beyond which you are just wasting chlorine unless you are shocking.

But in ALL chemical studies there's always a target range because a single number isn't necessarily meaningful.

[KurtV]

Richard,
Thanks. I'd seen that explanation from you before but had forgotten it.

I know you probably don't want to make a definitive statement like this, but I take from your explanation that, unless you're swallowing a pretty good amount of pool water, swimming at shock level (per the Best Guess Table) is at least as safe as swimming in an indoor pool.

[chem geek]

That is my opinion based on what I know about the chemistry. It's also consistent with my wife's swimsuit degradation experiences (indoor no CYA pool vs. outdoor CYA pool) and with corrosion issues in indoor pools and with the salt study (here). It is not standard industry practice and manufacturers would likely disagree since, according to them, only FC matters in "real pools" (according to the Pinellas County study discussed in this thread).

Richard