Recurrent algae...chlorine question

[ddas]

I slammed the pool a few weeks ago after an algae outbreak. Thought I got it all, but now the algae is re-blooming, so I'm starting a new slam. ARGH.

This time I'm trying to follow the slam instructions to the T. I have a SWG and my CYA is around 80. According to the chart, the shock level is 31. I've never put the chlorine so high ever. Plus -- I wonder whether this is mustard algae (it's a burnt orange color when I brush my pool) which PoolMath says I should go up to 44 after the slam is done.

I've just never let the chlorine go that high. Is it dangerous for my pool at all? (Note: no one is swimming in the pool yet; the water's too cold. So I'm not worried about human interaction. I'm more worried about the equipment.)

I'm going to do my best to follow the slam instructions very carefully this time so it can be dealt with once and for all.

I'm using liquid chlorine. I don't mind the inconvenience of lugging gallons of it around. But feel free to advise me if there's a better solution in my case.

This morning I raised the FC from 4 to 25 as a starter step. I was just nervous about adding significantly more, since I've never done it before and it makes me nervous to be adding so much. Can someone make me feel a little better about what I have to do?

 

[JasonLion]

With CYA around 80, raising FC into the 30s is fine. CYA reduces the activity level of chlorine, so you need to use more to get the same effect. With CYA around 80 and FC at 44 you have a way lower chlorine activity level than you would in the typical indoor public pool with CYA at 0 and FC at 2.

 

[JVTrain]

When you test for FC, you're testing for all the FC in the sample, even what is bound to CYA if it weren't being chemically changed by the testing. You measure 30 FC, but the actual FC ppm in the pool that is not bound to CYA is much less than 1 ppm at that high CYA.

This chart and data may be confusing to you if you're not used to looking at this kind of data, but at a CYA of 80 (just look at the 70 line on the first chart, close enough for discussion), and 30 ppm measured FC, your actual effective HOCl- concentration, what's actually free and unbound to CYA in your pool is about 0.5 ppm. Like Jason's example that's 1/4 the amount of chlorine activity of many indoor commercial/public pools with 0 CYA, 2 FC. See the first chart on this page: http://www.troublefreepool.com/threads/558-Pool-Water-Chemistry

 

[ddas]

Yeah, the chart is a little over my head, but I can tell from the way both of you are writing that it sounds like it's not a serious concern.

OK, boosting FC to 31-35 now, and when the slam is done, I'll raise it to 44 before letting it drift back down naturally. Thanks for the valium.