how long will chlorine last under cover?

[gtnos]

probably going to close my pool weekend of the 27th. by closing, i mean one final vacuuming, assemble my dome support and cover, draining down to below the water outlet and skimmer (i have cover for inlet i will place on it later), i will still add bleach daily and keep pump running a few hours a day for a week or so until temp is below 60, at which time i plan on making sure everything is in line with chemistry, bringing pool to shock level (probably a little higher) before covering inlet, pulling pump and 6 way valve and covering sand filter.

how long will chlorine last under the winter cover? i figure with no sunlight, covered and a high level, i should have quite a bit of chlorine left even after a week? :?

just curious... thanks!

 

[pwrstrk]

You might be good all winter. As the water temp falls so does the FC loss. Wait as long as you can to bring it to shock level, then your water temp will be really low.

Plus like you said it will see no sun.

 

[chem geek]

For my pool with a mostly opaque cover the chlorine loss rate at 50ºF is roughly 1 ppm FC every 14 days or around 1.7% loss per day. One would get from a shock level that is a 40% FC/CYA ratio down to a regular 7.5% FC/CYA ratio after (1-1.7/100%)^days = 7.5/40 gives us 98 days or a little over 3 months. That's the theory, anyway. One can also use Polyquat 60 which will last longer than the chlorine and will help prevent algae growth. See this post for closing instructions by shocking with chlorine, letting the level drop somewhat, then using Polyquat 60.

 

[Pauls234]

I closed sometime in October last year after bringing it up to shock level, opened a week or so before Memorial Day this year and tested around 4ppm IIRC. Of course, my pool was frozen solid for close to half that time.

 

[gtnos]

For my pool with a mostly opaque cover the chlorine loss rate at 50ºF is roughly 1 ppm FC every 14 days or around 1.7% loss per day. One would get from a shock level that is a 40% FC/CYA ratio down to a regular 7.5% FC/CYA ratio after (1-1.7/100%)^days = 7.5/40 gives us 98 days or a little over 3 months. That's the theory, anyway. One can also use Polyquat 60 which will last longer than the chlorine and will help prevent algae growth. See this post for closing instructions by shocking with chlorine, letting the level drop somewhat, then using Polyquat 60.

as usual chem geek... awesome answer! Thanks!!!!

another question... *roughly* how many ppm can i bring my FC up to without bleaching liner or hurting any equipment? i thought about tossing in 4 jugs of 8%... which would bring me up to about 50ppm.... that should last a long time.. but will it hurt pool (CYA is about 90, a little high i know, ill take care of that next spring )

 

[chem geek]

The active chlorine level is lower at lower temperature given the same FC/CYA ratio. 50 ppm FC with 90 ppm CYA at 50ºF has about 6 times the active chlorine as 7 ppm FC with 90 ppm CYA at 80ºF. However, the lower temperature has chlorine reaction rates be around 5 times slower so overall the rate of vinyl deterioration is about the same as during the summer season. On the other hand, if you were not to use as high a chlorine level, you'd have a lower rate of degradation compared to the summer season.

Personally, I wouldn't go above shock level and I'd instead use the Polyquat 60 as your backup. The most important factor is to close with the water as cold as reasonably possible and open before it warms up too much.

 

[gtnos]

The active chlorine level is lower at lower temperature given the same FC/CYA ratio. 50 ppm FC with 90 ppm CYA at 50ºF has about 6 times the active chlorine as 7 ppm FC with 90 ppm CYA at 80ºF. However, the lower temperature has chlorine reaction rates be around 5 times slower so overall the rate of vinyl deterioration is about the same as during the summer season. On the other hand, if you were not to use as high a chlorine level, you'd have a lower rate of degradation compared to the summer season.

Personally, I wouldn't go above shock level and I'd instead use the Polyquat 60 as your backup. The most important factor is to close with the water as cold as reasonably possible and open before it warms up too much.

thanks chem geek!