shocking with chlorine versus potassium monopersulfate

MikeInTN

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May 27, 2007
1,335
Middle Tennessee
#1
Hey gang,

I had to shock my pool the other day (con of the Chlorease SWCG - it doesn't superchlorinate near as well as the inline units), and it got me to thinking about what exactly is the highest "safe" level of chlorine that I can have in my pool and still swim, how long it would take for the shock levels of chlorine to degrade to this safe level, and if I wouldn't be better off using potassium monopersulfate to shock the pool with, as it dissipates fairly quickly? My CYA is between 70-80, and my pool is in direct sunlight for 95% of the day.

TIA,
Mike
 

NWMNMom

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Apr 8, 2007
1,582
Waaay NW MN
#2
I believe you really don't want to go too much over 5, but 3 is the desired level. You have pretty high CYA so have to have that higher FC level to get the sanitation right too. In full sun, I'm guessing it can dissapate fairly rapidly. I think there are actual calculations involved here - I don't do math or windows....
 

JasonLion

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May 7, 2007
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Silver Spring, MD
#3
I have run into two schools of thought about safe to swim chlorine levels. One group says use the max column from Ben's best guess chart, 10 for this pool, and the other group says that you can go much higher than that safely.

With a CYA level of 70-80 you want to keep the FC level between 5 and 10 normally. The old school don't swim above 3 recommendations are from before pools had CYA in them. CYA greatly reduces the activity level of chlorine.
 

Rangeball

Well-known member
May 25, 2007
785
#4
JasonLion said:
I have run into two schools of thought about safe to swim chlorine levels. One group says use the max column from Ben's best guess chart, 10 for this pool, and the other group says that you can go much higher than that safely.

With a CYA level of 70-80 you want to keep the FC level between 5 and 10 normally. The old school don't swim above 3 recommendations are from before pools had CYA in them. CYA greatly reduces the activity level of chlorine.
Jason, I thought SWG pools with higher CYA ran traditional 1-3 ppm chlorine and not the higher amounts recommended by the BG chart?
 

JasonLion

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May 7, 2007
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Silver Spring, MD
#5
Hum, catch me not paying attention eh. It was early in the morning :)

Rangeball is right, a goal of 2 or 3 for FC should be fine with a SWG for normal operation. Some people say 1 is alright as well.

At the other end of the scale, there isn't any problem with swiming as high as 10 given your CYA level and some people swim higher than that with no obvious problems.

By the by, no SWG is particuarly good for shocking a pool. When shocking you want the chlorine level to climb rapidly. A SWG can't do that, it will take at least hours and possibly days to get up to shock level. When shocking you can use bleach to get the initial shock and then put the SWG on boost to try and hold the FC level. Or you can do it all with bleach to save on cell life.
 
G
#6
MikeInTN said:
Hey gang,

I had to shock my pool the other day (con of the Chlorease SWCG - it doesn't superchlorinate near as well as the inline units), and it got me to thinking about what exactly is the highest "safe" level of chlorine that I can have in my pool and still swim, how long it would take for the shock levels of chlorine to degrade to this safe level, and if I wouldn't be better off using potassium monopersulfate to shock the pool with, as it dissipates fairly quickly? My CYA is between 70-80, and my pool is in direct sunlight for 95% of the day.

TIA,
Mike
MPS does not 'shock' the pool in the same way as chlorine does. It will not beak down cloramines already in the water but if a residual is kept in the water at all times by weekly addition it helps to oxidize ammonia before it can combine with the chlorine to form chloramines. Downside is that the MPS will test as CC unless you get the special reagent from Taylor or LaMotte (depending on whose test kits you are using) to remove the interferance. Or just add the MPS weekly and ignore any CC since it's probably just the residual MPS in the water you are reading.
 

MikeInTN

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May 27, 2007
1,335
Middle Tennessee
#7
Thank you to all that replied! :)

Jason - I thought the inline cell SWCG's helped keep the CC's down in a pool by basically superchlorinating the water as it flowed through the cell, so the need to shock is greatly reduced. The Chlorease unit I have hangs over the side of my pool, so I don't get the benefit of having most of my water flow through the cell as one does with an inline cell. As soon as the price drops a bit more on the inline systems, I'm sure I'll go that route. Thanks for the info on the max chlorine levels. I've been shocking during the first of the week when needed so that the chlorine levels will be back down to safe levels by the weekend, and just wanted to make sure what "safe" was.
 

JasonLion

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May 7, 2007
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Silver Spring, MD
#8
Yes, an inline salt cell will superchlorinate a small portion of the water as it passes through the cell. Not all of the water will be shocked, only the water that passes very close to the plates. The Chlorease must have the same thing happening to some extent. There has to be some flow past the unit to distribute the chlorine.

It is very rare that you need to shock an outdoor pool, even without a SWG, if you have been diligent about maintaining your chlorine level. You only really need to shock if the CC level gets above 0.5, which won't normally happen if you keep the FC level up and there is some direct sunlight.