Why higher FC?

[JamieP]

Both. A jug of 8.25% bleach adds 4 ppm FC to my pool. I always add a jug before and after. And if there are lots of kids make them take a potty break and test during the day and another jug sometimes. My pool is at 15.5 ppm right now with 80 CYA, just about perfect for lots of sun and swim!

Remember chlorine is your friend. Chlorine level below minimum at any time during the day leaves your pool unsanitary with increased risk of person to person disease transmission. It is safe to swim up to shock level for your CYA, Chlorine CYA Chart

I'm curious since our pool is new this season and I'm just getting the hang of dialing in the SWG... Why do you choose to keep your FC so high for a CYA of 80? I thought the target range was 6-11 with a minimum of 4. Do you keep it that high all the time or are you expecting lots of people? Is having it higher just precautionary? Do you notice any increased chlorine smell on suits and towels?

 

[pooldv]

No, I normally keep my FC at 8 to 9. Usually when we have folks coming over for a big swim weekend I will add a jug of bleach which adds 4 ppm FC to my pool. Then I add another after. My FC is higher than normal right now because we added a solar cover a couple of weeks ago and it has reduced FC consumption in the pool and it crept up. Since we spent a lot of today in the pool and there are a herd of folks coming over tomorrow I am happy for the extra FC.

Last summer we had to do a little SLAM and we swam every day with FC at 28 ppm and nobody noticed. CYA does a great job of buffering the harshness of chlorine.

The way I manage my SWG is to dial it in to maintain FC at slightly above target and then use bleach to cover any additional needs. This time of year is challenging as temps ramp up and and swimming ramps up FC need goes up. When I test FC and it drops down to 6 I add a jug of bleach and increase SWG run time or percentage.

I always keep 3 or 4 jugs of bleach around and add them anytime I am nervous about FC dropping below target. One SLAM is enough for me!

 

[JoyfulNoise]

Jamie,

My suggestion to you is to do careful measurements of your FC. For a CYA of 80ppm, your minimum FC is 4ppm. You need to exceed that minimum by the amount of daily FC loss you get. So if your pool is super clean and you only lose 2ppm FC per day, then you really don't need more than 6ppm. It takes some careful monitoring in the beginning to sort it all out but your pool's pattern will emerge and you'll practically be able to guess your FC everyday and come within 0.5ppm. Everyone's pool is different so don't feel like you're doing something wrong if your parameters are a little different than others.

As others have said - we're not mixing up rocket-fuel here, it's just pool water...

 

[JamieP]

No, I normally keep my FC at 8 to 9. Usually when we have folks coming over for a big swim weekend I will add a jug of bleach which adds 4 ppm FC to my pool. Then I add another after. My FC is higher than normal right now because we added a solar cover a couple of weeks ago and it has reduced FC consumption in the pool and it crept up. Since we spent a lot of today in the pool and there are a herd of folks coming over tomorrow I am happy for the extra FC.

Last summer we had to do a little SLAM and we swam every day with FC at 28 ppm and nobody noticed. CYA does a great job of buffering the harshness of chlorine.

The way I manage my SWG is to dial it in to maintain FC at slightly above target and then use bleach to cover any additional needs. This time of year is challenging as temps ramp up and and swimming ramps up FC need goes up. When I test FC and it drops down to 6 I add a jug of bleach and increase SWG run time or percentage.

I always keep 3 or 4 jugs of bleach around and add them anytime I am nervous about FC dropping below target. One SLAM is enough for me!

Thanks for the explanation. Another question I have is what I should expect my FC to be in relation to my target each night when I test if my SWG is dialed in properly?

I.e., is the goal with a SWG to have the FC at target levels or above each night I test and this would show that my run times/percentages are correct? If I were manually dosing, I would expect the FC to be lower at night (but still above minimum) and I'd bump it up again with bleach to be ready for the next day. But the SWG doses throughout the day so I probably don't want to see an FC drop by the end of the day, right?

- - - Updated - - -

Jamie,

My suggestion to you is to do careful measurements of your FC. For a CYA of 80ppm, your minimum FC is 4ppm. You need to exceed that minimum by the amount of daily FC loss you get. So if your pool is super clean and you only lose 2ppm FC per day, then you really don't need more than 6ppm. It takes some careful monitoring in the beginning to sort it all out but your pool's pattern will emerge and you'll practically be able to guess your FC everyday and come within 0.5ppm. Everyone's pool is different so don't feel like you're doing something wrong if your parameters are a little different than others.

As others have said - we're not mixing up rocket-fuel here, it's just pool water...

Thanks, Matt. It's nice to have your reassurance. This is probably a corollary to my questions above, but how do you figure out daily FC loss if the SWG is running throughout the day to make up for it?

 

[JamesW]

The goal is to always maintain the fc at, or above, the minimum. Typically, the target is a little higher to give some buffer. As long as the fc never goes below the minimum, you’re good.

 

[JoyfulNoise]

Jamie,

Your IC60 theoretically creates 2lbs of chlorine gas per day. In your 28,000 gallon pool, that equates to 8.6ppm FC per day or 0.358 ppm per hour.

So, once you've dialed in your SWG so that you get a constant FC every time you measure, then you know what your FC loss per day is because you know how many hours per day you are running your SWG and at what percentage output. So, if you measure your FC everyday at 5pm and you always get 5.5ppm and your IC60 ran for 6 hours at 75% output, then -

0.358 X 6 X 0.75 = 1.6ppm FC added

If your FC has not changed from the day before, then you added as much FC to it as the day consumed.

As I said, just keep some good records of FC and SWG runtime/output and the pattern will emerge.

 

[cowboycasey]

I measure same time every night and make sure I am at 7FC, if it is lower I will add run time or % SWG if over 8 I will turn down.. I also keep 4 gallons around just in case

 

[pooldv]

If your SWG runs most of the day and is off at night then theoretically it will be highest in the evening when the SWG shuts off and lowest in the morning when it turns on. The goal is for it always be higher than minimum, especially when it turns back on.

And don't you have a herd of kids? That is a lot of potential variation in chlorine demand, 2 or 3 kids for 2 or 3 hours or 5 or 6 kids for 6 or 7 hours or no kids because they might now swim every day. I don't know what the difference would be but could be a couple of PPM from one day to the next.

My theory is a little more from an extra gallon of bleach now is better than a SLAM needing 20 gallons of bleach later. I didn't use to think that until I had to SLAM last summer. I think around here most people only SLAM once and then they pay a lot more attention to their chlorine level! :-D

And there is no downside other than a few bucks for some bleach or a couple of months off the 5 or 6 year life span of your SWG. And with the perfect water balance you have from TFP you will gain more than that anyway!

 

[JoyfulNoise]

Bather load is not really an issue for swimming pools. A rough rule of thumb is that every person-hour of bather load requires about 4 grams (0.14 oz) of chlorine to oxidize. So 10 people swimming in a 28,000 gal pool for 1 hour would reduce the FC by 0.4ppm. And most of her "people" are kids.

The bigger issue is if one her kids urinates in the pool. Urine (mostly urea) requires a much higher dose of chlorine to neutralize it and that will cause huge FC demand.

So Jamie, teach your kids from the get-go that peeing in the pool is not allowed and that you'll be testing the water everyday. You'll know if someone is relieving themselves in the pool and pool privileges will be revoked from all if the no-pool-potty rule is broken. Snitching on those that violate the rule will be rewarded with candy. Nothing like a little familial peer-pressure to keep everyone in line....

 

[pooldv]

Mmmm, candy! LOL! Definite +1 on testing mid day and end of day if you think there is any chance of pee in the pool. It can take your FC to 0 in hurry. We enforce mandatory potty breaks for all younger kids who we don't know really well.

Interesting, I haven't seen much discussion on bather load oxidation for a pool. So, 10 people 6 hours would be 2.4 ppm more than yesterday with no swimming. What about dirty kids? Run around, play in dirt, get sweaty and then jump in the pool? Another 1 or 2 ppm? We all think kids are dirtier than adults (or is it just me?), but maybe not. We don't use sunscreen but what if you lather them up a couple of times?

 

[JamieP]

Thanks everyone. This gives me a better understanding of what I'm working towards. I am keeping good records, so it shouldn't be long until I get it all worked out. I have a better game plan now that the goal is clear.