What does the math say???

[Hermskii]

Cut to the chase and just read the bottom paragraph or read all of this for details:

Here are my current water specs:

FC: 8.0 (This is my minimum/target level. I usually take it up to 11 each night and lose 3.0 FC over the next 24/hrs. leaving me back at my target level.)
CC: 0.0
pH: 7.6
TA: 100
CYA: 70
Salt: 3,200
Borate: 50
CH: 120

As stated in my signature, the pool is a Intex 18' x 48" round inflatable ring AG pool. See all the other pool info there too please. I just replaced the SWG a few days ago. The specs indicate the SWG can produce a max of 12 grams/hr. of chlorine output. The manual for it also indicates that I'm supposed to run it for 3 hours a day which I always have. The filter pump turns on. Then, the SWG turns on and runs for 3 hrs. and then it turns off and the filter pump turns off shortly after. I set it to boost today at about 10:00 AM. The FC was at 7.5 at that time. I checked the FC about 1 hour ago ( 7:00 PM), after had been running for about 9 hours. The FC was now at 10.0 FC. During this time the pool had 3 bathers and got rained on a few times by a sky that was overcast all day long. On a regular long, sunny and fun day in the pool, it seems to eat about 3.0 FC a day. How long to I have to run the little SWG at night at 12 grams per hour at best to produce 3.0 of FC?

I started at 7.5 FC this morning and I figure I lost 2.0 FC while my wife and kids played in the pool for much of this totally cloudy day. My math says that takes it from 7.5 FC down to 5.5 FC. Then my little SWG generated 4.5 over 9 hours to get it back up to 10.0 which is a net of 4.5 FC over a 9 hour period. That would indicate it makes 1.5 FC each 3/hrs. during the active swimmers and rainy day. Thus 3.0 FC would take 6 hours and not just 3 hrs. as the book indicates that I need for this little 5,455 gallon pool. Is that because it is summer and the book doesn't take temps into account? Temps don't matter anyways though right when it comes to how much the SWG generates? This model can only generate more by way of running longer. I'd rather not run my pump and filter 6 hours a day to get 3.0 FC added to my total FC. I thought I did everything I needed with CYA and borates and great water management to keep my pump and SWG times to a minimum. Am I missing something or does the new SWG just not work efficiently?

What is the formula? How long should it take a SWG producing 12 grams of chlorine an hour to raise the FC exactly 3.0 FC? That is what I'm wanting this one to do each night starting at about 8:00 PM.

 

[JasonLion]

While it is possible to make a rough approximation with math (try the chlorine gas setting on PoolMath), by far the best approach is to set the percentage somewhere, let it run for a while, test the level, and adjust the percentage as needed to get closer to your goal. You repeat that cycle a couple of times and narrow in on the correct adjustment.

With CYA at 70 the recommended FC level is between 3 and 5 and never below 3. I would aim for 5 just so you have some safety margin. Running with FC up at 8 to 11 uses up the SWG cell lifetime more quickly and increases the amount of PH drift you will experience.

 

[DogsHouse_MyPool]

I plugged your numbers into PoolMath and it says you need to run about 5.2 hrs to generate 2.2 oz CL @ 12 grams/hr (0.423288 oz/hr). These numbers seem to be more in line with what you are experiencing, rather than the chart in your manual.

I couldn't find the manual for your unit (12 grams/hr.), but I found a similar unit ( CS7111 - 5 grams/hr.) . I looked at their run-time estimate for your pool @ 5455 gals. The run-times for that unit and your pool appear to be low as well. I can only assume they are figuring far less CL loss during the day than you are seeing. Their chemistry levels are also somewhat different than what is recommended at this site. From the manual:

Preferred Water Chemistry Reading
Minimum Ideal Maximum
Copper Ions 0 0.1 - 0.2 ppm 0.2 ppm
Free Chlorine 0 0.5 - 3.0 ppm 5.0 ppm
Combined Chlorine 0 0 ppm 0.2 ppm
pH 7.2 7.4 - 7.6 7.8
Total Alkalinity 100 ppm 100 - 140 ppm 140 ppm
Calcium Hardness 150 ppm 200 - 400 ppm 500 - 1000 ppm
Stabilizer (Cyanuric Acid) 10 ppm 20 - 40 ppm 50 ppm

I can only assume that based on their recommended levels, they are letting the FC drop to 0 during the day, and only regenerating it up to 1.5-2ppm. With this 5gr/hr unit, it would take 6.25 hrs to generate 1.1 oz CL required to raise CL level from 0 to 1.5 ppm. That is exactly what they publish in the CS7111 manual, 6-7 hours run-time /day.

I would follow Jason's recommendation above and lower the CL to a minimum of 3, and generate what you need to keep the level above that minimum.

 

[Hermskii]

Beautiful DogsHouse_MyPool. I sent JasonLion a private message asking him for some info and he had suggested I post my question here and that someone like you would come along and post a likely good answer as you did.

I want to complete this topic so I dug up my owner's manual and pulled the same portion of info from mine. Here it is.

Power: 110 - 120 Volt AC
Amperage: 1.1A
Wattage: 125W
Ideal Salt Level: 3000 ppm (parts per million)
Maximum Chlorine Output/hour: 12 grams/hour
Copper Ionizer Output Current: 175 mA
Filter pump minimum flow rate: 700 ~ 4000 gallons/hour (2650 - 15140 liters/hour)
Limited Warranty: see “Limited Warranty”

I too went ahead and did the best I could using the PoolMath. Here is the data I came up with:

If I want my SWG to replenish the 3.0 FC I lose each day PoolMath dictates I need it to produce 2.2 oz. of chlorine gas. There are 28.3495 grams in 1 oz. of chlorine gas. If I multiply that by 2.2 oz.
it equals 62.3689 grams of Chlorine gas. I then need to divide that by the amount of chlorine my SWG can produce being 12 grams/hr. which is the same as saying 62.3689 divided by 12 = 5.19 hrs. hours.

Hahaha! Re-reading it, I see we came to the same conclusion pretty much! Awesome! Thank you for your time and effort into this. let me proceed and ask another question:

Using a little SWG like mine, is it harder for it to produce 3.0 of FC when increasing it from 2 to 5 or from 7 to 10? I ask because I was told my cells had to work harder the higher my FC was when they started. Several replies have suggested to bring my chlorine levels back down from 8 to the 3-5 area. I will continue to run the SWG for 3 hours and supplement the FC with Clorox back to 5.0 to save or extend the life of my SWG cells. Thank you very much all and I'm all about more suggestions if you have them! Thanks everybody!

 

[jblizzle]

Depends on your CYA level. It will protect a % of the FC. The higher the CYA, the higher the % protected. So at as given CYA, if you have a higher FC level, you will lose more ppm to the sun than at a lower FC level.

So, it would be harder to maintain 10ppm than 6ppm because more would be lost to the sun. The SWG would still produce the same amount, but would have to run longer maintain 10ppm.

Another way without a SWG, if you target 6ppm it may drop to 3ppm the next day when you add bleach. But if you target 10ppm it may drop to 6ppm by next day which is a higher amount that has to be added each day.

 

[Hermskii]

Whooop! There it is.

You're saying that losing 30% of 10 FC being 3 FC is harder for my SWG to replace than losing 30% of 5 being 1.5 FC, It all seems so simple now! Yep! My SWG would only have to work 1/2 as hard to replace 1.5 PPM of FC than 3.0 of FC.

Thank you!