Calculating FC from SWG output

[TreeFiter]

So I've been wondering just how much FC is produced by a typical SWG. I was able to find out that one of the SWGs we typically use produces 36 grams/hr. So if I convert that to ounces, I get about 1.3oz. If I plug this into the pool calculator, it says it equates to about 0.4 ppm FC. All of this is based on a 24,000 gallon pool (what I would consider "average" size) with CYA between 50 and 80 ppm. I'm assuming that 36 grams/hr is the maximum output. This seems kind of low. If a pool runs 12 hours/day, it would only generate 4.8 ppm FC in a day. Most of my pools run at 50% or less. So that would mean only 2.4 ppm FC is added in a day. So in order for these pools to maintain FC levels, they must only be losing 2.4 ppm or less.

Is my math wrong, or do pools really only lose about 2 ppm FC in a day?

 

[aussieta]

my cell is 25 grams per hour at 100% in 13,000 gallon pool
2 hours at 100% equals 1ppm free chlorine for me
1 run 8 hours at 100% in the middle of summer
equals 4 ppm that is with lots of use and unfortunately the dog gets in to
on your equation i would suggest the cell is too small for the pool

 

[jblizzle]

Your math sounds correct. My t15 puts out 1.4 lbs per day.

A properly clean pool should normally only lose 2-4ppm each day.

 

[TreeFiter]

I suppose I could test this out using the same principle behind the OCLT. Once the sun goes down, there should be no FC loss, so if I were to run the SWG overnight, I could find out how much the FC increased overnight, then divide it by how many hours it ran for to get the rate of FC/hr. Now I just need to figure out what pool I can try this on.

- - - Updated - - -

Your math sounds correct. My t15 puts out 1.4 lbs per day.

A properly clean pool should normally only lose 2-4ppm each day.

Did you mean to say 1.4 oz?

 

[jblizzle]

No. The SWGs are rated in terms of pounds of chlorine gas.

Also, the OCLT could acceptably have a FC loss of 1ppm, so that is going to skew your test.

 

[timerguy]

My circupool is rated at 2lbs which I've found to be pretty accurate I've developed a excel spreadsheet to determine approximately how much FC will be generated for 1-100 percent based on number of hours running time. Anyone interested in getting a copy of it just pm me with your email addy and I'll send it to you.

 

[TreeFiter]

Hmmm...lbs/day makes sense, but the brochure for the Compupool Infinity series gave it as grams/hr. 36 grams/hr is a little over an ounce in 12 hrs, so it would be somewhere between 2 and 3 oz in a day. The brochure says the system is rated for something like up to 150,000 gallons. Maybe its a typo. I did find where the old systems were rated in lbs. The old systems we put on most of our pools, which typically range from 22,000 to 50,000 gallons are rated at 2.5lb.

 

[timerguy]

I doubt it'd be too difficult to convert grams to ozs to use in my spreadsheet if you're still interested. Just pm me your email addy.

 

[chem geek]

Since 24,000 gallons is 90,850 liters, then (36 g/hr) * (1000 mg/g) / (90,850 liters) = 0.4 g/hr (ppm FC per hour) as you had calculated. Over an 8-hour day that's 3.2 ppm FC. In 150,000 gallons that would only be 0.5 ppm FC over 8 hours so too undersized during the day. They may be figuring running 24 hours for 1.5 ppm FC but that's still pretty slim and unlikely to keep up.

 

[TreeFiter]

Since 24,000 gallons is 90,850 liters, then (36 g/hr) * (1000 mg/g) / (90,850 liters) = 0.4 g/hr (ppm FC per hour) as you had calculated. Over an 8-hour day that's 3.2 ppm FC. In 150,000 gallons that would only be 0.5 ppm FC over 8 hours so too undersized during the day. They may be figuring running 24 hours for 1.5 ppm FC but that's still pretty slim and unlikely to keep up.

I went back and looked at the brochure, and I realized its all in metric. It kind of makes sense that if they are giving grams/hr the volume would be in Liters. So it was 150,000 liters, which is somewhere around 40,000 gallons.

 

[mas985]

36 grams/hr = 1.9 lbs/24 hr so fairly typical output for a SWG.

 

[chem geek]

And 150,000 liters and 36 g/hr would be 1.9 ppm FC over 8 hours so that sounds about right for a barely sized SWG which is why we recommend getting one with a volume claim that is 2-3 times your actual pool volume.

 

[timerguy]

Does anyone make a swg that outputs over 2lbs in a 24hr period? Compupool says theirs is 2.5, but I've not heard of anyone getting that kind of production. Even IC 60s are rated at 2.0 lbs/day

 

[chem geek]

The AutoPilot Pool Pilot Professional strings up to 6 cells together for 15 pounds per day but that's still 2.5 pounds/day per cell at around $2000 per cell in a 6 cell system. The Saline Generating Systems (SGS) SG 5000 outputs 2.3 pounds per day for around $1500. The Hayward Saline C 6.0 Commercial Salt Chlorine Generator outputs 6 pounds per day but it's around $4000 (discounted).

 

[timerguy]

Thanks, was just wondering. That said, how can the IC 60 be rated for up to 60000 gal pools if the chlorine production is only 2lb/ 24 hr day? I guess technically, 4ppm every 24 hrs is sufficient?

 

[chem geek]

Over 8 hours that would be 1.33 ppm FC and no that is not normally sufficient. The ratings that the SWG manufacturers give are barely on the edge and really would only be possible if one were using supplemental algaecide or phosphate removers. This is why we suggest oversizing the SWG by a factor of 2-3. It's also more economical since the price is not proportional to SWG output, at least for the residential units. So the IC60 would be more appropriate for a pool up to 30000 gallons and would be economical for a 20000 gallon pool since the cell will last longer compared to a unit rated for up to 40000 gallons.