# Thread: Trouble with SWG Math

1. ## Trouble with SWG Math

Having a hard time figuring this out........

My SWG claims a production rate of 1.38 lbs. per day. ( I assume this means when the dial is set to 100%)

I've had good success running my pump 10 hours per day with the SWG set at 50%. So I did some math.......

1.38 lbs./day = 22 oz./day of chlorine gas (this is the term used in the PoolMath calculator)

So, according to PoolMath, 22 oz. Chlorine gas/day should raise FC by 7ppm each day if I run the unit at 100%

Now, back to what I actually do....run 10 hours at 50% output.

10 hours @ 100% should produce about 9.2 oz of chlorine gas
Therefor 10 hours @ 50% should produce 4.6 oz of chlorine gas

According to PoolMath, 4.6 oz of chlorine gas should raise the FC by a little less than 1.5ppm.

However, I'm pretty sure I have more than 1.5 FC loss each day as I measured it (without the SWG turned on) to be closer to 2.5 - 3.0 FC loss per day. But the 50% setting seems to be maintaining my FC at around 7.0ppm over the last several days.

Can anyone point out where my math or any assumptions I use are incorrect?

2. ## Re: Trouble with SWG Math

Nothing wrong with the calculations.

So either the pool is much smaller than you think, your SWG is bigger than it says it is, or poolmath has an error somewhere.

When you add other chemicals, does poolmath figure things correctly? If you want to be dead certain of the volume: A Chemical Way of Calculating Pool Volume

3. ## Re: Trouble with SWG Math

Hi Run!
Re:
However, I'm pretty sure I have more than 1.5 FC loss each day as I measured it (without the SWG turned on) to be closer to 2.5 - 3.0 FC loss per day. But the 50% setting seems to be maintaining my FC at around 7.0ppm over the last several days.
You're using the same math I've been using to dial in/ down my new swg and you're experiencing similar results.

I seem to only be "using" about 1.5 ppm per day whereas before swg I was def more like an average of 2.5 with many days closer to 3.

I was debating earlier today to myself that this either meant my math was bad, OR maybe the beauty of constant chlorination is that with the higher cya and lower "dose" levels, much less chlorine is "wasted" via UV exhaustion.

I'm hopeful the latter is true...and it makes sense to me.

4. ## Re: Trouble with SWG Math

Daily FC loss rate can look a lot like the stock market Dow Jones data - one day it's up, one day it's down then back up again, etc, etc. it's the trends and averages that are important. Bather load, temperature and sunlight all have a part to play. In a clean, unused pool with high CYA and the standard TFP recommended levels for an SWG, FC loss can be as low as 1ppm/day. Throw in some wind-blown foreign matter and a few kids and that loss will increase.

Also, as Swampwomam points out, the consistent dosing of FC and oxidation caused by an SWG cell tends to show lower daily FC loss than the fluctuating chlorine levels found in a manually dosed pool.

5. ## Re: Trouble with SWG Math

It is true that the higher the chlorine level the faster the rate of decay. One of the reasons why we don't recommend going over shock level during a SLAM. Was your CYA at the same level for the 2.5 ppm use vs the 1.5 ppm rate?

6. ## Re: Trouble with SWG Math

Originally Posted by pooldv
It is true that the higher the chlorine level the faster the rate of decay. One of the reasons why we don't recommend going over shock level during a SLAM. Was your CYA at the same level for the 2.5 ppm use vs the 1.5 ppm rate?
Yes.....CYA has been 45 throughout. When I opened, I slammed so kept the CYA where it was from over the winter. Now that I've passed all 3 SLAM tests (my annual opening ritual) I will start to bump CYA to about 70-80.

7. ## Re: Trouble with SWG Math

Raising CYA will further reduce your daily chlorine loss. It just keeps getting better!

8. ## Re: Trouble with SWG Math

Originally Posted by pooldv
Raising CYA will further reduce your daily chlorine loss. It just keeps getting better!
Yes

Will be doing that this weekend

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