# Thread: scale factor for chloring gas to calculate PPM

1. ## scale factor for chloring gas to calculate PPM

I'm creating a table (for "fun") using excel to calculate the amount of FC increase for various pump run times and % levels of my SWG. Treating 1 oz chlorine gas "like" 1 oz of water volume, my numbers were close, but not exact to those of the pool calculator. My SWG puts out 32 oz of chlorine in 24 hours according to the manual. Is there a scale factor I don't know about? Here is my spreadsheet.

2. ## Re: scale factor for chloring gas to calculate PPM

Use the Effects of Adding Chemicals section of PoolMath. Enter 32 oz of chlorine gas and that is how many ppm of FC you could expect running at 100% for 24 hours.

3. ## Re: scale factor for chloring gas to calculate PPM

That's what I've been doing. The purpose of the spreadsheet is to avoid having to refigure ratios every time I decide to change the SWG setting or pump run time. For example, I've been running at 100% for 4 hours for the last few weeks, but I've been gaining gradually and I'm now at 9.5 PPM of FC. Not crazy high considering my CYA is >100 PPM due to goof on my part. (I'm not worried about an algae bloom, though, and it seems to actually be decreasing overall FC consumption as compared to last year, when 100% SWG for 4 hours/day was just about right on.) Anyway, I dialed the SWG back to 90%. It's easy enough to calculate the ratio every time I do this, but the pool calculator values are rounded to the nearest 0.1 anyway, so I'm calculating a fraction of an already rounded number. Hope this all makes sense.

In short, I'm well aware of pool calculator and have used it quite frequently, but I'm searching for more convenience and precision with my personal spreadsheet. Most folks just want to see pretty, clear water. For a math nerd, though, it's fun to calculate average daily FC consumption, which I can't do very accurately (or precisely) unless my input values are accurate (and precise). I didn't share my full-blown spreadsheet, but only the sheet I just added.

4. ## Re: scale factor for chloring gas to calculate PPM

I do not follow what you do not follow.
With the PoolMath calculation I just recommended. Take that number as the max and then scale it down for % setting and fraction of 24 hours.

PoolMath says 13ppm at 100% for 24 hours.
So: 13 * % * hours/24 = ppm at that % and number of hours in a day

If you are looking for more precision on the 13ppm number, then you will have to dig into the HTML code of the calculator to see how it arrived at that number.

Although given the uncertainty of the pool volume itself ... I see no reason to try to be more precise in the ppm number

5. ## Re: scale factor for chloring gas to calculate PPM

You can use this formula which is simplified.

ppm = lbs of chlorine gas / lbs of water = 2 / (18000*8.34) = 13.32 ppm (100% at 24 hours)

You can then scale that number by hours and percentage as JB mentioned.

But all these estimates depend on the pool volume which is likely an estimate as well. Also, the output of the SWG is an estimate too. The actual output is dependent on salt content and water temperature. The output quoted is usually at a specific water temperature and salt level.

6. ## Re: scale factor for chloring gas to calculate PPM

Like I said, I understand how to calculate that ratio of 13 ppm, but that 13 ppm is only precise to the nearest whole number, because Pool calculator rounds all results greater than 10 to the nearest whole number. My spreadsheet comes up with 13.9 instead of 13, which tells me there must be some scale factor I am not aware of, which I am guessing involves chemistry, which is why I put the post here.

7. ## Re: scale factor for chloring gas to calculate PPM

Mas985 posted while I was writing my last post, so my last post was not taking into account the reply from mas985, which contains great information about additional variables, as well as the scale factor I was looking for, which is 8.34 oz./8 fl. oz. I was considering a gallon of water to be 8 pounds. Should have googled it! Thanks!

8. ## Re: scale factor for chloring gas to calculate PPM

I know nothing about pools, but if you have salt in your pool, the weight is higher than fresh, which is 8.34 ppg. Sounds like you are a precise person... so I thought I'd throw that in.

9. ## Re: scale factor for chloring gas to calculate PPM

While true, that doesn't change the ppm formula. In the formula posted above there is an implicit 1 g/ml of water density which divides the 8.34 lbs/gal factor which is also a density. So if you were to adjust the 8.34 for the added weight of the salt, the 1 g/ml density goes up by the same percentage so the ppm ends up being exactly the same. The full formula is as follows:

ppm = g/ml = lbs of chlorine gas * 453.592 g/lbs / (gallons water * 8.34 lbs/gallon * 453.592 g/lbs / (1 g/ml))

At 3200 ppm of salt, the formula would become

ppm = lbs of chlorine gas * 453.592 g/lbs / (gallons (water + salt) * 8.36 lbs/gallon * 453.592 g/lbs / (1.0024 g/ml))

8.36 / 1.0024 = 8.34 so it results in exactly the same ppm.

10. ## Re: scale factor for chloring gas to calculate PPM

If you want to get more accuracy from the pool math, multiply the input number by 100. For example, if you want to know what 1 oz will do, put 100 oz in the box and then divide the answer by 100.

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