Rule of thumb on CYA drop due to dilution by rain?

[jmattdoss4]

Just wondering...is there a formula or rule of thumb for estimating the amount of CYA drop due to rain. (I realize that an actual test is the only way to be sure.) At last check my CYA level was around 65-70 (checking weekly since opening and adding each time to get to 70). We have had about 4inches of rain in the last 7 - 10 days, so I'm thinking the level has dropped due to dilution and having to pump off about 2", but since my test kit has a limited number of CYA tests, I'm trying to estimate CYA without having to recheck every time it rains. Any thoughts?
Thanks!

 

[Richard320]

Figure out your average depth.

Say it's 60". You drained off 2". So you have 58" of the old water left. ⁵⁸/₆₀ * 70 CYA = 67.7 Not even enough to necessarily catch with the tester.

 

[jtaves85mcss]

Interesting thought, and I will play along for a minute.

Please note that your CYA is not something you should be worried about testing weekly. Remember the testing error is something near plus or minus 15 ppm.

The dilution by rain all depends on your pool size and shape, but an inch or two of rain is really nothing in the total. If you think about it really hard, in a 16x36x 5ft deep pool which is 21000 total, 1 inch of rain is about 400 gallons. 400/21000 = .019 -1 = .98 of the total volume. 70 x .98 = 68.7 Thus you have basically the same answer as Richard provided above. So lets say you pump out the 1 inch of rain = 1 ppm CYA loss. Given that you have could have rain replacement of 10 to 15 ppm in a season. You may also loose 10 ppm over a season due to degradation. With this scenario you could plan on adding 25 ppm over a 3-4 month season.

Pool math tells us that we need 70 ounces of CYA to raise the total by 25 ppm, so you could simply add 23 ounces of stabilizer each month if everything goes as expected.

 

[chem geek]

Also note that there isn't any rain dilution if the rain doesn't overflow the pool water. If it just fills up the pool and then evaporation lowers it again there will be no net change. In your case, the dilution is only the 2" you removed, not the 4" the rain added since presumably the other 2" will drop from evaporation.

 

[jmattdoss4]

You guys are awesome. Thanks.