Testing Margin of Error

[Kjp300]

Around here it has been rainy, cloudy and cool for the past 4-5 days. With the kids back in school and the weather, the pool has not gotten any use and I have mostly left my cover on. With the SWG, my FC has drifted up. I tested on Wednesday, 09/12/18 at 5:30pm and FC was 12.5. I opened the cover and left it off for a few hours and shut off the SWG. I closed the cover and it was closed all day yesterday with the SWG off. I tested again, yesterday, 09/13/2018, at 8:30pm and my FC was at 13.5. Both tests read CC of 0 and my CYA is 40-50. (Could see the dot at 50 and could not see it at 40). Water is Chrystal clear. This is obviously some sort of testing error. What is the margin of error for a FC test? Not worried now, but would be a little concerned as I let my FC drift to the lower end of the recommended amount. Thanks!

 

[Dirk]

Margin of error for the FC test is about 10%.

Some possible sources of test errors:

Are you circulating your pool well before testing?

Are you sampling the water about 18" down? (Elbow in the water?)

Are you testing the same time each day?

Are you sampling from the same spot in your pool each time?

Are you sampling well away from the returns?

Are you rinsing everything thoroughly in both fresh water and then again with your pool water before pulling your sample and before testing?

Are you holding the reagent straight up vertically to form consistent sized drops?

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