Taylor 2006 test kit experiment

[carli]

I took a sample in a plastic bottle from the pool water and experimented with the FC - CC test using the 25 and 10 mark.

(25 mark) 36 drops x 0.2 = 7.2 FC
(10 mark) 17 drops x 0.5 = 8.5 FC

On both test CC was zero because water stay clear didn't change to pink. why the results are different? They should
show the same result or not?

 

[Richard320]

I took a sample in a plastic bottle from the pool water and experimented with the FC - CC test using the 25 and 10 mark.

(25 mark) 36 drops x 0.2 = 7.2 FC
(10 mark) 17 drops x 0.5 = 8.5 FC

On both test CC was zero because water stay clear didn't change to pink. why the results are different? They should
show the same result or not?

In a perfect world, they should be the same. I can think of a few things that could throw things off.
1) The stamping on the side of the tube is not right. Maybe you tested 9ml and 24 ml. That 1 ml difference is a bigger percentage of the sample. So the testing error will be greater, too.
2) The amount of powder might not have been sufficient. There should be one or two undissolved crystals in there when you start testing.
3) You didn't swirl long enough. I know that using a speedstir, sometimes I get a color change on a test and a few seconds later it reverts.

 

[JamesW]

Also, the drops might not be consistent. When holding the bottle, it's important to hold it vertically to ensure consistent drops.

 

[JasonLion]

Those results agree within the precision of the tests. The error is +- one drop and +- an additional drop for every 10 drops you use in the titration. So the first result is +- 4 drops or +-0.8, while the second result is +- 2 drops or +- 1.0. That means the first result is 6.4-8.0, while the second result is 7.5-9.5. Those ranges overlap, so the results are considered compatible/equivalent/what ever you want to call it.

That said, if you do the test carefully, measure carefully and hold the dropper bottle vertically and let the drops drop off slowly, you should be able to get a little closer together than that, but don't expect them to match up exactly.

There isn't much point in doing the test by steps of 0.2. The additional precision is minimal, doesn't make any difference in the long run, and uses up your reagents more quickly.

Repeatability is actually more important than absolute precision. Repeatability means that when you get the same reading you know you are in the same situation you were in the last time you had that reading and treating the pool the same way from there should give you the same result.

 

[carli]

Thank everybody for your replies. The knowledge that you share here is very much appreciated.
Also if I misspell something let me know I am not use to write in English.