K-2006 - Inconsistent FC/CC results on subsequent tests.

[lalittle]

I've been using a Taylor K-2006 kit for several weeks now, and I just noticed that I get consistent results when I do multiple FC/CC tests even though I'm doing the tests right after each other, and I'm using the same spot in the pool for the water (which is as far out and as deep as I can reach in the mid-deep area. I would ignore a 0.1 or 0.2 ppm difference, but I'm seeing a full ppm difference sometimes. I'm following all the instructions carefully, and everything seems "normal" other than the fact that one test will indicate (for example) 2.2 ppm FC, a second test 3.0 ppm, and a third 3.2 ppm. I'm also noticing a less dramatic (yet still somewhat troubling) difference in CC -- anywhere from 0 to 0.6 ppm or so on any given test.

Is this normal? What could explain this?

Thanks,

Larry

 

[257WbyMag]

Welcome to TFP!

These variances could be caused by a couple of different things. You could be having differences in the drop size of the reagents due to static electricity build-up on the tip of the dropper. This can be remedied by wiping the tip of the dropper with a moist cloth or paper towel prior to testing.

You can also get variable results by innacuracy in sample size. Miniscule differences in the amount of water collected can make a difference.

Testing right after the addition of chemicals can make a difference in how your results read. This, as well as testing without the water moving (pump running) can make things a little off as well.

Is there any reason why you are using the 25 ml sample testing instructions as opposed to the 10 ml sample instructions? There really isn't any reason to need to have your results in 0.2 ppm increments. By testing with the smaller sample (which results in 0.5 ppm increments), you save lots of reagent and for the purposes of running a pool, are getting results that are plenty accurate for the purpose. Results of +/- 0.5 ppm are virtually irrelevant.

 

[lalittle]

Thanks for the response.

I actually do use the 10ml sample much of the time. I like to do the more accurate testing for "super" accuracy once in a while -- just to see "exactly" where I stand.

I think I'm pretty much dead on with the water amounts -- I'm always very careful to get it just right.

The only thing that I can think of that may vary is how long the pump has been running when I test. Is this alone enough to make the differences I'm seeing?

Thanks again,

Larry

 

[JasonLion]

Yes, how long the pump has been running does make a difference.

Two sequential tests on the same water with the same reagents should be plus or minus two drops from each other. If you are doing the test by 0.2s then +-0.4.

Make sure you are holding the dropper bottle vertically and gently squeezing the drops out so they seem to hang for a moment on the tip and drop off on their own rather than being "sprayed" out.

 

[JamesW]

Another issue to note is that you should be double rinsing before and after each test. Any leftover residual can interfere with the accuracy of the tests.

 

[lalittle]

Yes, how long the pump has been running does make a difference.

Two sequential tests on the same water with the same reagents should be plus or minus two drops from each other. If you are doing the test by 0.2s then +-0.4.

Subsequent tests can be two drops off from one another? That would mean that the 10ml sample is actually only accurate to 1 ppm since 2 drops difference would be 2 x 0.5 ppm.

Make sure you are holding the dropper bottle vertically and gently squeezing the drops out so they seem to hang for a moment on the tip and drop off on their own rather than being "sprayed" out.

I believe I'm doing this correctly -- I'm very careful with this sort of thing.

Larry