Fas-DPD and DPD testing significantly different

[jlppool]

Hi
First off I am having no trouble with my pool -water is clear everything is okay. I am trying to understand why there is a significant difference between my fas-dpd test and my dpd test. I am using the taylor K-2005 and the TFTtestkit TF-100 testkits. Using new chemicals purchased this year (with expiration dates in 2022) the fas-dpd fc is currently 5.5 (which is my target as my CYA is approx 70). The dpd test clearly shows the fc is in excess of 7.5. I know from my reading fas-dpd is supposed to be more accurate, but why the significant difference? The color matching on the taylor K-2005 is pretty easy so I don't think it is a visual thing.

I treat my pool based on the fas-dpd test, but I wanted to know why the difference.

Thanks
John

 

[reggiehammond]

General consensus I’ve read is that everyone sees the results of tests using a comparator block differently; I’ve found that if I blink a few times and move the sample around under a light that I can get the color via comparator to look different. Thus why the drop-based FAS-DPD is recommended as the accuracy isn’t reliant on interpretation, but simple counting.

 

[BowserB]

What Reggie said. I've used the advice of holding the thing up to white or gray clouds outside, but I'm never 100% confident and have gone to daily drop test. Trying to be consistent, I pull a sample halfway down the deep end in the morning around 7-8am. At the same time I use a calibrated meter for pH. Like Reggie said, there is no color judging when you just count drops. Bonus is it only takes another 30 seconds to test for CC.

It should go without saying that a Speed-Stir is part of the setup.

 

[PoolEric]

Funny this should come up, as I've encountered something similar... I have the TF-100 test kit which comes with the Taylor K-1000 mini-test-kit. I've been using the K-1000 to check regularly that there's any chlorine since, hey, it came with the kit. Today, it was reading bright bright yellow (5ppm+), definitely off the scale. To get a more accurate reading I used the FAS-DPD test in the TF-100. It tells me 3.5ppm.

So, should the K-1000 test only be used for "Yes"/"No" as to whether there's ANY chlorine? If it shows the palest of yellow (matching the .5ppm block) is that a "Yes" or "No"? I didn't realize there'd be that much of a difference.

I'm guessing I'm better off using the FAS-DPD test like my K-2006 has.

 

[HermanTX]

Funny this should come up, as I've encountered something similar... I have the TF-100 test kit which comes with the Taylor K-1000 mini-test-kit. I've been using the K-1000 to check regularly that there's any chlorine since, hey, it came with the kit. Today, it was reading bright bright yellow (5ppm+), definitely off the scale. To get a more accurate reading I used the FAS-DPD test in the TF-100. It tells me 3.5ppm.

So, should the K-1000 test only be used for "Yes"/"No" as to whether there's ANY chlorine? If it shows the palest of yellow (matching the .5ppm block) is that a "Yes" or "No"? I didn't realize there'd be that much of a difference.

I'm guessing I'm better off using the FAS-DPD test like my K-2006 has.

I only use the comparator in the K-1000 to test pH. I have used it to test my tap water to test both pH and Chlorine as that should be under 5ppm (actually hopefully under 1ppm TC for tap water). Also the K-1000 is for Total Chlorine (FWIW).
You should always use FAS-DPD to test your pool FC and then follow that up with CC test. That is by far the most accurate.

 

[PoolEric]

I only use the comparator in the K-1000 to test pH. I have used it to test my tap water to test both pH and Chlorine as that should be under 5ppm (actually hopefully under 1ppm TC for tap water). Also the K-1000 is for Total Chlorine (FWIW).
You should always use FAS-DPD to test your pool FC and then follow that up with CC test. That is by far the most accurate.

Hmmmmm... I don't remember seeing that the K-1000 is measuring TC (doesn't mean it's not so, just that I need a stronger magnifying glass ) Anyway, I learn something new every year, just when I think I know everything I need to.

Thanks!