Question on FAS-DPD test kit

[hsmyers]

I just got the Taylor FAS-DPD test kit. I already have a regular Taylor DPD test kit (chemicals are a few months old). My target FC level is 6 ppm (I have a SWG with 80 ppm CYA).

First I measured the chlorine with the regular DPD kit. Because it measures 0-5 ppm, I diluted the pool sample 1:1. I measured exactly 2 ppm FC. In the 1-3 ppm range it's pretty easy to match the colors, so I'm pretty confident in this number.

Then I measured with the FAS-DPD kit. Using the .2 ppm resolution I got 6.8 ppm FC. Using the .5 ppm resolution I got 7.5 ppm FC. Any idea why I'm seeing such a huge difference between the regular DPD kit, and the FAS-DPD kit?

 

[JohnT]

First I measured the chlorine with the regular DPD kit. Because it measures 0-5 ppm, I diluted the pool sample 1:1. I measured exactly 2 ppm FC. In the 1-3 ppm range it's pretty easy to match the colors, so I'm pretty confident in this number.

Diluting causes loss of accuracy. While your match may be good, the measurement for dilution may be off enough to affect the reading. If your FC is only 2ppm, why dilute it?

 

[hsmyers]

I diluted it because I find it hard to match colors between 3 and 5 ppm FC on the normal test kit. Plus my target FC is 6.

Diluting seems pretty straightforward. First I fill the left side of the two tube tester with pool water and then dump it in the larger TA test tube. Then I fill the left side of the two tube tester with tap water and pour it in the TA test tube. I swirl to mix, then pour the diluted sample back in the left side of the tester, and then I do a normal FC test. I don't see where there could be a large enough error to have the readings off by as much as I'm seeing (DPD = 4.0, FAS-DPD = 6.8)

 

[hsmyers]

I tried the normal DPD test, but this time without diluting the pool sample. I find it hard to match the colors between 3 and 5 ppm FC, but I got a reading somewhere in between the two. So, the DPD kit is measuring about 4 ppm FC, and the FAS-DPD kit is measuring 6.8 ppm FC. This seems like a really large difference. I'm just trying to figure out which one to believe.

 

[JohnT]

Tap water isn't good for diluting, as it usually contains chloramines, which should be measurable as chlorine on the test.

 

[hsmyers]

Tap water isn't good for diluting, as it usually contains chloramines, which should be measurable as chlorine on the test.

If that were the case, then my measurement of 4.0 would actually be high, which would make the difference between the two measurements even larger.

Let's pretend I never mentioned the word "dilution".

With the Taylor DPD kit I measure 4 ppm FC (without diluting the sample). With the Taylor FAS-DPD kit I measure 6.8 ppm FC. The FAS kit is new. The regular DPD kit is only a few months old.

This seems like a really large difference. I don't know which test to believe. I could go to a pool store to get verification, but whenever I've tried this in the past I get such wildly different numbers (on the same day with the same water sample), that I've stopped trying to do that.

Thanks for your help.

 

[JasonLion]

The FAS-DPD chlorine test is by far the most precise and accurate chlorine test available. There are various mistakes you might have made while doing the tests, but assuming that there weren't any problems of that kind, I would always believe the FAS-DPD test over any of the others.