Odd results with FAS-DPD test

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Hilton

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In The Industry
Jun 15, 2010
76
Recently when testing a pool using a K-2006, the water sample would barely change colour when the DPD powder was added. But when the titration agent was added, the colour would get darker and darker with every drop until after about 5 drops, at which point it would lighten again as is normal. On a 10mL sample test, total drops was 10 before it went clear again.

In subsequent re-tests, I tried 10mL and 25mL samples, and I tried adding several extra dippers of DPD powder (to make sure there was enough in case of high chlorine), all with similar results. A test strip showed somewhere around 8-10ppm, but not dark enough that it looked >10.

What was happening?
 
That's a mystery. You might try calling Taylor Technologies on that one. Be sure to thoroughly stir after adding the powder since it normally keeps getting darker as it mixes. I've seen it do a flash of pink at high FC, but adding more powder shows up as red. You did everything right.
 
It was thoroughly mixed - after a couple of frustrating tries I even added the cap and shook the heck out of it just to be sure! In matter of fact, during the part where adding drops made the water pinker, the water would start to lighten back up if left for a few seconds.

If it happens again I will try to call Taylor direct.
 
I spoke directly with Taylor Technologies today. They say that what I am seeing can be normal with an excessively high chlorine level; simply make sure there is enough DPD powder in the tube (add more until there is some crystallized and left over at the bottom) and keep adding reagent until the colour clears up again, and stays clear for 20-30 seconds.

I had a similar effect today, but today the test was only 6.5ppm FC - hardly an excessive chlorine reading. So I'm still confused, but taking Taylor's word that the test is still accurate.
 
For whath it is worth, I just did the same test tonight with a result of FC of 6.5 with CYA of 45 and had the "normal" results - not what you are having. I have never seen that even when my FC was 40 with a CYA of over 200.
 
Hilton,

I hope you eventually solve this puzzle. The trouble is that you are caring for pools that are not your own. Heaven knows what in the world the client is putting in their pool water unbeknownst to you---fun water colors not to mention the myriad things people toss into a pool during a party.

How does this water look to you? How often are you called upon to service this pool?

How many different sets of reagents do you have?

Just wondering.....I love a good mystery....

Lana
 
I routinely test much higher with normal response as well. I maintain these pools once per day Mon-Fri, with a volunteer doing evening and weekend testing (per my other recent thread).
 

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The FAS-DPD test can bleach out if you don't use enough powder. What you describe sounds like there was not enough powder, and the test was just starting to bleach out but hadn't bleached out completely. Adding drops progressively lowers the FC level, which first went to eliminating the partial bleaching out, and subsequently worked normally.

That only fits what you described if you accidentally used quite a bit too little powder. Between the tiny spoon and the occasional fairly large grains in the R-0870 that can happen sometimes, though it doesn't really seem likely. This kind of problem is much more common when the FC level is really high, say 25+, where extra powder is always required. Still, it seems like something along these lines must have been happening.
 
Hilton said:
Kits are about ten weeks old. R-0870 and R-0871 were refilled about two weeks ago from 470mL bottles purchased three weeks ago.
Click to expand...
I don't think that R-0870 comes in a 470 ml size. R-0870 comes measured by mass (grams) or weight (pounds), not volume.

R-0871-E does come in a 16 ounce bottle, which is 473 ml.
 
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