nitpicky fas-dpd test question

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tltmom

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Aug 5, 2011
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Urbana, IL
My pool hazed up slightly last night after a pool party, so I assumed that similar to another recent post, I had an increased biologic load in the pool, from the extra swimmers. I added 2 extra ppm last night to the usual dose of Cl, and the water looks better this morning. I can't even tell if the water has any haze left or not. That's easier to tell when it's sunny.
My question is this: I can't seem to get a consistent number when I use the fas-dpd test.
For example, last night I got:
7.0, 0
6.5, .5
7.5, 0
I added 5ppm Cl after testing
This morning, results were:
10.5, .5<cc<1.0(took 2 drops to clear)
11.0, .5
I try to be very precise when I run the test. I follow the extended directions to the letter. The only thing that is hard to keep the same from test to test is that sometimes I end up using a scoop of powder that seems to be all powder, sometimes there are a lot more "chunks" in it. (I warned you this question was nitpicky! :wink: )
I can't decide from all these results if I need to go through the shock process or not.
(This was not a true oclt since it was already fairly light outside when I tested this morning)
Btw the swg in my signature is not turned on right now.
 
I got tripped up by the description of a "heaping scoop". I wish there was a picture to go along with the directions so I knew exactly how heaping. I started using the alternate "two level scoops" just to be more consistent.
 
Yes, I've read that too: a little too much, within reason, is fine.
What I can't seem to figure out is why I'm getting slightly different results when I do multiple tests on the same sample. Normally I don't run more than one test, but if I'm doing an oclt or trying to see exactly how much Cl the pool is using, the accuracy starts to matter.
I'm sure the inconsistency is with me, since I am fully on board with this being the best Cl test.
Thanks for the input.
 
Could it be static causing the drops to be different sizes? Maybe slightly different sample sizes?

If I were trying to check less than 1 ppm change with my K-2006 then I would use the 25mL sample. Maybe this would give you a more accurate reading.
 
The chunks vs. powder thing bothers me as does the "heaping spoonful"........I just heap up the spoon with chunks or powder and assume all is well.....but i don't do the test more than once on the same sample but you can be assured I WILL now and see if i have issues replicating results.

I also would like to know why one is supposed to wipe with a wet cloth the tip of the purple stuff????
 
R-0870 powder is the indicator dye that causes the pink color in the FAS-DPD test. You need to add enough to bind to all of the chlorine in the sample. Technically, the higher your TC level, the more R-0870 powder you need. In practice, two level tiny spoons, or one "heaping" tiny spoon generally does it. However if your TC level is very high you may need more. Using too little R-0870 will give you false low results. Using too much won't have any effect (unless you use a really huge amount). Generally if you need quite a bit more R-0870 the sample will turn pink for a moment and then go brown or clear. If that happens you certainly need more R-0870. However, it is possible to add just a little too little and not notice and get an incorrect result. To avoid problems it is simplest to always add a bit too much R-0870 just to be safe.

If the R-0870 gets too clumped it won't dissolve properly. It always clumps a little, more over time. Generally this isn't an issue, but if it gets really clumped you can just crush the clumps back into powder.

Variations in test results can come from many sources. First, make sure to fill the test vial to the same level each time. A 1 ml difference in initial filling will cause a 10% error, often noticeable. Second, hold the dropper bottle vertically and squeeze gently so that the drops appear to fall off the tip "naturally" under their own weight, instead of being "sprayed" out of the bottle. That gets the most consistent drop size.

Wiping the tip of the dropper bottle with a damp tissue helps prevent static electric buildup on the tip. If a charge does build up on the tip it can cause the drops to be smaller than usual. This is normally not an issue, and when it does come up it is usually only on the TA test. However, it is possible for this to affect any of the tests where you are counting drops.
 
JasonLion said:
R-0870 powder is the indicator dye that causes the pink color in the FAS-DPD test. You need to add enough to bind to all of the chlorine in the sample. Technically, the higher your TC level, the more R-0870 powder you need. In practice, two level tiny spoons, or one "heaping" tiny spoon generally does it. However if your TC level is very high you may need more. Using too little R-0870 will give you false low results. Using too much won't have any effect (unless you use a really huge amount). Generally if you need quite a bit more R-0870 the sample will turn pink for a moment and then go brown or clear. If that happens you certainly need more R-0870. However, it is possible to add just a little too little and not notice and get an incorrect result. To avoid problems it is simplest to always add a bit too much R-0870 just to be safe.
Click to expand...

I'm frankly frustrated by the ambiguity of these instructions, and I don't think the original poster is being "nitpicky" at all. What does "if your TC level is very high" (how high?), "you *may* need more" (how much more?), "simplest to always add a bit too much" (how much is "a bit"?) really mean?

I'm also getting very inconsistent results for trying to measure whether or not I'm losing only 1ppm of FC overnight--it all depends on which measurement I count, and I'm trying to be consistent. With this much ambiguity how can I really know when I'm done shocking? Please help me understand how this is really "trouble free"? It sure doesn't feel like it. :|
 

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The reason to use more DPD powder when the FC is very high is if you add powder and you see a flash of pink and it disappears -- that's how you'll know it's too high. You add more powder until you get a distinguishable color (i.e. not so faint a pink that you wonder if it's there at all). You don't have to add enough powder to react with all the chlorine because the FAS-DPD drops will be reacting with the chlorine so by the time you get to where it goes from pink to clear there is very little chlorine left no matter how much you started with. The only effect on measurement error with how many scoops you use is in how much of a transition you see from pink to clear so is the error in that last drop.

If the FC level is very high such that adding powder shows a flash of pink but goes clear, then if you add more powder so you get a consistent pink then adding FAS-DPD drops during the test will first have the color intensity increase as the FC gets used up by the FAS-DPD and the dye becomes less bleached out. As you continue to add FAS-DPD, the color intensity will then decrease and you'll get to the pink-to-clear transition at the end of the test. See this link for technical details about the test and in particular Figure 2.1 that shows how chlorine reacts with the dye to produce a colored product and a colorless product where at high oxidizer levels there is more of the latter product (i.e. the bleaching out effect). When you add FAS-DPD drops, this reaction is reversed (goes to the left) as shown in Figure 2.3.

If you are wanting more precision than 0.5 ppm, then use a 25 ml sample size where each FAS-DPD drop represents 0.2 ppm. For normal chlorine testing you typically don't need this sort of precision.

As for being trouble-free, that will only happen if you realize that the criteria are rough rules-of-thumb and not absolute cutoff points where magically the quality of the pool goes from being horrid losing 1.2 ppm FC overnight to absolutely pristine when losing 0.8 ppm FC overnight.
 
I don't know if I'm on the right track, but if I add a pretty hefty scoop of powder (with little if any clumps in it) that my results are much more consistent than I had been getting when I first posted. I may have been using just a little too small an amount of powder, as one of the moderators had suggested. So I feel more comfortable with my results now. I hope that helps other posters in this thread.

As always, I appreciate all the help I get here, especially since as far as I know there are no other BBB'ers in my area, my pool builder gave me zero and I mean zero guidance when it comes to water chemistry, and the local pool store folks think I'm nuts (since I won't purchase any of their magic pool chems and just use liquid chlorine) :wink:
 
If you do a search on youtube of "taylor fas-dpd test" you will find a video demonstrating the test. I found it helpful from the standpoint that One could see they did what looked like 2 heaping scoops to me.

I too was all over the place when testing with it and found i became consistent when i began using the speedstir. for the fas test, that thing is the best.
 
chem geek said:
If you are wanting more precision than 0.5 ppm, then use a 25 ml sample size where each FAS-DPD drop represents 0.2 ppm. For normal chlorine testing you typically don't need this sort of precision.
Click to expand...

This is the first I've heard of the 25 ml sample size option. Are there any other instructions for using this option? How much of the powder is recommended with this amount? 2.5 heaping spoonfuls? ;)
 
I didn't think that was nitpicky at all. I had the same question in my mind for a while.
I watched the YouTube and Low and behold... I am using the WRONG END of the scooper. :hammer:

I am using about the same amount of powder as the video shows,so I am not concerned. It is nice to know how I SHOULD be using the scooper so I know for sure I am using the right amount. The TF-100 Kit only has you adding 1 heaping scoop, not 2.

Thanks for being nitpicky! :goodjob:
 
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