fas-dpd test

dbarker1

Member
May 1, 2010
18
alabama
I've been using this test for a while but since I've started shocking the pool I'm getting odd results with the test. It normally goes completely clear when adding the drops but the last few times I've tried it never gets clear no matter how many drops I put in it. I've also had to add about 2 scoops to get the sample to turn pink but I think I read on here that thats not uncommon. Has anyone ever had this happen? FC should be about 30. CYA is 70. Could water temp affect it or am I doing something wrong?
 

dmanb2b

TFP Expert
LifeTime Supporter
Apr 4, 2009
3,734
NY
How much chlorine have you added recently? From the sounds of it, your FC is really high :shock:

Try using 5ml of pool water and 5ml of bottled water and then count each drop as 1ppm...see if that gets you there
 

257WbyMag

TFP Expert
LifeTime Supporter
Feb 23, 2008
5,061
Denton, TX
Not uncommon to have to add two scoops. Are you using the 10 ml sample? 60 drops to clear would equal 30 ppm. Did you stop at 60 drops? It may be higher than you think.
 

dbarker1

Member
May 1, 2010
18
alabama
I've been trying to maintain a FC level of 30 for the past 2 days to shock. I have only added a little here and there as it has not lost much. I'm having to guess a little because I can't read the test correctly. Its gets almost clear and just stays there. I used the pool calculator and a large measuring cup to add the bleach so I don't think I overshot it but I could be wrong!
 

dbarker1

Member
May 1, 2010
18
alabama
Yes I am using the 10ml sample and I dont remeber where I stopped at like 75 or something but there was no change after about 52 drops. I've done the test for months and have never seen it do this before its usually a definate change to clear.
 

257WbyMag

TFP Expert
LifeTime Supporter
Feb 23, 2008
5,061
Denton, TX
My guess is that your FC is so high that although you may be clearing it with the drops, it may be reverting back to a light pink shade so quickly that you cannot notice it. Any sample that has enough drops the clear will go back to pink again if left standing. I think that this may be happening, albeit much faster than usual, because your FC is so high. This is just a theory but I am going to see what others say.
 

dbarker1

Member
May 1, 2010
18
alabama
I'm mixing by hand. That is a possibility of the sample changing faster than I can tell or at least that would make some sense.
 

dbarker1

Member
May 1, 2010
18
alabama
Shocking because the water has remained a little cloudy and I saw what appeared to be some algae growing in several spots. When testing I usually stir by hand for about 2 seconds maybe then add another drop and repeat.
 

JasonLion

TFP Expert
Platinum Supporter
LifeTime Supporter
May 7, 2007
37,880
Silver Spring, MD
When your FC level is very high, you need to move along a bit more quickly for the first segment of the FC test. As long as the sample remains a solid dark pink you want to be adding drops at the rate of about one per second, or just a little faster. Then you can slow down when the pink starts getting lighter so you don't miss the endpoint.

Another way to help make the FC/CC test easier at extremely high FC levels is to mix equal parts of pool water and chlorine free water (typically distilled water) together and then test that. When done this way each drop counts as 1.0 ppm of FC or CC. That halves the number of drops you need to use.
 

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