Inconsistent TC results

[koxta]

Hi, everyone! First-time poster here!

Before I go on asking questions, I'd like to thank everyone on this forum for the vast amount of knowledge shared here. As a first-time, first-year pool owner, this has been really invaluable to me and my journey to understanding pool maintenance. I can proudly say my pool is very much trouble-free and crystal clear thanks to all the information and advice I was able to find here.

Now on to my issue: I am noticing quite a discrepancy when measuring chlorine levels using the OTO method vs. using the FAS/DPD method. Here are my numbers from the past 2 tests using the TF-100 test kit:

8/16/17:

TC (OTO) 5 - 10
PH 7.5
CYA 70
TA 100
CH 350
FC 3
CC 0
TC 3

8/23/17:

TC (OTO) 5 - 10
PH 7.5
TA 100
CYA 70
CH 350
FC 4.5
CC 0
TC 4.5

What got me really concerned is that during the 8/23/17 test, the OTO test result was showing a darker shade of yellow, yet the FAS/DPD test turned white only after 9 drops, which would indicate an FC level of 4.5. On top of that, when measuring TA, the solution has turned yellow instead of red, which from what I understand indicates a very high level of chlorine. I'm trying to wrap my head around this and thinking that maybe I'm making a mistake somewhere during testing? I follow the directions very precisely. I did take a water sample to my local Leslie's, and they seem to use Taylor's reagents to perform all their tests as well. Unfortunately, they only test for TC using the OTO method, yet it was also showing very high levels of chlorine (definitely in the 5 - 10+ range). I'm thinking the OTO test, in this case, is correct, but I'd love to know where does this discrepancy come from and how to fix it.

Note: I'm using jugs of Sodium Hypochlorite to maintain the pool's chlorine levels.

 

[jblizzle]

Welcome to TFP!!!

How much powder are you using for the FAS-DPD chlorine test? You can't use too much, but too litter could cause inaccuracies.

At a normal FC level with the 10ml water sample, 1 scoop should be plenty. But at higher FC levels, more powder might be good.

I would be much more likely to trust the FAS-DPD over the OTO though.

 

[OTPirate]

Hey! The OTO test is a test for the presence of Chlorine, it does not give an exact number like your FAS-DPD test does. Use the FAS-DPD test for preciseness and the OTO for presence of CL. I use my OTO test quickly in the morning to see if I am going to be stopping by Dollar General that day to buy more bleach. Then, when I have more time, I can sit down and do the FAS-DPD test for a precise reading. Have you added a bunch of bleach recently to get a high CL? You are correct about TA turning yellow with high CL. Make sure you wipe the R-0009 with a damp cloth between each drop (hold bottle vertically) when testing TA.

 

[koxta]

Wow, such quick responses! Thanks, guys!

I'm adding a heaping spoon (included in the test kit) of the powder, per the instructions. I noticed that during the 8/23/17 test, the water has turned very dark pink, nearly red, after adding the powder. This combined with the TA test turning yellow makes me think that the TC levels are much higher than 4.5. Please correct me if I'm wrong, though. Also, I have added half a gallon of 10% sodium hypochlorite an hour before testing the water.

Not sure if that matters, but the R-0871 bottle is bigger (by design I think) and the nozzle produces noticeably bigger drops than the TA or CH reagent bottles.

 

[Mr Bruce]

Are you using a 10ml sample and counting each drop of 871 as 0.5?

As for the pink, a good description is "Barbie Doll" pink. What you describe sounds normal to me.

Also, I just wanted to note that your FC is too low for your CYA in both tests Pool School - Chlorine / CYA Chart

 

[koxta]

Are you using a 10ml sample and counting each drop of 871 as 0.5?

Yes and yes!

Also, I just wanted to note that your FC is too low for your CYA in both tests Pool School - Chlorine / CYA Chart

I am familiar with this chart and I am aware of that, but I don't want to keep adding chlorine without being certain as to what the actual level is. Thanks for pointing it out, though!

 

[JoyfulNoise]

Add two scoops of powder to your 10mL sample and see if that changes the number of drops needed to go from pink to clear. You can also add a thrid drop of the R-0007 thiosulfate chlorine neutralizer to the TA test to see if that restores the color. If you have a SpeedStir, add the R-0007 and then let the water swirl for 10-15sec, then add the R-0008 indicator.

 

[koxta]

Add two scoops of powder to your 10mL sample and see if that changes the number of drops needed to go from pink to clear. You can also add a thrid drop of the R-0007 thiosulfate chlorine neutralizer to the TA test to see if that restores the color. If you have a SpeedStir, add the R-0007 and then let the water swirl for 10-15sec, then add the R-0008 indicator.

I will try this. My concern, however, is that if the amount of powder added was to alter the test results, wouldn't that make the test results bogus unless the exact needed amount of powder has been added? This would then contradict the test instructions.

I am not worried about the TA color, I am only pointing it out as an indicator of high chlorine concentration. Does it really get yellow, instead of red, at 4.5 TC?

 

[jblizzle]

Like I said above, you do not have to be exact with the powder, but you do have to add enough

Should be a deep pink to start (unless the FC happens to be very low).

 

[Caco]

The other thing to note is are you testing for CC's.
I might be wrong, because I dont have the OTO test, but I think this shows TC - total chlorine
Whereas the FAS-DPD shows FC and CC, which you have to add to get TC
I would be surprised if you have high CC's, but you may want to check this

 

[koxta]

Thanks everyone! I will run some more tests later this week and report my findings!

The other thing to note is are you testing for CC's.
I might be wrong, because I dont have the OTO test, but I think this shows TC - total chlorine
Whereas the FAS-DPD shows FC and CC, which you have to add to get TC
I would be surprised if you have high CC's, but you may want to check this

Thanks for bringing that up! I have mentioned in my OP that the CC level is zero (or very close to zero), so that's not it!

 

[koxta]

Back with some more info! It looks like you were right on the money with the amount of powder to use! I was adding one single heaping spoon before, but this time I have added two and the test results are much closer to what I would expect:

8/25/17@6pm

FC - 4.5
CC - 0
TC - 4.5
PH - 7.5

Note that this test result shows FC = 4.5, the same level as my test from two days ago. Since I have not added any chlorine to my pool within the past two days, it would have been impossible for the pool to have virtually zero chlorine loss. Thus I conclude that my FAS/DPD test from 8/23 was under-reporting FC/TC.

To confirm that the results were accurate this time and the testing method dependable, I added 1/2 gal. of 10% sodium hypochlorite right after the test and let it mix for an hour or two. The pool calculator tells me that this should have increased my chlorine level by 4ppm. So, testing once more:

8/25/17@7:30pm

FC - 8.5
CC - 0
TC - 8.5
PH - 7.5+

Right on! Exactly as expected, with two heaping spoons of the powder. Then, I read this post and it all made even more sense. Since the powder reagent's role is to bind to all available chlorine, I am going to need more of it with higher chlorine levels. I'm assuming the reagent remains inert without any chlorine left to bind with, hence adding more is not doing any harm and skewing the test results.

I hope I got that right Thank you!