FAS-DPD testing Difference between Reagents

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K

Karthas

Gold Supporter
Jun 30, 2022
16
Tucson AZ
Pool Size
20000
Surface
Plaster
Chlorine
Liquid Chlorine
I am just finishing up a SLAM. During the SLAM, I had to order more of the R-0871 reagent. There was a significant difference with the results of 1st bottle and the replacement. I have since run out of the first bottle and continued with the second bottle. Today I received a new bottle, and the results are as follows: Bottle in use FC 14 new bottle 20. All of these bottles have been purchased in the last 2 weeks. One bottle is from Taylor purchased on Amazon for fast delivery and the other bottle from TF testkits. Any thoughts? Thank you in advance for your opinions.
Pattie
 
The Taylor bottle from Amazon exp 6/24. The bottle from TF test kits does not have it however I just received it last week. I just got back from the pool store and the FC reading was17.7. also, my CC was .5 on both and the pool store was 1.1 ppm. I use 10mm of pool water for testing chlorine. The drops come out fairly fast at the beginning and the slnow down. I try to keep it at 1 second per drop. I also try to keep the drops the same size letting them form and drop off by themselves. I did send an email to TF test kits asking if the reagent degrades in heat. The TF bottle was in the mailbox for a few hours on a hot day and the bottle was warm when I brought it in the house. Thank you all for your thoughts.
 
Karthas said:
I am just finishing up a SLAM. During the SLAM, I had to order more of the R-0871 reagent. There was a significant difference with the results of 1st bottle and the replacement. I have since run out of the first bottle and continued with the second bottle. Today I received a new bottle, and the results are as follows: Bottle in use FC 14 new bottle 20. All of these bottles have been purchased in the last 2 weeks. One bottle is from Taylor purchased on Amazon for fast delivery and the other bottle from TF testkits. Any thoughts? Thank you in advance for your opinions.
Pattie
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Hi Pattie,
One explanation I could think of is technique. Are you holding both dropper bottles completely vertically and allowing the drops to form and fall naturally off the tip with minimal squeezing of the bottle?
The date on the R-0871 currently being sold at TFT is best by 9/24. There shouldn't be a serious loss of reagent efficacy even after the temperatures you're describing.
Double check that the tip is seated properly - snap the tip out by applying sideways pressure to the tip with your thumb, then reinsert by pressing it down until it snaps into place. Then retest.
I will follow up via email. I am sorry that you're seeing these discrepancies but we will get you sorted one way or another!
Nate @ TFT
 
Thanks Nate.
I have removed the tip and reseated it. It came off easily so that might have been the issue. The drops were pretty big, and I think the reagent may have been coming out below the tip as well, but I am not sure. Yes, I keep the bottle vertical. I will be testing a little later today and will let you know the results of the test again. Thanks for your suggestions.
Pattie
 
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Try swapping dropper tips between bottles, and see if one of the bottles has a dodgy dropper.

In which direction was the difference between the very first bottle and the second bottle, you didn't give values comparing those two?
 
I cannot swap out the tips as they are different sizes. The garbage pickup was today or I would have fished the first bottle out and swapped the tip as it is the same type as the bottle that is reading low. I retested FC and CC again at sunset and FC is either 12 or 17 and CC is .5. I could not compare the first bottle to the to the second as I did not use the second bottle until the first was used up. I will say that the OCLT FC level was 30 PPM in the evening with the 1st bottle and in the morning, it was 19 PPM and .5 with the second bottle. I will be more diligent with my notetaking in the future. I have recorded my additions on paper. I am performing another OCLT and if it passes then I will have the FC drop to normal levels and will retest pH and keep FC level between 5-9 as current CYA is 60. I am hoping for the best.
 
There is naturally a wider variance of testing the higher your FC is. At lower levels you can get 0.5-1.0 ppm drop variance IIRC but at higher levels it can be higher variance, esp if not using a speed stir.

Just something to think about if you are measuring at 30
 
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Thank you. I wondered what the margin of error is in testing. I am going to let the chlorine level drop to 5ish and use the other chlorine test that is side by side with the pH test and see if the results are any different.
 
Once you get down to low teens or high single digits it should be minimal. I wouldn’t drop below your target
 
Karthas said:
I am hoping to lower CYA level to 50 prior to allowing chlorine to drop to 5.
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5 is flirting with minimum for CYA of 50, which leaves you at risk of dropping too low if life gets in the way. I would shoot for 8 or even 9.
 

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