FAS/DPD vs. DPD results

[CharliesPool]

I've been a little casual with my testing recently and haven't done a FAS/DPD test in a couple of weeks. My DPD test was consistently aroound 1 or 2. I bumped up the SWG and even added some liquid chlorine. Today I tested using FAS/DPD and I'm at 11 ppm and the DPD test still shows about 1 or 2. All of the reagents were purchased within the last two months.

I read some of the other posts that said it's not uncommon to have a slight difference between the two tests, but this is a significant difference. Anyone have ideas on what could be causing this?

Charlie

 

[Texas Splash]

but this is a significant difference.

I agree. Since you confirmed all reagents are new, our next questions might be how exactly are you performing the FC test with the powder & drops. Are you using the 10 ml water sample size with one scoop of powder? Adding drops until clear and dividing the result in half? Example: 10 drops equals an FC of 5?

 

[CharliesPool]

I agree. Since you confirmed all reagents are new, our next questions might be how exactly are you performing the FC test with the powder & drops. Are you using the 10 ml water sample size with one scoop of powder? Adding drops until clear and dividing the result in half? Example: 10 drops equals an FC of 5?

Thanks for the quick reply. Yes ... one (fairly) level scoop of powder and then divided the total number of drops by 2. I think I might ask one of my neighbors if I can try their DPD reagent. The pool is clear and no signs of anything growing.

 

[JoyfulNoise]

The DPD reagents can get bleached out when the FC is high and over 10. If you’re not using the high range comparator block, the sample volume is only about 4.5mL and the dye will get bleached. DPD is not like OTO where the yellow goes to a progressively darker orange at high FC. DPD will simply start to bleach out at high FC and give a false low reading. If you need to measure high levels of FC with DPD then you need to get the high range comparator block and add double the amount of R-0001/R-0002 to compensate.