High amounts of DPD Powder needed!! To high or no Clorine?

[KaseyNewman]

We are first year pool owners and recently winterized the pool, but kept the connected spa open. Because the water volume is so much less, i am still trying to figure out how to keep the chemicals balanced. Today while testing with the K-2006, I needed 4 scoops of DPD Powder to turn the sample pink, at which point it turned a deep pink (not light pink like it has been in the past). After about 30 drops of Reagent (15ppm Cl), it was still deep pink and I stopped adding more. Is my problem too high of clorine, or no clorine? I have a SWCG, but still trying to figure out the right mix of pump time and SWCG time.

[chem geek]

The chlorine is high. That's why it took more DPD powder before the color stayed. High chlorine levels well above 10 ppm can bleach out DPD. Your chlorine level may be well above 20 ppm. You should set your SWCG off to 0% output and if your pool is exposed to sunlight then that should lower the chlorine level over time. If you want to lower the chlorine level faster, then you can add a chlorine neutralizer. You can use a 5 ml sample size for your FAS-DPD test so that every drop is 1 ppm FC (saves on reagent).

When the water is colder, chlorine usage drops, especially if the pool is no longer exposed to sunlight.

[CarlD]

OK, I'm confused. DPD is usually either a drop or a tablet. The OP said DPD powder. That sounds like FAS-DPD powder. If so, it won't bleach out at 10ppm, but regular DPD will. Sounds like apples and oranges to me.

Carl

[chem geek]

DPD powder used in the FAS-DPD test can indeed bleach out at levels > 25 ppm. One gets a "flash of pink" and then it goes clear. You then add more powder until you get a color. It sounds like the Free Chlorine (FC) level is very, very high -- perhaps above 50 ppm given that it took 3 scoops before a color was seen. To test such a high FC sample, one can dilute with distilled water so that one can save on reagent and one can also use a smaller sample size (10 ml is 0.5 ppm per drop; 5 ml is 1 ppm per drop).