I have the standard comparator to test FC up to 6 ppm (or so it's marked) with DPD1 tabs. I've known for some time that there are DPD2 and 3 tabs available, but I've ignored them. Now I understand that the 2 and 3 tabs can measure combined chlorine, which DPD1 doesn't. Can I use these tabs in my standard test kit?
As I understand it, one would use a DPD1 tab in the kit, mix and wait two mins, then take the FC reading. Then add a DPD2 tab, mix and wait as before, and take the now intensified colour reading. Take the second reading from the first and there's the CC.
I'm not too sure about DPD2. I think you use those on their own in the test kit, take the reading and multiply by 1.4 to give the CC level. All this info is from Dryden Aqua literature.
The problem I have is: if it is so easy to take a CC reading with DPD3 tabs then why aren't these tabs included in the kits? They seem to be widely available but I can't find much info on them.