OK, if it's a cylinder of paper, like the inside of an oil filter, it's a cartridge. If it look like overlapping fan blades, it's a DE filter. And if it's full of sand it's either really dirty or it's a sand filter!

You can't read an FC of over 5 with a DPD test kit. What we suggest is an FAS-DPD test kit, and FAS-DPD is not the same thing at all as a DPD test kit. If you have LOTS of chlorine, your DPD may read "zero" because it's actually bleached out! The other tests in the kit should be fine. You can order just the FAS-DPD test from Taylor if you want.

Carl