FAS-DPD test
Also called: FAS-DPD, FAS-DPD drop kit, drop-titration chlorine test.
A FAS-DPD test is a drop-titration chlorine test: you count drops until the sample turns from pink to colorless. A FAS-DPD kit measures free and combined chlorine precisely, accurate to 0.2 or 0.5 ppm, and it reads the high chlorine levels that test strips and the DPD color block cannot.
Why it matters
It is the only kind of kit that can run a SLAM, because SLAM levels run into the high teens or twenties and a DPD block comparator bleaches out and stops reading up there. The Taylor K-2006 is the standard example; it also tests pH, total alkalinity, calcium hardness and CYA. The near-identical K-2005 uses the DPD color-match block and tops out around 5 ppm. The kit doesn't wear out, the reagents do: Taylor states its reagents are formulated to remain effective for at least one year. No standard drop kit tests for metals, and borates need their own strip.
In numbers
With a 25 mL sample, each drop of titrant counts as 0.2 ppm of chlorine. With a 10 mL sample, each drop counts as 0.5 ppm.