Quote Originally Posted by prh129
It looks like in my case there was something in the water that caused a problem with the 2-reagent test but not with the 3-reagent test (that's probably why the third reagent was added)
You win the prize!

I was careless, and didn't notice on my quick read through earlier. The '2-reagent' system combines the buffer and the indicator. But, on pools with lower pH, and relatively high alkalinity, there's not enough buffer in the indicator. That's why extra drops of indicator -- which add extra drops of buffer -- works, when the 'normal' number of drops don't.

(The pH of the sample has to be pretty high, for the indicator to work, and the 'buffer' tries to move the pH up high enough to work. But, pools with high alkalinity are resistant to pH change, thus the problem.)

Ben