Evan, thanks for the info! Would a couple of drops of thiosufate correct the Rainbow kit and vice versa, could the Rainbow kit be used to test REALLY LOW pH if a few drops of bleach were added? (every year we lose a few copper heat exchangers due to low pH/ alk and I'd like to know how bad the water is so I can properly adjust it - as for why this happens, the owners use only tri-chlor pucks and [the antithesis of CarlD's advice] 'spend 2 - 5 minutes a year to buy a new heat exchanger' - no joke, there are some we replace every year and tell them how to avoid it.)