OTO is a reliable test -- not super accurate, but very reliable. However it tests for TOTAL chlorine (free chlorine + combined chlorine).

DPD color match testing is NOT reliable: at levels above 5 ppm, the reading may begin to decrease, reading ZERO somewhere around 15 ppm. BUT, DPD tests for FREE chlorine only, without a 2nd added step.

Most color strips are yellow=>purple syringaldazine, which usually tests for free chlorine, which really only reads to about 5 - 7 ppm. It doesn't bleach out like DPD, but water with 8, 15, 30 ppm of chlorine will all three produce identical readings.

. . . membership updated.