This OTO business is one thing on this forum I have never understood. I can run my pool for weeks without testing and not be outrageously far off just by adding my standard dose every other day. I sure won't get to zero. Every time I read an explanation I see no reason I should add OTO capability to what I have. (I don't have an OTO test for at least fifteen years; my ancient K-2000 came with regular DPD. Then I upgraded to add TA, CH, CYA and finally FAS-DPD.) I'm familiar with OTO; I used it exclusively for at least 20 years back in the stoneage.) I'd just rather do FAS-DPD and get accurate data. I shoot for every other day but I frequently skip. It's plenty.