[mike31154]

Thanks for the advice Mindy. Where's a good source for the calibration solution?

Not sure it's even worth calibrating the handheld, I think I paid less than $20 for it. My RO/DI system is barely six months old so I'm fairly confident in the Dual TDS meter that came with it, especially since both probes on that yielded identical readings for my source water. Being relatively new, I trust the RO and DI TDS readings since the filters should be nowhere near exhausted. My DI resin has only changed colour to about one quarter of the canister height.

Besides, even if the readings are a little off at the low end, I'm not too concerned since I was using treated tap water up until six months ago.