I just have a cheapo hand held as well and haven't bothered with calibration. My RODI system has the standard dual TDS meter and I've never cal'd that one either. One time I pulled a probe from the dual TDS job and compared the reading to the handheld, difference was about 10-15 ppm when reading my tapwater. If memory serves, the handheld read lower. My tap is around 210 with the handheld and 220 with the dual TDS probe. I think I tried both the dual TDS probes and they were both reading the same. That's the one I'm more concerned about, not the handheld, since the dual TDS is on my RODI. I'm pretty ok with trusting the dual to tell me when the TDS out of the DI stage starts to creep above 0. I'm kind of with Binare on this one, for our purposes do we really need to be tweaking these things for a few tens of ppm? Of course if I had access to a more accurate/pricey instrument, I'd certainly give it a go to compare.
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