If the meter is out the other direction it won't say -10ppm in the bottled water, but it will still test your RO water at -10ppm of whatever it is, so your RO water could be 9ppm and the meter will still read 0ppm. That's one of the reasons why you need to use calibration fluid not distilled water.
Of these meters that you can't calibrate, I would like to see what they read when someone puts them into 342ppm calibration solution just to see how far off they really are. I just don't see the purpose of using something to test when the tester itself isn't accurate. That makes no sense at all to me.
