I would never trust any inline meter - they are notoriously inaccurate. I have one on my 40-40 and it can be as much as 25ppm out.
I always use a handheld, and check it regularly with calibration fluid. If you don't do this you could be blissfully unaware that the water you think is 000ppm is actually 10, 15 or even 20 or more.
You only find out when the complaints start coming in.