I have 3 now all the same make.
All 3 give different readings in calibration fluid and all 3 are about 30 out either way and one is about 40 out.
Only 1 reads consistently 000 after having changed resin so I use that one....it also reads true from the tap.
I tested it in a marine fish shop that produces and sells "pure water".......they have a Ro permanently producing pure and their pure was 24...they said my meter was out but also admitted they never ever check or service the Ro.