TDS meter or Conductivity Meter? 

Often one can hear that his TDS meters are not measuring correct. One reason is it need recalibration, one will need calibraion solution to do this. Another reason is, basically TDS meters from different manufacturers are calibrated with different conversion factor.

TDS meter base on reading the conductivity of the water and convert it to TDS with a known conversion factor, it is known to be a conductivity meter of 200uS has a TDS value of 100ppm of NaCl. So very simple for manufacturers, they just get the meter to show the conductivity meter value by half and now become a  TDS meter. But some manfacturers use CaCO3 solution to calibrate and this will result in for the same water, the TDS values are different.

To avoid all the confusion, one can simply use a conductivity meters, and most TDS meter comes with conductivity measuring function.