Some of the Electrical Conductivity meters we carry read out in two different units of measurement: EC (μS and mS) and TDS (ppm and ppt). The TDS is a variable unit of measurement and for this reason, we do not like to use it. The meters can be set to a wide range of conversion factors for TDS (anywhere from 0.45 to 1.0). In other words, the meter multiplies the EC reading in mS by 0.45 (or by 0.75, etc., depending on what setting you have used for the conversion factor) and then displays it as ppt. The default setting for the Dist6, for example, is 0.50.  Therefore, the reading in ppt will always be one half of the reading in mS (unless you change the setting in the meter). Because various meters use different conversion factors depending on which salt they use as a reference, you can have a lot of confusion with the TDS readings. For this reason, we advise people to always use EC because mS andμS are universal.