A TDS reading is a number derived from an EC test. The TDS is calculated based on a conversion factor, usually ranging from 0.5 to 0.7. If the EC reading is 1000 µS, the PPM reading will be 500 on a 0.5 conversion, and 700 on a 0.7. So for one solution with one EC reading, different TDS meters will read differently.

You will need to find out if the conversion factor of your tester is the proper one for the nutrient schedule you intend to follow. If the schedule has EC readings, use those instead of TDS so you do not have to worry about conversions.