The discussion about EC and TDS has been an ongoing issue for as long as I can remember. The two measurements are used to determine the strength of Hydroponic nutrient solution. Although they are widely used they should only be used as a guideline and you should always follow mixing instructions on the label of you nutrient to start with.

EC stands for Electrical Conductivity and is measured in mS/cm or miliSiemens per centimeter.
TDS stands for Total Dissolved Solids and is measured in ppm or parts per million.

Taking the EC value and performing a math function to determine the TDS value calculate TDS. TDS as a calculation it is really only a guess (but a good close guess) at what the nutrient concentration is. There are three different conversion factors that can be used to determine TDS. Different manufacturers use different conversion factors and that is where the confusion starts. In other words you could test the same solution with two different meters and get two different readings. But the EC is read the same by all meters the only difference is the conversion factor. That does not mean EC is the best either it just means that TDS is more of a math formula.

There is the 442 conversion (40% sodium sulfate, 40% sodium bicarbonate, and 20% sodium chloride) which some say is the closest thing to a Hydroponic solution. The 442 conversion is approximately 700 x EC in miliSiemens (mS). Then there is the NaCl conversion (sodium chloride) which others say is the closest thing to a Hydroponic solution. The NaCl conversion is approximately 500 x EC in miliSiemens (mS).

So, if you do not calibrate your meter using the correct calibration solution your meter could give you a very inaccurate reading.

Electrical Conductivity (EC) Parts Per Million (PPM)
miliSiemens (mS) microSiemens (mS) NaCl Conversion 442 Conversion
1.0 1000 = 500 ppm 700 ppm
1.5 1500 = 750 ppm 1050 ppm
2.0 2000 = 1000 ppm 1400 ppm
2.5 2500 = 1250 ppm 1750 ppm
3.0 3000 = 1500 ppm 2100 ppm
(sorry but the forum does not allow tables just spread out the above info)
No matter how you do the math or what you use EC or TDS they all are only accurate for the time you first mixed the nutrients. After about 4 hours the plants have changed it anyway. That one reason I say to do a full change the nutrients every 7-10 dayís and adjust your Ph a minimum of twice a day.

There is a direct correlation between EC & TDS EC is used more by the rest of the world and TDS is mostly used in America. You get more of a range using TDS over EC but thatís about it. Nutrient manufactures mostly display their nutrients in terms of PPM and that is a TDS measurement. I guess you can blame them for all the confusion.