TDS vs. EC.- pure water does not conduct electricity.  Even a small amount of salts allows electricity to be conducted through water.  Both TDS and EC sensors measure how easily electricity flows through water, which tells us how much total salts are in the water.  Unfortunately these sensors cannot tell us what those salts are- like how much potassium is in the water.  Also, many organic fertilizers like urea don’t show up because they do not effect the salt level in the water.

EC meters measure in mS/cm or uS/cm- 1000 uS/cm is 1 mS/cm.

TDS stands for Total Dissolved Solids.  There’s two ways to measure this, through conductivity and through gravimetric testing.  Labs often use gravimetric testing because it is more accurate.  It involves boiling away all of the water and weighing the salts and organic compounds that are left.  This process is not very convenient for growers, so we generally use TDS conductivity sensors.  These sensors are really EC meters in disguise.  TDS sensors use a conversion factor to get a measurement from mS/cm to ppm.  They often multiply the EC reading by either 0.5 or 0.7 to get an approximated ppm value.  We call these readings either the 500 or 700 scale.  For an EC reading of 1, one meter will read that as 500, and the other as 700.

Why don’t we just use EC?  It doesn’t really matter what you use, as long as you know where you’re at.  Conductivity testing just helps us stay consistent and make sure that our feed concentrations are what we’re after.

Some pro tips for using conductivity sensors:

Know what scale your meter reads in.  Take note of scale your feed chart is in.  If your meter is reading in 500, and your feed chart is in 700, you’re gonna have to use a conversion chart.

Temperature affects these readings, so using a meter that also tells temperature is preferable, as these often compensate for temperature and have more stable readings.

Keep your testers calibrated periodically.  Use a calibration solution on the same scale as your meter, or use a conversion chart.

Also, it helps to test the waste water after feeding.  If the value coming out of the plants is very much higher than what you put in, it’s a good idea to put less nutrients into your next feeding.  If the value coming out of the plants is very much lower than what you put in, your plants are feeding heavily and you can feed them more concentrated nutrients next time.