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EC, PPM, TDS: A Guide Understanding Nutrient Concentration

New in the Hydroponic series

EC, PPM, TDS: A Guide Understanding Nutrient Concentration

  Many people new to hydroponic growing get confused by the variety of terms and acronyms used to discuss nutrient strength and the various means used to calculate them. I'm going to attempt to dispel some of that confusion with a little mini guide. Lets start from the top!

EC: Electrical conductivity
TDS: Total Dissolved Solids
PPM or Parts Per Million: a measure of TDS
TDS or PPM Meter: An electronic device used to test EC in a nutrient solution

How the meter functions:
  Most nutrients in a hydroponic fertilizer dissolve into ions which your plants can absorb and use to grow. Consequently, the ions in solution change the electrical conductivity of your nutrient. When you use a digital meter to check your nutrient concentration, you are testing the EC of the water.

Why we use TDS at all:
  We as growers are generally not all that interested in EC by itself because the EC doesn't really matter to the plant, all that matters is how much of the various nutrients are in the solution. Now I say EC isn't important to the plant because many different ions will affect EC in different ways IE, one gram of Sodium Chloride (table salt) in one liter of water will have a different EC than one gram of Potassium Chloride in a liter of water. Also, some organic nutrients like green sand, blood/bone meal, and fish emulsion produce very little change in EC, but when broken down by soil bacteria can yield usable nutrients.

  To try to mitigate this problem to some degree, most American TDS meters try to change EC to PPM. Converting EC to PPM is generally a very simple calculation involving a constant (either .5 or .7) multiplied by the EC in microSiemens. A quick aside, EC as read on most meters, is in milliSiemens, very important to remember.

Let Constant = .7 or .5
Siemens: 1,000 microSiemens(µS/cm) = 1.0 milliSiemens(mS/cm) = 1.0 EC(as read by the meter) = .001 Siemens(S/cm)
PPM = Constant * (EC * 1000)

Which constant should I use:
  The reason there is so much confusion about which of the two constants to use is because the manufacturers of these devices cannot agree on a reference solution. Some manufacturers use NaCl as a reference solution (.5 constant) and others use a "442" solution (40% sodium sulfate, 40% sodium bicarbonate, and 20% sodium chloride) (.7 constant). The disagreement stems from which of the calibration solutions most resembles a hydroponic nutrient solution in its makeup of ions.

When talking about nutrient concentrations, which should I use? EC or PPM:
  When two people give you the same EC readings, of the same nutrient formula, the concentrations will be exactly the same. On the other hand, if two people with two different ppm meters (one NaCl and one 442) give you the same ppm reading they will not be the same concentration. For that reason it is almost always a better idea to use EC when talking to people on these forums.

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