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!
Definitions 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.
Calculations: 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.
