EC: Electrical Conductivity (of an aqueous solution) is
its ability to conduct electricity based on ionic activity
and content.
TDS: Total concentration of Dissolved Solids – mostly
inorganic salts/minerals and small amounts
of organic matter – in water.
Why Measure EC-TDS?
Knowing and controlling EC-TDS is
important in agriculture, gardening,
and hydroponics for
determining nutrient
concentration; in
aquariums and
operations for
simulating a
natural environment;
in water/wastewater
treatment; and in keeping
plumbing, pools and spas in
good condition. TDS is one of the
oldest known water measurements
and is generally considered to be a
measure of water purity.
EC and TDS? EC or TDS?
Digital meters determine Total
Dissolved Solids (TDS) by measuring
Electrical Conductivity (EC), that is,
how much current is passing from one
of the probe’s electrodes to the other
as a gauge of the ionic activity in a
solution. Some industries use the EC
measurement directly, expressed in
microsiemens (µS/cm) or millisiemens
(mS/cm), while others use readings
that have been converted to TDS,
which are expressed in parts per
million (ppm) or parts per thousand
(ppt). While ppm is more popular,
TDS is sometimes given as mg/L.
Getting to TDS
TDS can be measured directly by
weighing a sample of water, letting the
water evaporate, then comparing the
weight of the residue with the original
sample weight. Modern meters use
another method; it’s not perfect,
but much quicker and easier:
mathematically converting the EC
to TDS. There are three different
conversion factors to get from
EC to TDS; NaCl, 442™, and
KCl. These conversions are done
automatically by your meter, so you
need to determine which one works
best for your particular application. (For
instance, NaCl, with a .5 TDS conversion
factor, is designed to correlate closely
with nutrient solutions.)
Temperature Matters.
Temperature of the solution
when measured can alter
measurements up to 4%
for every degree Celsius.
However, most meters
automatically compensate
for temperature (ATC) and
bullseye on 25° Celsius, 77°
Fahrenheit, adjusting for
temps above and below.
Less expensive models require users
to compensate manually, utilizing a
conversion table.
Buying the Right Meter
It’s simple advice, but very important.
If your industry uses EC, buy an EC
meter. If it uses TDS, buy a TDS meter.
Combo meters are available, but
generally reserved for lab use where
many different types of readings may
be required. In addition, be sure to
buy the meter with the range most
tightly centered on your anticipated
readings. Common sense might tell
you to buy the meter with the widest
range, but this will decrease the
accuracy of your measurements.
So… you can utilize conversion
tables until your face turns blue, but
why not buy the meter that displays
the reading you want? No math
Caring for Your Meter
After each use, rinse the probe in tap
water or soapy water, and store dry.
Most EC-TDS units hold calibration
well. If used every day, calibrate once
a week. If readings become suspect,
you should calibrate.