Download Principles of Conductivity Measurement

Principles of Conductivity Measurement
Importance of conductivity in process
technology
The Electrolytic conductivity is important for
the characterization of liquids in laboratories
and factories. Measurements of single samples
or a continuous process facilitates monitoring
the health of the system.
Conductivity is a measure for the total ion
concentration in a sample. The more acids,
bases and salts present in a solution, the
higher the conductivity. In water and waste
water mainly ions of dissolved salts are present.
Therefore, conductivity is a likely indicator for
the salt contamination of water and waste
water. Additionally the determination of the
salt content in food is very important.
Conductivity measurement
The cell constant
Electrolytic conductivity is determined by
a resistance measurement. In the easiest
case the measurement cell consists of two
metal electrodes. Between these alternating
voltage is applied. The more ions that are in
the sample solution, the higher§ the current
between the electrodes. With help of Ohm’s
law the conductivity is calculated. The unit
of the electrolytic conductivity is Siemens per
centimeter (S/cm).
There are countless types of conductivity
sensors whose measurement values vary by
a great margin - depending on the electrode
assembly.
1 S/cm
=
1000 mS/cm
1 mS/cm
=
1000 µS/cm
The cell constant is either known or it is
determined by means of conductivity standards
(view page 10). The cell constant has to be put
into the transmitter prior to measurement.
To compensate for the geometry of the
conductivity cell a cell constant is used:
Conductivity [S/cm] =
Measurement [S] x Cell constant [1/cm]
Conductivity ranges of aqueous solutions:
1000 mS/cm
100 mS/cm
10 mS/cm
Conductivity measurements help to determine
the purity of water
Why liquids are conductive
The electrolytic conductivity of liquids is
caused by the decomposition of dissolved
acids, bases or salts into positive cations
and negative anions. For example, common
salt (NaCl) decomposes into Sodium ions
(Na+) and Cloride ions (Cl-). The conductivity
of ultra pure water is very low, because
approximately one molecule of water (H2O) in
a billion decomposes into H+ and OH- ions. This
intrinsic conductivity of water also represents
the lower border of the conductivity scale. It is
of greatest importance in the control of pure
water in power plants, electronics and the
pharmaceutical industry.
1 mS/cm
100 μS/cm
10 μS/cm
1 μS/cm
0.1 μS/cm
0.01 μS/cm
Ultra
pure
water
– 24 –
THE MEASURE OF EXCELLENCE
TM
Pure
water
Drinking
water
Waste
water
Process
water
Conc.
acids