Instruction manual

Instruction Manual CyberScan PC5500/ 5000
167
The four-cell electrode
Traditionally, conductivity measurements were made with a “two cell”
electrode. This electrode used two metallic sensors, an anode and a
cathode to which ions migrated. Under the influence of DC current
the electrodes quickly became polarized. In this situation, molecules
formed at the electrode surfaces and ions migrating to the area
collect around the respective anode or cathode and actually screen it
from other ions. In essence the flow of ions stops, and current ceases
to flow. Polarization and associated errors can be minimized by using
AC voltage, the appropriate cell constant, and a large electrode
surface area. The influence of polarization can also be minimized by
the use of a four-cell electrode.
The four cell configuration consists of two cells, an outer cell and an
inner cell. Voltage is applied to the sensors of the outer cell, which in
turn generates a voltage across the sensors of the inner cell. The
inner cell is connected to a high impedance circuit and, unlike the
outer cell generates no current. Since no current is generated across
the inner cell, polarization cannot occur at the inner cell. By
measuring the voltage of the inner cell, which is adjusted to match the
reference voltage by increasing or decreasing the current through the
inner cell, one obtains a true picture of conductivity minus the
influence of polarization.
Cell Constant
Optimum Conductivity Range
4-cell
0.1
1.0
10.0
Not Available
0.01 to 20 mS/ cm
1 to 200 mS/ cm
Conductivity and Temperature
Conductivity in aqueous solutions reflects the concentration, mobility,
and charge of the ions in solution. The conductivity of a solution will
increase with increasing temperature, as many phenomena
influencing conductivity such as solution viscosity are affected by
temperature.
The relationship between conductivity and temperature is predictable
and usually expressed as relative % change per degree centigrade.
This temperature coefficient (% change per degree) depends on the
composition of the solution being measured. However, for most
medium range salt concentrations in water, 2% per degree works
well. Extremely pure water exhibits a temperature coefficient of 5.2%,
and concentrated salt solutions about 1.5%.