Conductivity Basics

The term Conductance refers to the readiness of materials to carry an electric current. Liquids which carry an electric current are generally referred to as electrolytic conductors. The flow of current through electrolytic conductors is accomplished by the movement of electric charges (positive and negative ions) when the liquid is under the influence of an electrical field. The conductance of a liquid can be defined by its electrical properties - the ratio of current to voltage between any two points within the liquid. As the two points move closer together or further apart, this value changes. To have useful meaning for analytical purposes, a dimension needs to be given to the measurement; i.e., the physical parameters of the measurement.

By defining the physical parameters of the measurement, a standard measure is created. This standard measure is referred to as specific conductance or conductivity.

It is defined as the reciprocal of the resistance in ohms, measured between the opposing faces of 1 cm cube of liquid at a specific temperature.

What is Conductivity ?

Conductivity is the ability of a material to conduct electric current. The principle by which instruments measure conductivity is simple - two plates are placed in the sample, a potential is applied across the plates (normally a sine wave voltage), and the current is measured.  Conductivity (G), the inverse of resistivity (R) is determined from the voltage and current values according to Ohm's law.
G = I/R = I (amps) / E (volts)

Principle of Measurement : 

An electrolyte solution contains positive ions, each of which has a positive electrical charge, and negative ions, each of which have has a negative electrical charge. As illustrated in Fig. (A), a pair of metal plates placed at opposites sides in an electrolyte solution, and a battery is connected. The positive ions move toward the plate connected to the negative terminal of the battery, and the negative ions move toward the plate connected to the positive terminal of the battery, and thus electric current flows through the solution. When a voltage is applied, the ions move straight toward the respective oppositely charged metal plates, as illustrated in Fig. (B). Since conductivity is inversely proportional to resistance, the conductivity can be known if the resistance is measured as per the  Ohm's law.

The voltage (E) of the battery being constant, the conductivity (k) and the current (I) are proportional; therefore, the conductivity can be obtained if  the current is measured. 

Therefore 

Conductivity K = 1 / Resistance(R) * Length(L) / Area(S)

As per the ohm’s law Resistance(R) = Voltage(E) / Current(I)

Substituting for R

We get 

Conductivity K = Current(I)  / Voltage(E) * Length(L) / Area(S)

Conductivity Measurement :

Conductivity measures the ability of a solution to conduct an electric current between two electrodes. In solution, the current flows by ion transport. Therefore, with an increasing amount of ions present in the liquid, the liquid will have a higher conductivity. If the number of ions in the liquid is very small, the solution will be "resistive" to current flow. 

Conductivity Cell: 

A conductivity measuring cell is formed by two 1-cm square surfaces spaced 1-cm apart. Cells of different physical configuration are characterized by their cell constant, K. The flow of current through conductors is accomplished by the movement of electric charges (positive and negative ions) when the liquid is under the influence of an electrical field. 

This cell constant (K) is a function of the electrode areas, the distance between the electrodes and the electrical field pattern between the electrodes. 

Often, for considerations having to do with sample volume or space, a cell's physical configuration is designed differently. Cells with constants of 1.0 cm-1 or greater normally have small, widely spaced electrodes. Cells with constants of K = 0. 1 or less normally have large closely spaced electrodes. Since K (cell constant) is a "factor"which reflects a particular cell's physical configuration, it must be multiplied by the observed conductance to obtain the actual conductivity reading. 

For example, for an observed conductance reading of 200 µS using a cell with K = 0. 1, the conductivity value is 200 x 0. 1 = 20 µS/cm. 

The cell constant is defined as the ratio of the distance between the electrodes, d, to the electrode area, A. 

The most commonly used standard solution for calibration is 0.01 M KCl. This solution has a conductivity of 1412 µS/cm at deg C 

The Effect of Temperature   

The conductivity of a solution with a specific electrolyte concentration will change with a change in temperature. The temperature compensated conductivity of a solution is the conductivity which that solution exhibits at the reference temperature. This temperature is chosen to be either 25oC or 20oC. A measurement made at reference temperature, therefore, needs no compensation. 

Types of Conductivity Sensor.

Two Electrode Sensor Technology: Two electrode sensors provide a simple, time-proven method for conductivity measurement. Precision machined electrodes of various sizes (cell constants) are matched to the process based on their measurement range.
 Two electrode sensors are recommended for use in clean (non-coating) applications such as the following: 
 • Ultrapure Water
 • Demineralized / Deionized Water 
 • Reverse Osmosis • Water for Injection
 •Boiler Water 

Four Electrode Sensor Technology 
As the name suggests, four electrode sensors add an additional pair of electrodes to the two electrode sensor design. This second pair of electrodes provides sensor diagnostics which can then be used to compensate the measurement if scale or particulate build-up occur on electrodes.
 Four electrode conductivity sensors can withstand coating and scale which might otherwise foul a traditional two electrode sensor.

Typical applications include the following:
• Leak Detection 
• Condensate Return 
• Salinity 
• Chemical Concentration 
• Clean-In-Place

Sensor Technology (How it works) 

Two electrode conductivity measurement is based on the ability to conduct a current between two electrodes.
 The concentration of ions in the liquid are directly proportional to the conductance of the liquid. 

Pros 
• Simple, time-proven electrode design.
• Industry standard cell constants determine measurement range.
• Works best for clean applications where electrodes do not get fouled. 
• High accuracy and repeatability. 

Cons 
• Susceptible to coating and scale (no compensation).
 • Susceptible to corrosion. 
• No diagnostics. 

Four electrode sensor designs keep a constant current through two of the electrodes and let the drive voltage change. If fouling occurs then the drive voltage can be increased to compensate the measurement. 
Pros 
• Compensation for coating and build-up. 
• Wide measurement range. 
• Sensor diagnostics if fouling is too great. 
• No polarization affect. 
Cons 
• Not as accurate as two electrode sensors at low conductivity 
• Susceptible to corrosion.
 • Limited availability of analyzers. 
 • Conductive field can be distorted by pipe walls and flow cells.