3.1 Electrical Conductivity analyze

        Electrical properties are their ability to conduct electrical current. Various electrical properties are resistivity, electrical conductivity, conductance, temperature coefficient of resistance, dielectric strength and thermoelectricity.

             Some of electrical properties of materials are as below:

• Conductivity : 

It is property of material which allow flow of electric current through material. It is parameter which indicates that how easily electric current can flow through material. Conductivity of material is give and take of resistivity. Electrical conductivity measure of how well material accommodates movement of an electric charge. It is ration of current density to electric field strength.

Electrical conductivity is very useful property since values are affected by such things. Therefore, electrical conductivity information can be used for measuring purity of water, checking for proper heat treatment of metals and inspecting for heat damage in some materials.

• Conductance :

The degree to which an object conducts electricity, calculated as the ratio of the current which flows to the potential difference present. This is the reciprocal of the resistance, and is measured in siemens or mhos.

• Cell Constant :

Cell constant of particular cell is determined as the ratio of the distance between the electrodes of the cell to the area of the electrodes.

cell constant K = l/A.

Applications

Some important applications in industries are

• Water treatment : process that improves the quality of water to make it appropriate for a specific end-use.  

• Leak detection : used to determine if and in some cases where a leak has occurred in systems which contain liquids and gases. 

• Clean in place : method of cleaning the interior surfaces of pipes, vessels, process equipment, filters and associated fittings, without disassembly. 

• Interface detection : to examine the ballast water before the discharge to ensure that the height of the interface is safe enough to avoid traces of oil. 

• Desalination : the process of removing salt from seawater. 

Electrolytic conductivity can be measured as the ability of a solution to carry an electric current. When a voltage in applied between two electrodes immersed in the solution, an electric current flows due to the migration of ions under the influence of the electric field. 

There are various techniques by means of which conductance of a solution may be determined. These techniques are broadly classified as - 

• null method

• direct reading method

Null method :

        Null method is a method of measurement using an electrical device, as a Wheatstone bridge, in which the quantity to be measured is balanced by an opposing known quantity that is varied until the resultant of the two is zero. 

        In this technique, the effect of an unknown resistance must be compared with the effect of a variable standard resistance under nearly identical conditions as possible. This can be realized by keeping the unknown and standard resistances can be kept in two arms of a wheat stone bridge, and varying the standard resistance until the difference between the voltages or current between the two circuits is zero. Such an arrangement is shown in fig. 3.1

When the bridge is balanced (the well-known condition is P/Q=R/S),

 We have Rc/Rv = RA/RB i.c. Rc=Rv(RA/RB)

Therefore, the unknown resistance R- can be determined by knowing the values of standard resistance values RA, RB and the variable resistance Rv.

By adjusting ration in above equation we can extend the range of conductivity.

Direct reading method

Widely used than null method.

No need to convert measured resistance in conductance.

The unbalanced current is amplified in an electronic amplifier in calibrated meter.

It is easy to work with direct method.

Figure above shows typical conductivity metre.

It applies an alternate current I at an optimal frequency to two active electrodes and measures the potential V.

Both the current and potential are used to calculate the conductance I/V.

The conductivity metre then uses the conductance and cell constant to display conductivity.

Conductivity = Cell Constant x Conductance

The current source is adjusted so that the measured potential V is equal to the reference potential Er.

A conductivity cell comprises two electrodes which may be two parallel sheets of platinum fixed in position by sealing the connecting tubes into the sides of the measuring cell.

In order to reduce the polarization effects which produce a large cell capacitance , the effective area of the electrode is greatly increased by coating the electrode with platinum black.

 Most of conductivity cells are of such a design that the solution completely surrounds the electrodes. In such cases, the conductance of the cell is given by,

1/R=σ [A/d]

Where, 

                A = Cross section area of electrodes

                d = distance between two electrodes

so, σ = [1/R]*[d/A]

Where, 

                                                                d/A = Cell Constant, K.

so, σ = [k/R]

Conductivity cells are available in different size, shapes and types.