Conductivity Measurement
The SI unit of conductivity is the siemens/metre (S/m) and this is defined as the conductance at a specified temperature between opposite faces of a cube of material with edges 1 m long. Conductance is the property which allows a material to conduct current when a potential difference is applied. The unit of conductance is siemens (S) which is the reciprocal of the ohm
The conductivity equipment used in this application indicates the value of conductivity in microsiemens per cm ( S/ cm). The relationships between these various units are as follows:
1 S/ cm = 10-6 S/ cm = l0-4 S/ m
As a reciprocal, 1 S/ cm is equivalent to 1 M / cm.
In the application described in this chapter, the conductivity of the sample liquid is determined by the measurement of the current flow when a voltage is applied between two electrodes in contact with the liquid. The electrodes are mounted in a conductivity cell which is positioned so that the sample liquid flows continuously through it.
The dimensions of the electrodes, the distance and the volume of water between them determine the cell constant (K). This constant is a factor by which the conductance of the cell is multiplied to obtain the value of conductivity of the liquid. The units of measurement and formulae are as follows:
= K.106 = K.G.106 Where: is the liquid conductivity in S/ cm
R G is the cell conductance in S
R is the cell resitance in
K is the cell constant in cm-1
A cross-section of a simple fireline conductivity cell is shown in Fig. 1. The cell includes three ring-shaped electrodes spaced equally in a loaded epoxy resin moulding. The moulding is accurately bored and therefore a precise volume of liquid is contained by it. The cell is threaded at each end and is intended for vertical mounting as an integral part of the liquid pipeline.
Conduction through the solution takes place between the central electrode and the two outer electrodes. The outer electrodes are connected to an earthed terminal of a monitoring instrument and electrical conduction is confined to within the cell and is not influenced by adjoining metal pipework.
The current flowing between the electrodes is kept to a minimum value and is alternating, as opposed to direct, to prevent polarisation. This is necessary, because gases formed by polarisation result in a loss of contact area between electrodes and liquid and hence a reduction in the accuracy of conductivity measurement.
The use of carbon composition material for the electrodes substantially eliminates electrode-to-solution contact error, which often contributes to the polarisaion effect. The carbon electrodes also provide conducting surfaces that do not require special maintenance other than periodic cleaning by means of a bottle brush. A feature of the stainless steel cells is the frosted surface of the electrodes which is essential to prevent polarisation. It is most important that this frosting is not polished away by the use of abrasive cleaners.
The electrical supply to a conductivity cell originates from a transmitter monitor type TX2/ 3P. The TX2/ 3P also includes the circuitry for measurement of the current flow through the cell and amplification to provide indication, alarm and 4 to 20 mA signal current facilities.
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