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How to Measure Conductivity?
Conductivity measurement is an extremely widespread and useful method, especially for quality control purposes so it is important to know How to measure conductivity?
What is conductivity?
Conductivity is the ability of a solution, a metal or a gas – in brief all materials – to pass an electric current. In solutions the current is carried by cations and anions whereas in metals it is carried by electrons. How well a solution conducts electricity depends on a number of factors:
- Mobility of ions
- Valence of ions
All substances possess some degree of conductivity. In aqueous solutions the level of ionic strength varies from the low conductivity of ultra pure water to the high conductivity of concentrated chemical samples.
Importance of Conductivity measurement
Conductivity measurement is an extremely widespread and useful method, especially for quality control purposes. Surveillance of feed water purity, control of drinking water and process water quality, estimation of the total number of ions in a solution or direct measurement of components in process solutions can all be performed using conductivity measurements. The high reliability, sensitivity and relatively low cost of conductivity instrumentation makes it a potential primary parameter of any good monitoring program. Some applications are measured in units of resistivity, the inverse of conductivity. Other applications require the measurement of total dissolved solids (TDS), which is related to conductivity by a factor dependent upon the level and type of ions present. Conductivity measurements cover a wide range of solution conductivity from pure water at less than 1×10-7 S/cm to values of greater than 1 S/cm for concentrated solutions. In general, the measurement of conductivity is a rapid and inexpensive way of determining the ionic strength of a solution. However, it is a nonspecific technique, unable to distinguish between different types of ions, giving instead a reading that is proportional to the combined effect of all the ions present.
There are two technical approaches to measure conductivity – inductive and contacting.
Inductive Conductivity Measurement:
Inductive sensors are using two electromagnetic coils usually encased in a polymer ring. An alternating voltage is applied to the driving coil, which induces a voltage in the receiving coil.
The induced current is influenced by the conductance of the solution. Due to the sealed casing, inductive sensors can be used in aggressive environments, but they are not suited for conductivities below 15 µS/cm
Contacting Conductivity Measurement:-
Contacting sensors consist of conducting electrodes being in direct contact with the medium. The primary parameters measured are voltage and the applied current. The interdependence of both parameters, described by Ohm’s law, allows for the calculation of a resistance R (measured in Ohm [Ω]) or the inverse – the conductance G (measured in Siemens [S]).
The contacting-type conductivity sensor usually consists of two electrodes, insulated from one another. The electrodes, typically 316 stainless steel, titanium-palladium alloy or graphite, are specifically sized and spaced to provide a known “cell constant.” Theoretically, a cell constant of 1.0 describes two electrodes, each being one square centimetre in area and spaced one centimetre apart.