August 2017Analytical Instrumentation & Environmental Monitoring
In order to measure conductivity across a wide measuring range using a single sensor, it is safe to say that that ‘four wins’. Only sensors with four electrodes deliver reliable measurement results spanning from low to high conductivity values.
In layman’s terms, sensors with four electrodes have two electrodes opposite one another to which an alternating voltage is applied. This ensures that positively and negatively charged ions are attracted to ions of the opposite charge, causing them to move toward each other. If the charge of the electrodes switches, the ions move back to where they started. A higher number of ions moving in a liquid results in higher conductivity.
But, if there are too many ions, they repel each other, thereby inhibiting movement to some extent, which distorts the conductivity measurement: the actual value would be higher than the measured value. This is called the polarisation effect. This is where the four-electrode measurement comes into its own. In such measurements, the two additional electrodes act as observers. These two conductivity measurement watchmen keep a close eye on the polarisation and report the deviation between the measured and actual values to the transmitter. Then, the corrected measured value is output from the transmitter, which thus compensates for the polarisation effect.
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