Conductivity is the primary measurement of mineral contamination in water and has been in use for this purpose for well over a century. As an on-line measurement it provides inexpensive, reliable monitoring of water quality through the various stages of treatment. The technology for conductivity measurement has improved through many generations of analog and microprocessor-controlled measuring circuits and has greatly improved accuracy and temperature compensation.
When a voltage is applied to the two electrodes of a conductivity sensor immersed in a solution, ions between the electrodes are attracted to the oppositely charged electrode and move toward it as the current flow in the solution. (Current flow in the wiring is carried by electrons.) This current flow is used for the conductivity measurement. However, if the ion migration and electrochemical reaction at the electrode surfaces are significant, these effects will interfere with the conductivity measurement. These effects are often referred to as polarization.
To minimize polarization, AC voltage is always used for conductivity measurements. This changes the polarity of the electrodes frequently enough that ions don't move or react significantly. In addition, in state-of-the-art instrumentation, the voltage and/or frequency of the applied AC are selected by internal auto-ranging to achieve the optimum compromise between obtaining adequate signal and minimizing polarization and other interferences. This auto-ranging is invisible to the user.
The sensor cell constant, determined by the geometry of the electrodes, is another factor critical to the measurement. Lower conductivity measurements generally require a lower cell constant, with relatively large cross-sectional areas of solution and closer electrode spacing in order to obtain a good signal without having to measure too high a resistance.
Higher conductivity measurement generally requires sensors with higher cell constant. This means smaller cross-sectional areas of solution with higher current density and wider electrode spacing to prevent having to measure too low a resistance. The exact cell constant requirements for a particular range depend also on the measuring circuit, the cable and the surface condition of the electrodes. An integrated measuring system is required to assure achieving specified performance across the entire measurement range.
Most process conductivity sensors in the low to moderate conductivity range now use the coaxial cell design with inner and outer electrodes which provides the same effective ratio as the easier to visualize parallel plate electrode design.
Previous work has detailed the determination of the precise cell constant with traceability to ASTM and NIST standards, how this has been incorporated into factory calibration facilities, and many other aspects of conductivity measurement.
In real installations, the vulnerability to interference from both bubbles and particulate contamination is greatly reduced if the spacing between sensor electrodes can be made wider. In addition, wider spacing gives less flow restriction and faster response which can be important in deionizer rinse-down processes. Therefore there is an advantage to providing a measuring circuit that is capable of measuring pure water accurately with a higher cell constant.
Integrated sensors described here use a nominal 0.1 cm-1 cell constant for the full range, including very accurate ultrapure water measurements. Other instrumentation must use lower cell constants with more electrode surface area spaced more closely together. In some applications, the reliability of measurement can be improved significantly by using the higher cell constant but the measuring circuit must be able to accommodate it. Range specifications should be reviewed carefully.