There are three main technologies used to measure conductivity. Selecting the proper sensor technology is dependent upon the type of application and the expected conductivity measurement range. The chart below provides some examples.
2-pole (or 2-electrode) conductivity sensors are perhaps the simplest technology to understand. These sensors consist of two plates of metal with an insulating material between them. An AC voltage is applied to the plates and current flows through the process liquid. The voltage and current are known values; thus the resistance liquid to be calculated. The distance between the electrodes and their surface area must also be considered. These physical dimensions are expressed as a value known as a “cell constant”. Smaller cell constants (0.1) are best for low conductivity liquids such a purified water. Larger cell constants (1.0) work well for general applications such as potable water.
2-pole sensors are limited in range due to the polarization effects of the charged ions in the liquid at higher conductivity levels. Since 2-pole conductivity sensors have exposed metal electrodes they should only be used on clean, non-coating liquids. These sensors are also more vulnerable to corrosion and bubbles.
The Hamilton Conducell UPW Sensor is best suited for applications such as purified water
4-pole (4-electrode) conductivity sensors function in a much the same way as 2-pole sensors however they offer a much larger measurement range. This is accomplished by using a second pair of electrodes. This second pair of electrodes produce a known current which allows for measurement of the voltage drop across the electrodes. This fluctuating voltage drop is then used to compensate for polarization errors found in the two electrode design.
4-pole sensors are well-suited for applications where the conductivity can vary greatly. The electrodes are offered Titanium and Hastelloy alloys for better compatibility with strong chemicals. The compensating design provides some ability to correct for particulates and bubbles; however liquids with heavy build-up tendencies should be avoided. The exposed electrode design should also be considered in the installation. A small electrical field is generated from the electrodes thus the tip of the sensor should be fully exposed to the process liquid by 1 cm in all directions.
Two of the four electrodes present on the Conducell 4US sensor can be seen in this photo
Toroidal and electrodeless sensors are common names for inductive conductivity sensors. Embedded within the sensor are two insulated magnetic coils known as toroids. A fixed AC voltage is applied to the primary coil. The excitation of the primary coil generates a corresponding current flow within the surrounding liquid. The secondary coil reacts to the current flow and creates a magnetic field. The strength of the magnetic field directly corresponds with the conductivity of the liquid. The magnetic field from the second coil generates its own current flow which can then be measured as liquid conductivity.
The fundamental design of inductive conductivity sensors makes them well suited for applications with coating and scaling properties. The non-conductive sensor body is made from corrosion-resistance PEEK plastic which is well suited for strong acids and bases. Toroidal sensors lack the sensitivity to accurately measure low conductivity liquids such as pure water. Their physical size combined with the electrical field effect requires substantial spacing away from pipe and tank walls to avoid measurement error.
Inductive sensors are well suited for corrosive and coating liquids due to their plastic body