4-pole or 4-electrode sensors consist of two current and two voltage electrodes. Between the two current electrodes there is a constant electric current. With the two voltage electrodes a voltage drop is measured across the sample. This voltage drop is directly related to the conductivity of the sample. Because of this measurement principle, 4-electrode sensors have a much broader linear measurement and are excellent in applications with large variations in conductivity. They also have the advantage of not being affected by contamination in the process stream.