There are three types of conductivity probes manufactured by Hanna: two electrode probes, four ring probes, and toroidal probes.
Two Electrode Probes
The simplest design is a two electrode probe that utilizes an amperometric approach to make the measurement; a known AC voltage is applied at a specific frequency between a pair of electrodes in solution. The current produced is measured and reported in conductivity units referenced to a calibrated standard. Electrodes are made of graphite or metal. Fouling due to mineral deposits and polarization at high concentrations are drawbacks of this technology. Two electrode probes are best used in clean water applications when the conductivity remains less than 5 mS/cm. These probes have a limited range, therefore you would have a dedicated probe/meter for each range.
Four Ring Probes
Four ring probe conductivity (four ring conductivity) utilizes a potentiometric approach to taking a measurement; an alternating current is applied to the outer two “drive”electrodes to induce a current in the solution. The voltage is measured between the inner pair of electrodes in solution. The voltage is proportional to the conductivity. Electrodes are made of graphite, stainless steel or platinum. Polarization effects are reduced or negated. A risk of fringe field effect is greater with this type of probe. This happens when the measurement field, the constant current, extends outside of the probe. You only need to worry about the fringe field effect if your probe is too close to the sides of the container or pipe where you are taking an EC measurement. A good rule is to keep the probe at least an inch away from all surfaces. The distance you need to keep the probe varies, reference the manual.
NOTE: Both two and four electrode probes may incorporate an outer sleeve over the cell channel. The sleeve MUST stay in place during the measurement as this defines the volume of solution measured and the cell factor of the probe.
The third type of conductivity probe manufactured by Hanna is often found in industrial processes connected to a controller. An inductive, electrodeless or toroidal conductivity probe, uses two or more toroidal transformers which are inductively coupled side by side. These are encased in an inert plastic sheath. By applying a high frequency voltage to the drive toroid, a magnetic field develops that induces a current in the surrounding solution. A receiver toroid on the other side of the sensor measures the strength of the induced current. The strength depends on the conductivity of the solution. One benefit of this technology is that there are no polarization effects. Another benefit is in the choice of material encapsulation. This can produce chemical resistant and relative immunity to fouling. Also, solutions are not needed for calibration.