Conductivity Sensor / Resistivity Sensor

What does a conductivity sensor measure?

Conductivity sensors measure the ability of a solution to conduct an electrical current. It is the presence of ions in a solution that allow the solution to be conductive: the greater the concentration of ions, the greater the conductivity.

How do you calibrate a conductivity sensor?

A conductivity sensor can be calibrated against a solution of known conductivity (much like calibrating a pH sensor against a solution of a known pH). Alternatively, you can use a device that contains a range of very precise resistors that duplicate known conductivity measurements.

How many types of conductivity sensor are there?

There are two types of conductivity sensor: contacting and inductive. With contacting sensors the electrodes that measure the conductivity are in direct contact with the solution. An alternating voltage is applied to the electrodes. This causes the ions in the solution to move back and forth between the electrodes, creating a current which is measured and converted to a conductivity measurement. This type of sensor is very good for measuring low conductivity solutions where there are very few solid particles that could collect around the electrodes and interfere with the measurement, e.g., pure water. For high conductivity solutions or if the solution would corrode electrodes or contains a large amount of solid particles, inductive sensors are required. These sensors use two coils of wire encased in a plastic body. Current flowing through one coil induces a current to flow through the other. The amount of induced current is dependent on the conductivity of the solution.

What unit is conductivity measured in?

Conductivity is measured in siemens per cm (S/cm). A conductivity of 1 S/cm is actually quite high, so most conductivity measurements involve solutions where conductivity is measured in mS/cm (thousandths of a S/cm) or in μS/cm (millionths of a S/cm). Drinking water is generally between 50 and 1500 μS/cm.