What are Toroidal / Inductive Conductivity Sensors and How they Work

What is Conductivity?

Conductivity can be defined as the effectiveness of electricity conducted in water.  It is basically the inverse of resistance.  So the lower the resistance, the higher the conductivity.  The 2 types of conductivity measurement used in industry are:

• Contact/Electrode Conductivity Measurement
• Toroidal (or Inductive) Conductivity Measurement

These two have completely different constructions and methods of measurement.  They also are suited to different conductivity measurement applications as well as the range of conductivity in which you are measuring.

How do you measure this?

The most common method of conductivity measurement involves the insertion of 2 electrodes in the water, applying an AC voltage, thereby allowing an AC current to pass between the two stainless steel, platinum or carbon/graphite electrodes.  Due to the relationship between voltage, current and resistance, the resistance, and hence conductivity can be determined.  These measurements are ideal for low to medium conductivity water but can be prone to false readings if the electrodes are subject to coating or fouling. This usually results in a false lower than actual reading.  It is best not to use these in water with high levels of suspended solids, oils or grease, or where the water is corrosive.

How can I avoid this issue?

Toroidal / Inductive conductivity sensors are better suited to water that is fouled, dirty, greasy or contaminated, or if the water is corrosive, but do cost more. They are also much better suited to water with higher conductivity.

Inductive sensors utilize a transmission coil and a receiving coil to effectively measure conductivity.  The term toroidal conductivity or inductive conductivity can be interchanged, as they are the same type of conductivity measurement.  An inductive conductivity sensor incorporates 2 physical coils embedded in a hermetically sealed plastic housing.  This renders them impervious to most fouling, contaminated and corrosive water, and can be used in water with high levels of suspended solids.

The 2 coils are actually toroids (toroidal sensors).  When connected to a conductivity controller suited for these types of sensors, an AC voltage is applied to the transmission (or drive) coil.  This creates a voltage in the water that surrounds this coil which results in ionic current flow in the water proportional to conductivity, thereby inducing a current in the receiving coil. The higher the conductivity, the higher the current induced into the receiving coil.

Differences between products: