This article will tell you how to measure conductivity and give some pointers on choosing the right type of conductivity sensor for the job.
Just like other parameters in process automation, you can measure this variable with more than one principle. The two main types are conductive and inductive sensors.
Conductivity sensor types
Let’s start with conductive or contact positioning sensors, the first type of conductivity sensor ever produced. Physicist Friedrich Kohlrausch made this discovery in 1874.
Kohlrausch showed that electrolytes have definite and constant electrical resistance, and his use of alternating current (AC) improved the accuracy of his results. If you measure a liquid’s resistance, then you can calculate its conductivity.
These days, standard conductive probes have two electrodes positioned opposite from each other. So applying an AC voltage generates a current in the product.
In this current, the cations move to the negative electrode and the anions to the positive. That means that higher free charge equals higher current flow as well as conductivity.
Now, the electrodes’ geometry, their surface area and distance from each other, can influence the reading. So for lower conductivity, you’ll likely have large surfaces and small distances. And for higher, you’ll have larger gaps and smaller surfaces.
If you have high conductivity (> 30 mS/cm), then the current density between the electrodes may disrupt your readings with polarization. The ions can create a mutual repulsion, leading to a reduced reading. So to prevent this effect, you can use a four-electrode sensor.
Here, two electrodes measure the free-ion current. The other two measure the potential difference in the product, disregarding the current. Thus, using both values to calculate conductivity negates the polarization issue.
Last but not least, we have inductive sensors. You’ll find these in applications with high conductivity or galvanic isolation.
The inductive sensor has two electromagnetic coils, one to send and one to receive, both housed in a plastic coating. Then an oscillator creates an alternating magnetic field in the transmission coil, inducing a voltage.
This voltage moves the free ions, creating a current flow. And this flow induces an alternating magnetic field and therefore flow in the reception coil. The current intensity depends on the number of free ions in the liquid. Again, if you know this value, then you can calculate the conductivity.
Because of the galvanic isolation of inductive sensors, you don’t have to worry about polarization here. And because you have no electric contact between the solution and the device, soiling has no effect either.
Which sensor you should use depends on your application and its needs. For lower conductivity, you may want a two-electrode sensor. For a wide measuring range, you should consider the four-electrode sensor. And for applications with high conductivity, inductive sensors might work better.