An entirely different design of conductivity cell called electrodeless uses electromagnetic induction rather than direct electrical contact to detect the conductivity of the liquid solution. This cell design enjoys the distinct advantage of virtual immunity to fouling (Note 1), since there is no direct electrical contact between the measurement circuit and the liquid solution.
Note 1 : Toroidal conductivity sensors may suffer calibration errors if the fouling is so bad that the hole becomes choked off with sludge, but this is an extreme condition. These sensors are far more tolerant to fouling than any form of contact-type (electrode) conductivity cell.
Instead of using two or four electrodes inserted into the solution for conductivity measurement, this cell uses two toroidal inductors (one to induce an AC voltage in the liquid solution, and the other to measure the strength of the resulting current through the solution):
The basic idea of this instrument is that a primary coil energized by AC power induces an electric current that passes through the sample liquid. This current, in turn, induces a measurable voltage in a secondary coil.
Since ferromagnetic toroids do an excellent job of containing their own magnetic fields, there will be negligible mutual inductance between the two wire coils.
The only way a voltage will be induced in the secondary coil is if there is an AC current passing through the center of that coil, through the liquid itself. If the liquid is non-conductive, the secondary coil will see no induced voltage at all despite being situated near the energized primary coil.
The more conductive the liquid solution, the more current will pass through the center of both coils (through the liquid), thus producing a greater induced voltage at the secondary coil. Secondary coil voltage therefore is directly proportional to liquid conductivity (Note 2 ).
Note 2 : Note that this is opposite the behavior of a direct-contact conductivity cell, which produces less voltage as the liquid becomes more conductive.
The equivalent electrical circuit for a toroidal conductivity probe looks like a pair of transformers, with the liquid acting as a resistive path for current to connect the two transformers together:
Toroidal conductivity cells are preferred over direct-contact conductivity cells whenever possible, due to their ruggedness and virtual immunity to fouling.
However, toroidal cells are not sensitive enough for conductivity measurement in high-purity applications such as boiler feedwater treatment and ultra-pure water treatment necessary for pharmaceutical and semiconductor manufacturing.
As always, the manufacturer’s specifications are the best source of information for conductivity cell applicability in any particular process.
The following photograph shows a toroidal conductivity probe mounted on a display board at a trade-show, along with a conductivity transmitter (to both display the conductivity measurement in millisiemens per centimeter and also transmit the measurement as a 4-20 mA analog signal):
