What is the principal of Inductive Conductivity sensor?
So two things to understand.

1) Toroids have a tight magnetic field, and so the two toroids don't couple to each other in air. (or if you put magnetic rod through the holes)

2) If you put a loop of wire through the hole of a toroid, it forms a 1 turn winding on that toroid, and it will try and force a current through it.

So the conductive fluid (in blue) forms a rather resistive loop of wire through both toroids, i.e it forms two secondaries tied together. 

(R1+R2) * N^2 = V/A 







simulate this circuit &ndash; Schematic created using CircuitLab

This is an alternative to using a single toroid, where the fluid forms a shorted turn secondary. In the two toroid case there is no current out the second toroid when there is no conductivity. 

In the single toroid case there is always magnetising current through the toroid, and you need to cancel out the magnetising current, usually done by having a synchronous detector that only detects the in-phase (resistive) component. Then you still have residual core loss. So the two toroid version probably has simpler circuitry, and works better for weaker conductivity.







simulate this circuit

Wish someone told me that before I developed the single toroid type.