The flow sensor consists of three coils wound on the soft Iron bobbin.
The primary coil is in the middle and is excited with a constant current (200mA) source at a constant frequency (400Hz to 700Hz).
The two secondary coils are wound symmetrically on either side of the primary coil on the same core.
Due to magnetic flux generated by primary coil, secondary coil will have induced voltages, due to transformer action.
When this flow meter is inserted in flowing Sodium, magnetic field interacts with Sodium and produces eddy currents which influences signal output.
When Sodium is static, voltage generated on secondary coil is same because only transformer voltage in the coil is available & the motion voltage is zero. Hence resultant differential secondary voltage is zero.
When flow is established across the flow meter, voltage induced in upstream secondary is less than downstream secondary due to effect of motion voltage.
The difference between the two secondary coil voltages is proportional to the Sodium flow/velocity through the ECFM.