An ink jet printing apparatus adapted to producing images using inks having predetermined concentrations of a magnetic label material therein, includes a printhead; an ink delivery system adapted to provide inks to the printhead; and a magnetic sensor associated with the ink delivery system, said sensor being sensitive to the magnetic label material in the ink and adapted to produce a signal which is characteristic of the concentration of the label material in the ink; wherein said magnetic sensor includes a horseshoe permanent magnet having first and second pole faces and a pair of magnetic field sensors located symmetrically between said pole faces having their axes of magnetic field sensitivity aligned perpendicular to the fixed field of said permanent magnet such that no signal is produced from said fixed field.