Drilling Tool Luhai non-magnetic drill collars are made from non-magnetic materials with low-magnetic permeability. API monogram, serial number, OD,ID, type and size of connections are stamped on recessed mill flats. Magnetic Properties of Non-magnetic Drill Collar When MPS=1×105/4πA/m Relative magnetic permeabilityμr≤1.1010 Magnetic Field Gradient△B≤0.05μT