An apparatus and method for estimating a parameter of a fluid flowing in a tubular is disclosed. A source of a primary magnetic field is coupled to the tubular and is configured to induce the primary magnetic field in the fluid to align nuclei of the fluid in the tubular along the primary magnetic field. A transmitter transmits an excitation signal into the fluid. A receiver detects a signal from the aligned nuclei responsive to the excitation signal. A processor estimates the parameter of the fluid from the detected signal. The source of the primary magnetic field is removable from the tubular. A coil may induce a secondary magnetic field to either enhance the strength of the primary magnetic field in the tubular or substantially cancel the primary magnetic field in the tubular, for example, to reduce particle build-up in the tubular.