A method and apparatus are set forth for measuring flow velocity of an element of interest in a compound within a pipe. In the preferred and illustrated embodiment, a static magnetic field is imposed on the flowing material of interest. Preferably, a uniform field of finite width is imposed. There is a first coil around the path for imposing an RF magnetic field at right angles to the static magnetic field. There is a relationship between the flowing material including the element, frequency of the RF field and static magnetic field intensity which aligns nuclei or electrons of the element. There is a second (detection) coil spaced downstream from the first coil, the second coil also being around the pipe. By turning the first coil off or on, the NMR or EMR signal observed at the second coil can be made to change abruptly as the "tagged" element flows to the second coil. By measuring the time of travel of the element of interest evidenced by the second coil signal, the flow velocity can be measured. An alternate embodiment utilizes a magnetic field gradient at the coils to narrow the coil field dimensions.
James D. King;Erroll S. Riewerts;William L. Rollwitz