Method for operating a nuclear magnetic flowmeter and nuclear magnetic flowmeter

A method for operating a nuclear magnetic flowmeter in which the necessary time for determining a longitudinal relaxation time constant of the medium is reduced in comparison to the prior art by at least one nuclear-magnetic measuring process being carried out on a magnetized medium in the measuring tube. The at least one nuclear-magnetic measuring process includes an inversion pulse and acquisition sequences, each acquisition sequence beginning at a starting point in time in respect to the at least one nuclear-magnetic measuring process and has an activating pulse, a first waiting interval, a first refocusing pulse, an echo interval with a duration, a second refocusing pulse, a second waiting interval and a deactivating pulse. An echo signal is measured in the echo interval, and a longitudinal relaxation time constant of the medium is determined using the echo signals.