Method for determining a liquid portion of a flowing medium with a nuclear magnetic flowmeter

A method for determining a liquid portion of a flowing medium with a nuclear magnetic flowmeter, wherein the nuclear magnetic flowmeter includes a measuring tube with a measuring volume, a measuring device and a magnetizing device, wherein the medium is made to flow through the measuring tube and has a liquid portion and a gaseous portion, wherein a magnetic field is generated along a magnetic field direction in the measuring volume by the magnetizing device and the medium is magnetized by the magnetic field, the method comprising:

• generating a gradient magnetic field with a strength gradient along a first gradient direction in the measuring volume by the magnetizing device;
• exposing the medium to the gradient magnetic field in addition to the magnetic field;
• carrying out the following measurement steps by the measuring device:
• generating and irradiating an excitation signal into the magnetized medium in the measuring volume for exciting nuclear magnetic resonances in the magnetized medium along the first gradient direction;
• measuring nuclear magnetic resonances excited by the excitation signal in the magnetized medium along the first gradient direction in the measuring volume as a measuring signal;
• determining frequencies of the nuclear magnetic resonances in the measuring signal along the first gradient direction;
• carrying out the following evaluation steps by the measuring device:
• assigning the determined frequencies to positions along the first gradient direction;
• assigning the nuclear magnetic resonances at the positions to the liquid portion and to the gaseous portion of the medium;
• and
• determining a liquid portion of the flowing medium in the measuring volume from the positions of the nuclear magnetic resonances of the liquid portion and the positions of the nuclear magnetic resonances of the gaseous portion of the medium;
• wherein the excitation signal includes a first excitation pulse followed by a first refocusing pulse and a second refocusing pulse followed by a second excitation pulse for compensating for nuclear magnetic resonances still present;
• wherein a spin echo is measured between the first refocusing pulse and the second refocusing pulse as a part of the measuring signal;
• wherein the second excitation pulse has a phase shift of 180° with respect to the first excitation pulse;
• wherein the excitation signal includes an MMME pulse sequence;
• wherein the MMME pulse sequence follows the second excitation pulse;
• wherein, between the second excitation pulse and the MMME pulse sequence, the gradient magnetic field is first reduced to zero and then increased again in a second gradient direction;
• wherein the second gradient direction is perpendicular to the first gradient direction;
• and
• wherein nuclear magnetic resonances excited by the MMME pulse sequence are measured and evaluated by determining at least one flow rate of the magnetized medium.