Nuclear magnetic flowmeter and method for operating a nuclear magnetic flowmeter

Abstract

A nuclear magnetic flowmeter (1) for determining the flow of a medium flowing through a measuring tube (2), having a magnetic field generator (3) having permanent magnets for generating a magnetic field interfusing the medium over a magnetic field section L.sub.M, having a pre-magnetization section L.sub.VM located within the magnetic field section L.sub.M and having a measuring device also located in the magnetic field section L.sub.M including a coil-shaped antenna (4) with the length L.sub.1 serving as a measuring antenna. At least one coil-shaped antenna (5) is provided in the pre-magnetization section L.sub.VM for generating a pulse or pulse sequence spoiling the magnetization of the medium in the direction of the magnetic field.

Claims

1. A nuclear magnetic flowmeter for determining the flow of a medium flowing through a measuring tube, having a magnetic field generator comprised of permanent magnets for generating a magnetic field interfusing the medium over a magnetic field section L.sub.M, having a pre-magnetization section L.sub.VM located within the magnetic field section L.sub.M and having a measuring device also located in the magnetic field section L.sub.M, the measuring device having at least one coil-shaped antenna with the length L.sub.1 serving as a measuring antenna, wherein at least one additional coil-shaped antenna is provided in the pre-magnetization section L.sub.VM for generating a pulse or pulse sequence spoiling magnetization of the medium in a direction of the magnetic field.

2. The nuclear magnetic flowmeter according to claim 1, wherein the coil-shaped antenna has a length L.sub.2 that has been set in accordance with the relationship: $L_2\ge 2d\left(\frac{v_{\max }-v_{\min }}{v_{\max }+v_{\min }}\right)+L_1$ wherein d is a distance between the measuring antenna and the coil-shaped antenna, v.sub.max is the maximum flow velocity, v.sub.min is the minimum flow velocity and L.sub.1 is the length of the measuring antenna.

3. The nuclear magnetic flowmeter according to claim 1, wherein the at least one additional coil-shaped antenna comprises a plurality of additional coil-shaped antennae provided in the pre-magnetization section L.sub.VM for generating a pulse or pulse sequence spoiling the magnetization of the medium in the direction of the magnetic field.

4. The nuclear magnetic flowmeter according to claim 3, wherein the coil-shaped antennae are arranged directly adjacent to one another in the direction of flow of the medium.

5. The nuclear magnetic flowmeter according to claim 3, wherein the coil-shaped antennae are arranged spaced from one another in the direction of flow of the medium.

6. The nuclear magnetic flowmeter according to claim 3, wherein the coil-shaped antennae are arranged partially directly adjacent to and partially spaced from one another.

7. The nuclear magnetic flowmeter according to claim 3, wherein the coil-shaped antennae have the same length.

8. The nuclear magnetic flowmeter according to claim 3, wherein the coil-shaped antennae have different lengths.

9. A method for operating a nuclear magnetic flowmeter for determining the flow of a medium flowing through a measuring tube, having a magnetic field generator comprised of permanent magnets for generating a magnetic field interfusing the medium over a magnetic field section L.sub.M, having a pre-magnetization section L.sub.VM located within the magnetic field section L.sub.M and having a measuring device also located in the magnetic field section L.sub.M including at least one coil-shaped antenna with the length L.sub.1 serving as a measuring antenna, comprising the steps of: generating a spoiling pulse or spoiling pulse sequence with at least one additional coil-shaped antenna being located in the pre-magnetization section that spoils magnetization of the medium in the direction of the magnetic field in the pre-magnetization section, waiting a time Δt before measuring the flow of the medium and subsequently, performing a nuclear magnetic measurement of the medium in the measuring device by exciting the magnetized medium with excitation signals and measuring measurement signals produced in the medium by the excitation signals.

10. The method according to claim 9, wherein the spoiling pulse or spoiling pulse sequence is one of P90 pulse, a P180 pulse and a saturation pulse sequence.

11. The method according to claim 9, wherein the medium flowing through the measuring tube has a flow profile with a maximum flow velocity v.sub.max and a minimum flow velocity v.sub.min, wherein the waiting time Δt is selected in accordance with the relationship: $\frac{\left(d+\frac{L_2}{2}-\frac{L_1}{2}\right)}{v_{\max }}\ge \Delta t\ge \frac{\left(d-\frac{L_2}{2}+\frac{L_1}{2}\right)}{v_{\min }}$ wherein d is the distance between the measuring antenna and the coil-shaped antenna, L.sub.2 is the length of the coil-shaped antenna and L.sub.1 is the length of the measuring antenna.

12. The method according to claim 9, wherein said spoiling pulse or spoiling pulse sequence for spoiling the magnetization of the medium in the direction of the magnetic field is produced by a plurality of coil-shaped antennae which simultaneously generate pulses.

13. The method according to claim 9, wherein said spoiling pulse or spoiling pulse sequence for spoiling the magnetization of the medium in the direction of the magnetic field is produced by a plurality of coil-shaped antennae which generate pulses staggered in time.

14. The method according to claim 9, wherein said spoiling pulse or spoiling pulse sequence for spoiling the magnetization of the medium in the direction of the magnetic field is produced by a plurality of coil-shaped antennae which are spaced from each other.

15. The method according to claim 9, wherein said at least one additional coil-shaped antenna is a plurality of coil-shaped antennae which are directly adjacent to each other for producing said spoiling pulse or spoiling pulse sequence for spoiling the magnetization of the medium in the direction of the magnetic field.

16. The method according to claim 9, wherein said at least one additional coil-shaped antenna is a plurality of coil-shaped antennae which are partially directly adjacent and partially spaced from each other for producing said spoiling pulse or spoiling pulse sequence for spoiling the magnetization of the medium in the direction of the magnetic field.

17. The method according to claim 11, comprising the addition step of performing further nuclear magnetic measurements with a respective time lag Δτ relative to one another.

18. The method according to claim 17, wherein a spin-lattice-relaxation time T.sub.1 is determined from measured values of said further nuclear magnetic measurements.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 shows a first embodiment of a nuclear magnetic flowmeter according to the invention,

(2) FIG. 2 shows a second embodiment of a nuclear magnetic flowmeter according to the invention, and

(3) FIG. 3 shows a third embodiment of a nuclear magnetic flowmeter according to the invention.

DETAILED DESCRIPTION OF THE INVENTION

(4) The essential elements of a first embodiment of a nuclear magnetic flowmeter 1 according to the invention are shown in FIG. 1. The nuclear magnetic flowmeter 1 has a measuring tube 2 with medium flowing through it, whose flow is to be determined. The medium can include one or several phases. For determining the flow, the nuclear magnetic flowmeter 1 has a magnetic field generator 3, which is arranged around the measuring tube 2. The magnetic field generator, which can comprise one or more permanent magnets, generates a magnetic field that interfuses the measuring tube 2 over a magnetic field section L.sub.M. A coil-shaped antenna 4 is provided at the rearward end of the magnetic field generator 3 in terms of flow direction of the medium, which is used for generating excitation pulses exciting the medium as well as for detecting measuring signals caused by the excitation signals in the medium. The coil-shaped antenna 4 has a length L.sub.1. The section interfused with the magnetic field in front of the coil-shaped antenna 4 is used for pre-magnetization of the medium flowing through the measuring tube 2 and is called pre-magnetization section L.sub.VM. A coil-shaped antenna 5, which is arranged around the measuring tube 2, is provided in the pre-magnetization section L.sub.VM according to the invention. The coil-shaped antenna 5 generates a pulse spoiling the magnetization of the medium in the direction of the magnetic field generated by the magnetic field generator 3 or a pulse sequence spoiling the magnetization of the medium in the direction of the generated magnetic field and has a length L.sub.2. The magnetization of the medium is destroyed for a section L.sub.3 of the medium. In FIG. 3, the situation is shown in which the section of the destroyed magnetization L.sub.3 corresponds to the length L.sub.2 of the antenna 5.

(5) After the magnetization of the medium has been destroyed, it is formed again in the magnetic field generated by the magnetic field generator 3. This occurs in an effective pre-magnetization section L.sub.VMeff, between the antenna 5 and the antenna 4. The antenna 5 generating the spoiling pulse is spaced from the measuring antenna 4 by a distance d.

(6) A further embodiment of the nuclear magnetic flowmeter 1 according to the invention is shown in FIG. 2. The same elements have the same reference numbers in both embodiments.

(7) The fundamental difference between the embodiment shown in FIG. 1 and the embodiment shown in FIG. 2 is that several coil-shaped antennae 5 are provided for generating a pulse spoiling the magnetization of the medium or pulse sequence spoiling the magnetization of the medium. The antennae 5 are arranged around the measuring tube 2. FIG. 2 shows five such antennae 5. The antennae 5 are arranged spaced from one another, around the measuring tube 2.

(8) FIG. 3 shows a third embodiment of the nuclear magnetic flowmeter 1 in which the same elements as shown in FIG. 2 have the same reference numbers. This embodiment differs from that of FIG. 2 only in that the coil-shaped antennae are arranged directly adjacent to one another in the direction of flow of the medium instead of being spaced from each other. If all antennae simultaneously generate a spoiling pulse, section L.sub.3 is the area of the pre-magnetization section, in which the magnetization of the medium has been destroyed.