METHOD FOR DETERMINING A PULSE DURATION T90 OF A 90° PULSE IN A NUCLEAR MAGNETIC MEASURING METHOD AND RESPECTIVE NUCLEAR MAGNETIC FLOWMETER

Abstract

A method for determining a pulse duration T.sub.90 of a 90° pulse in a nuclear magnetic measuring method. A signal generator has a known generator resistance R.sub.S, wherein a coil has a coil impedance Z.sub.L with a coil resistance R.sub.L and a coil reactance X.sub.L, wherein a coupling circuit has an adjustable matching capacitance C.sub.M and an adjustable tuning capacitance C.sub.T, and wherein the medium has a Larmor precession having an angular Larmor frequency ω.sub.P. The time needed for determining the pulse duration T.sub.90 of the 90° pulse is reduced by the matching capacitance C.sub.M and the tuning capacitance C.sub.T being set so that the angular resonance frequency ω.sub.0 corresponds to the angular Larmor frequency ω.sub.P and by power matching being present between the signal generator and the coil. The coil resistance R.sub.L is determined and the pulse duration T.sub.90 is determined as a function of the coil resistance R.sub.L.

Claims

1. A method for determining a pulse duration T.sub.90 of a 90° pulse in a nuclear magnetic measuring method with a circuit arrangement that has a signal generator for generating the 90° pulse that has a known generator resistance R.sub.S, a coil for transmitting the 90° pulse to a medium that has a coil impedance Z.sub.L with a coil resistance R.sub.L and a coil reactance X.sub.L according to Z.sub.L=R.sub.L+jX.sub.L, a coupling circuit has a matching capacitor having an adjustable matching capacitance C.sub.M and a tuning capacitor (13) having an adjustable tuning capacitance C.sub.T, and an angular resonance frequency ω.sub.0, the method comprising: magnetizing the medium by a magnetic field that has a Larmor precession having an angular Larmor frequency ω.sub.P, setting the matching capacitance C.sub.M and the tuning capacitance C.sub.T so that the angular resonance frequency ω.sub.0 corresponds to the angular Larmor frequency ω.sub.P and power matching is present between the signal generator and the coil, determining the coil resistance R.sub.L, and determining the pulse duration T.sub.90 as a function of the coil resistance R.sub.L.

2. The method according to claim 1, wherein the function of the coil resistance R.sub.L is $R_L=A+{\mathrm{BT}}_{90}-{\mathrm{CT}}_{90}^3+\frac{{\mathrm{DT}}_{90}}{E+T_{90}^2}$ wherein A, B, C, D and E are stored constants.

3. The method according to claim 1, wherein the coil resistance R.sub.L is determined using the generator resistance R.sub.S, the angular resonance frequency ω.sub.0, the matching capacitance C.sub.M and the tuning capacitance C.sub.T.

4. The method according to claim 1, wherein the coil reactance X.sub.L is assumed to be constant and the coil resistance R.sub.L is determined using the generator resistance R.sub.S, the angular resonance frequency ω.sub.0, and either the matching capacitance C.sub.M or the tuning capacitance C.sub.T.

5. The method according to claim 4, wherein the coil reactance X.sub.L assumed to be constant is determined using the generator resistance R.sub.S, the angular resonance frequency ω.sub.0, the matching capacitance C.sub.M and the tuning capacitance C.sub.T.

6. The method according to claim 1, wherein a measure is composed for power matching by the power of the 90° pulse at the signal generator and the power of the 90° pulse reflected at the coil are determined and a ratio between the reflected power of the 90° pulse and the power of the 90° pulse determined.

7. The method according to claim 2, wherein the constants A, B, C, D and E are determined from a system of equations, wherein the system of equations is formed by the resistance R.sub.L being determined for at least five different pulse durations T.sub.90 of the 90° pulse and the respective resistance R.sub.L, and by the respective pulse duration T.sub.90 being applied to the equation $R_L=A+{\mathrm{BT}}_{90}-{\mathrm{CT}}_{90}^3+\frac{{\mathrm{DT}}_{90}}{E+T_{90}^2}.$

8. The method according to claim 1, wherein at least one of the matching capacitance CM and the tuning capacitance C.sub.T is scaled in that a quotient formed of the angular resonance frequency ω.sub.0 as dividend and a reference angular resonance frequency ω.sub.0.sup.n as divisor is multiplied by at least one of the matching capacitance C.sub.M and the tuning capacitance C.sub.T.

9. The method according to claim 8, wherein the magnetic field is generated using permanent magnets and the reference angular resonance frequency ω.sub.0.sup.n is determined at a temperature of the permanent magnets.

10. A nuclear magnetic flowmeter for a nuclear magnetic measuring method, comprising: a circuit arrangement having a signal generator for generating a 90° pulse, a coil for transmitting the 90° pulse to a medium, a coupling circuit and a angular resonance frequency ω.sub.0, and a control unit wherein the signal generator has a known generator resistance R.sub.S, wherein the coil has a coil impedance Z.sub.L, with a coil resistance R.sub.L and a coil reactance X.sub.L according to the relationship Z.sub.L=R.sub.L+jX.sub.L, the coupling circuit has a matching capacitor having an adjustable matching capacitance C.sub.M and a tuning capacitor having an adjustable tuning capacitance C.sub.T, and wherein flowmeter is adapted to magnetize the medium by a magnetic field so that the medium has a Larmor precession with an angular Larmor frequency ω.sub.P, wherein the control unit is adapted for determining a pulse duration T.sub.90 of a 90° pulse, wherein the control unit is adapted for setting the matching capacitance C.sub.M and the tuning capacitance C.sub.T such that an angular resonance frequency ω.sub.0 corresponds to the angular Larmor frequency ω.sub.P and power matching is present between the signal generator and the coil, wherein the control unit is adapted for determining the coil resistance R.sub.L, and wherein the control unit is adapted for determining the pulse duration T.sub.90 as a function of the coil resistance R.sub.L.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0035] FIG. 1 schematically shows an embodiment of a nuclear magnetic flowmeter, and

[0036] FIG. 2 is a simplified circuit diagram of the circuit arrangement of the nuclear magnetic flowmeter of FIG. 1.

DETAILED DESCRIPTION OF THE INVENTION

[0037] FIG. 1 shows an embodiment of a nuclear magnetic flowmeter 1 for nuclear magnetic measuring methods in operation. FIG. 1 shows the measuring tube 2, the permanent magnets 3, the circuit arrangement 4 and the control unit 5 of the nuclear magnetic flowmeter 1.

[0038] The measuring tube 2 has a medium 6 flowing through it, wherein the medium 6 has several phases. The permanent magnets 3 generate a magnetic field that magnetizes the medium 6 in a volume so that the medium 6 has a Larmor precession with the angular Larmor frequency ω.sub.P. The nuclear magnetic flowmeter 1 is designed to carry out nuclear magnetic measuring methods, which, for example, determine the flow velocity of the phases of the medium 6 through the measuring tube 2 and the portions of the individual phases in the medium 6.

[0039] FIG. 2 shows a simplified circuit diagram of the essential elements of the circuit arrangement 4. The circuit arrangement 4 is comprised of the signal generator 7 for generating a 90° pulse, the coil 8 for transmitting the 90° pulse to the medium 6, and the coupling circuit 9.

[0040] The signal generator 7 essentially has the signal source 10 and the generator resistor 11, wherein the generator resistor 11 has the chosen and, thus, known generator resistance R.sub.S. The generator resistance R.sub.S represents the characteristic impedance of the signal generator 7 and is 50Ω in this embodiment.

[0041] The coil 8 has the coil impedance Z.sub.L with the coil resistance R.sub.L and the coil reactance X.sub.L according to Z.sub.L=R.sub.L+jX.sub.L.

[0042] The coupling circuit 9 in this embodiment consists of the matching capacitor 12 with the adjustable matching capacitance C.sub.M and the tuning capacitor 13 with the tuning capacitance C.sub.T and has the input 14 and the output 15. The input 14 of the coupling circuit 9 is connected to the signal generator 7 and the output 15 of the coupling circuit 9 is connected to the coil 8. Thereby, the tuning capacitor 13 and the coil 8 are connected in parallel and the signal source 10, the generator resistor 11, the matching capacitor 12 and the parallel circuit consisting of the tuning capacitor 13 and the coil 8 are connected in series.

[0043] The circuit arrangement 4 is a resonant circuit that has the angular resonance frequency ω.sub.0, wherein the angular resonance frequency ω.sub.0 is determined by the topology of the circuit arrangement 4, the generator resistance R.sub.S, the matching capacitance C.sub.M, the tuning capacitance C.sub.T and the coil impedance Z.sub.L.

[0044] The generator resistor 11, the matching capacitor 12, the tuning capacitor 13 and the coil 8 are all devices in the circuit arrangement 5, whereas the generator resistance R.sub.S, the matching capacitance C.sub.M, the tuning capacitance C.sub.T and the coil impedance Z.sub.L are all properties of the above devices.

[0045] The control unit 5 in this embodiment determines a pulse duration T.sub.90 of a 90° pulse in nuclear magnetic measuring methods using the method with the following method steps:

[0046] In a first method step, the matching capacitance C.sub.M of the matching capacitor 12 and the tuning capacitance C.sub.T of the tuning capacitor 13 are set so that the angular resonance frequency ω.sub.0 corresponds to the angular Larmor frequency ω.sub.P of the magnetized medium 6 and that power matching is present between the signal generator 7 and the coil 8.

[0047] In a second method step, the coil resistance R.sub.L is determined in that the equations for the circuit arrangement 4 according to Kirchhoff's circuit laws are solved for the coil resistance R.sub.L and the coil resistance R.sub.L is determined using the generator resistance R.sub.S, the angular resonance frequency ω.sub.0, the matching capacitance C.sub.M and the tuning capacitance C.sub.T.

[0048] In a third method step, the pulse duration T.sub.90 is determined from a function of the coil resistance R.sub.L, which is described by the equation

[00007]$R_L=A+{\mathrm{BT}}_{90}-{\mathrm{CT}}_{90}^3+\frac{{\mathrm{DT}}_{90}}{E+T_{90}^2}$

[0049] In this, A, B, C, D and E are previously stored constants. Determination is carried out in that the equation is solved for the pulse duration T.sub.90 and the determined coil resistance R.sub.L is used.

[0050] In this embodiment of the invention, the matching capacitance C.sub.M and the tuning capacitance C.sub.T are scaled with a reference angular resonance frequency ω.sub.0.sup.n determined at one temperature of the permanent magnets, whereby the impact of a fluctuation of the temperature of the permanent magnets on the matching capacitance C.sub.M and the tuning capacitance C.sub.T is compensated. For scaling, a quotient is first formed from the angular resonance frequency ω.sub.0 as dividend and the reference angular resonance frequency ω.sub.0.sup.n as divisor. The quotient is thus ω.sub.0/ω.sub.0.sup.n. Then, the quotient is multiplied by the matching capacitance C.sub.M or the tuning capacitance C.sub.T. Thus, the scaled matching capacitance is

[00008]$C_M^n=\left(\frac{{\omega }_0}{{\omega }_0^n}\right).Math.C_M.Math.$

and the scaled tuning capacitance is

[00009]$C_T^n=\left(\frac{{\omega }_0}{{\omega }_0^n}\right).Math.C_T.$

[0051] In the equations, the upper index n indicates the scaled matching capacitance C.sub.M and the scaled tuning capacitance C.sub.T.

[0052] The stored constants A, B, C, D and E have been determined in that a system of equations was formed using the above-described method. The system of equations was formed in this embodiment in that the coil resistance R.sub.L was determined for ten different pulse durations T.sub.90 of the 90° pulse and the respective, determined coil resistance R.sub.L and the respective pulse duration T.sub.90 were each applied to an equation according to

[00010]$R_L=A+{\mathrm{BT}}_{90}-{\mathrm{CT}}_{90}^3+\frac{{\mathrm{DT}}_{90}}{E+T_{90}^2}.$

[0053] The system of equations has five unknowns, namely A, B, C, D and E and has ten equations, wherein the coil resistance R.sub.L and the pulse duration T.sub.90 are known in each of the ten equations. Consequently, the system of equations is over-determined and the constants A, B, C, D and E were determined from the system of equations. Due to the over-determination, the accuracy of the determination of the constants A, B, C, D and E is increased compared to determination with the required at least five equations.