Nuclear magnetic resonance level gauging device

Details Nuclear magnetic resonance level gauging device Nuclear magnetic resonance level gauging device

1998-05-20

2000-12-26

Arana, Louis

Electricity: measuring and testing

Particle precession resonance

G01V 300

Patent

active

061665428

DESCRIPTION:

BRIEF SUMMARY
The present invention relates to a nuclear magnetic resonance (NMR) gauging device and in particular to one which may be employed in the non-invasive gauging of the quantity of a specific substance, for example fuel, present within a container, for example a fuel tank.
There exists a number of situations, for example in low or zero gravity environments; where the substance to be gauged is held in a neutrally buoyant suspension in another medium; where the substance is rapidly agitated so that its spatial distribution varies with time; or where the physical integrity of a container cannot be broken to permit a probe access to gauge the quantity of the contents; where the gauging of the quantity of a material cannot be carried out using conventional gauging devices which are based on, for example weighing, visual observation or volume measurements. Particularly there exists a problem in gauging the quantity of fuel in a fuel tank in a low gravity space environment such as may be found in orbiting satellite equipment.
A recent review of many current and proposed gauging methods is provided in "Propellant Determination Methods for Space Systems" by E. Kruzins and D. H. Parker presented at the CNES/ESA International Conference Propulsion des Vehicules Spatiaux, Toulouse November 1994 and which is incorporated herein by reference. The currently used gauges employ methods based on book-keeping, gas law or thermal methods or a combination of both book-keeping and one or other of the other two methods. Consequently these gauges have a common disadvantage that they are at their least accurate when the propellant is close to exhaustion, and the measurement accuracy is most critical.
It is the aim of the present invention to provide a gauging device suitable for operation in the low gravity environment and the other situations described previously and in which the measurement accuracy of the gauge is high even when the amount of specific substance to be measured within a container is at a reduced level.
The phenomenon of NMR is widely used in the chemical and structural analysis of materials and in medical imaging applications known as MRI. In these applications static magnetic fields of several Tesla are employed and the free precession or absorption frequencies are measured to determine what species are present.
Generally, NMR involves the detection of an induced magnetisation in an ensemble of nuclear spins of one or more species of interest within a substance which are to be determined when a resonance condition is established between an oscillating electromagnetic field and the nuclear spins in the presence of a static magnetic field of appropriate value. At this resonance condition the nuclear spins are able to resonantly absorb energy associated with the oscillating field of frequency, f and it is by monitoring the effects of the absorption of this energy on the ensemble of spins that the presence of otherwise of a particular spin species of interest may be determined.
In the present application any one of a free precession signal, a signal dependent on the change in amplitude of the oscillating electromagnetic field at resonance or an adiabatic fast passage signal may be monitored and processed to gauge the quantity of the substance of interest, for example fuel, present in a container, such as a fuel tank.
In the circumstances where the free precession signal is to be monitored then where an ensemble of nuclear spins of spin I is placed in a region of uniform static magnetic field of strength B applied parallel to the z-axis of a coordinate system a thermal equilibrium magnetisation arises from a resultant net polarisation of nuclear spins along the z-axis.
The pulsed application of an oscillating electromagnetic field of frequency w.sub.I, having its magnetic field perpendicular to the static field causes the spins to re-orientate into the x-y plane of the coordinate system, provided that the resonance condition given by equation [1] below is satisfied. The magnetisation vector then precesses about the z-axi

REFERENCES:
patent: 4166972 (1979-09-01), King et al.
patent: 5251482 (1993-10-01), Bates et al.
patent: 5818228 (1998-10-01), Menon et al.

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