EP1166081A1

EP1166081A1 - Fluid densitometer with excentrically supported float/weight assembly

Info

Publication number: EP1166081A1
Application number: EP00919001A
Authority: EP
Inventors: William Peter Stuart-Bruges
Original assignee: Sondex Ltd
Current assignee: Sondex Ltd
Priority date: 1999-03-31
Filing date: 2000-03-31
Publication date: 2002-01-02
Also published as: GB2348507B; GB9907453D0; WO2000058710A1; GB2348507A; AU3976400A

Abstract

A fluid density measurement device (10) comprises a float/weight assembly (20) with a float (22) and weight (24) pivotable about an axis (26). The float/weight assembly adopts an orientation about the horizontal axis which is dependent upon the density of the fluid in which the device is immersed. A reference wheel (74) is rotatably mounted freely and is eccentrically weighted so as always to adopt a defined orientation relative to the vertical direction. A follower wheel (72) is magnetically linked by magnets (70, 76) so as to follow the rotation of the float/weight assembly. A detector (80) detects the relative positions of the two wheels, thus sensing the orientation of the float/weight assembly regardless of the rotary position of the device as a whole. The two wheels are housed in a pressure-resistant compartment (60). As an alternative to the magnetic coupling and the detector wheels, the float/weight assembly may alter the position of a magnetic core so as to vary the transmission characteristic between two coils. The coils can be coupled to the detector in the pressure-resistant compartment by inductive windings.

Description

FLUID DENSITOMETER WITH EXCENTRICALLY SUPPORTED FLOAT/WEIGHT ASSEMBLY

Background of the Invention The present invention relates to a device for gravimetric fluid density measurement.

Fluid density measurement devices are known which are immersed in the fluid and provide a visual reading of the density of the fluid. Examples of such devices are described in detail in United States Patents US-A-4 , 353, 253 and 4,037,481. Other devices designed for total immersion in fluid can remotely measure fluid density inside pipelines, oil or water wells, or containers.

Devices in common downhole use suffer from various defects, namely sensitivity to the inclination of the device relative to the vertical, or sensitivity to local gravity effects, or, where the instrument has to be read while moving, to axial acceleration effects. A device for measuring fluid density is described in UK Patent Application No. 2 333 602, published 28^th July 1999, and

International Patent Application WO 99/37988, published 29^th July 1999, which overcomes the difficulties mentioned above. The device comprises a float and weight assembly, each mounted on links for up and down movement along an axis of movement of the device. Links connect the float and weight to a crank; the links transfer the forces from the float and weight to rotational forces acting in opposite directions on the crank, so that the position adopted by the crank will be dependent on the density of the fluid. The device will, however, only work when the device is not absolutely or close to horizontal.

US Patent 4 981 042 discloses an instrument for measuring fluid density that has two chambers, isolated from one another by a partition and a lever that is pivoted on a flexible portion of the partition. In one chamber, a float that is connected to one end of the lever is at least partially submerged in the fluid whose density is to be measured and rises or falls depending on variations in the fluid density. The float causes the part of the lever that extends into the other chamber to adopt an angular position that is indicative of the fluid density. The angle is measured by sensing circuitry housed in the other chamber .

US Patent 5 456 121 discloses a pivotable vane flow meter comprising a vane that is mounted on an axis parallel to the direction of fluid flow and that is biased, so that the vane adopts a rotary position about the axis dependent on the rate of flow of the fluid.

In the oil industry, an increasing number of wells are drilled substantially horizontally over long stretches. Previously-available gravimetric fluid density measurement devices have only been able to function in wells having a significant vertical gradient for so long as the well is not absolutely horizontal.

Summary of the Invention

The invention in its various aspects is defined in the appended claims to which reference should now be made. Advantageous features of the invention are set forth in the appendant claims. The invention has application to measurement of fluid density in oil wells. In such an operational environment problems are introduced, by the high pressures encountered. Preferred devices embodying the invention for use in such an environment are described in more detail below, and are constructed to withstand pressures of up to about 15000psi (pounds per square inch) or about 100 MPa (mega Pascals) . Both the preferred devices described below take the form of a fluid density measurement device which comprises a float/weight assembly with a float and weight, the assembly being pivotable about a longitudinal axis. The longitudinal axis can in use be exactly horizontal, and is illustrated in this orientation in the description below, but can also be inclined, provided it is not essentially vertical. The float/weight assembly adopts an orientation about the longitudinal axis which is dependent upon the density of the fluid in which the device is immersed. In the first embodiment a reference wheel is rotatably mounted freely and is eccentrically weighted so as always to adopt a defined orientation relative to the vertical direction. A follower wheel is magnetically linked or coupled so as to follow the rotation of the float/weight assembly. A detector detects the relative positions of the two wheels, thus sensing the orientation of the float/weight assembly regardless of the rotary position of the device as a whole. The two detector wheels, that is the reference wheel and the follower wheel, are housed m a pressure-resistant compartment .

In the second preferred embodiment, as an alternative to the magnetic coupling and the two detector wheels, the float/weight assembly alters the position of a cost assembly. An asymmetrical core is coaxially pivoted to act as a vertical reference, so as to vary the transmission characteristic between two coils. The coils can be coupled to the detector m the pressure-resistant compartment by inductive windings .

Brief Description of the Drawings

The invention will now be described in more detail, by way of example, with reference to the drawings, in which:

Figure 1 is a perspective view illustrating the operation of a fluid density measurement device embodying the present invention;

Figure 2 is a cross section through the weight/float assembly of the device, illustrating the principle of operation of the device of Figure 1;

Figure 3 is a perspective view illustrating the operation of the spring/bearing device of Figure 1; and Figure 4 is a perspective view illustrating a second fluid density measurement device embodying the present invention;

Figure 5 is a side sectional view illustrating a third fluid density measurement device embodying the present invention;

Figure 6 illustrates a detail in longitudinal sectional view taken on the line VI-VI in Figure 5;

Figure 7 is a view similar to Figure 5 of a fourth fluid density measurement device embodying the invention; and

Figure 8 illustrates a detail in longitudinal sectional view taken on the line VII-VII in Figure 7.

Detailed Description of the Preferred Embodiments

The preferred fluid density measurement device 10 illustrated in Figure 1 comprises an elongate housing 12 of a generally-cylindrical shape, and having a cylindrical portion 14 and an end 16. The housing 12 includes a part defining a chamber 13 which encloses a float/weight assembly 20. The float/weight assembly 20 consists of a float 22 constituting a buoyancy element, and a weight 24, all rotatable about a longitudinal axis 26. In the example illustrated, the float 22 is of prismatic shape and more particularly has an elongate quasi-cylindrical shape, as shown in perspective view in Figure 1 and in cross-section in Figure 2. The float may, however be constructed in any other convenient shape. The weight 24 is likewise of elongate shape, shown as a circular rod, mounted between circular end plates 28 which form extensions of the ends of the float in a plane perpendicular to the longitudinal axis 26.

The device has holes 18 or other apertures in the generally cylindrically-shaped housing 12 where it surrounds the float/weight assembly 20 through which fluid can enter to surround and act on the float 22 and weight 24. The float/weight assembly 20 is mounted for rotational movement about the longitudinal axis 26 by means of a stub axle 30 at one end and a rather longer axle 32 at the other. The stub axle 30 extends to and is received by a bearing 34 in the end 16 of the housing, as described in more detail below with reference to Figure 3, and the other, longer axle 32 extends through a bearing 35 m an intermediate wall 36 across the housing 12. Referring to the cross-sectional view of Figure 2, the float is of relatively low density and it is seen that the float will have a centre of buoyancy 38 which lies off the longitudinal axis 26. The float may conveniently be of plastics material, and may be hollow provided that its volume can be stabilised adequately over the required range of temperature and pressure. The weight 24 or counterweight has a density which s much greater than that of the float 22, and may conveniently be made of metal. The centre of gravity 40 of the weight lies at the centre of the weight cross-section and is likewise off the longitudinal axis 26. The float and weight are arranged at about 130° so that the notional line 42 joining the centre of the float and the centre of the weight does not pass through the axis 26. Consequently the centre of gravity of the float/weight assembly as a whole will be offset from the pivotal or rotational axis of the assembly. The assembly will tend in use to adopt an attitude in which the torque exerted by the float and that exerted by the weight are equal and opposite. The amount of buoyancy provided by the float will be dependent upon the density of the fluid, and thus the attitude of the float/weight assembly will vary in dependence upon the fluid density. The angular orientation adopted by the float/weight assembly is accordingly determined by the fluid density and it is used in this device to provide a measure of density. The device can be used over a range of angles to the horizontal; in particular it has been found to be operable up to about 20° from the vertical.

Reference is made to the above-mentioned United States Patent and the above-mentioned application for a fuller discussion of the theory. Briefly, the mathematical basis for the illustrated embodiment will now be described with reference to Figure 2 of the drawings. Consider two elements of mass Ml and M2, densities p_L and ρ₂, arranged for pivoting about an axis, where the two centres of the masses subtend an angle φ at the axis.

Let the point of attachment of the weight 1 make an angle θ with the vertical, the whole system being immersed in fluid of density p.

Then, for equilibrium:

M g sin . ^• p g sin (<

M₂ sin ( φ - - θ ) - M. sinθ

P =

M

— - sin ( φ - ^■ θ ) - — sin θ

P₂ Pi

Thus a response curve relating p to θ may be tabulated for any ratio M1/M2, and any angle φ, and any values of p, and p, . Note that the value of g is unimportant, and that, since the device is insensitive to longitudinal forces, the readings will not change with varying angles of inclination to the horizontal, since gravity may be resolved into an orthogonal component and an axial component.

The instrument illustrated is designed for use at very high pressures, for example greater than 10,000 psi (pounds per square inch) or 70 MPa (mega Pascals). When used in such fluids, i.e. liquids, at very high pressure, to be sufficiently strong the floats are generally denser than the fluids to be measured. In addition, the device may be designed to measure very low densities, e.g. of air or gas, where again the float will be denser than the fluid to be measured. The result of this is that the "float" provides a nett downwards force. This is in fact the situation illustrated in Figure 2. In other circumstances, for instance, the "weight" may in fact possess buoyancy compared with the fluid. The principles are nevertheless the same.

The bearing 34 supporting the stub axle 30 will now be described in more detail. The bearing 34 is shown in Figure 3 of the drawings and includes a compression spring 46 orientated with its axis aligned with the longitudinal axis 26 and providing some shock resistance against impacts in the longitudinal direction, that is along the axis 26. The stub axle 30 is received in a slidable compartment 48 providing a jewel bearing surface and which moves axially inside a retainer 50 which is fixed to the end 16 of the housing 12. A set screw 52 allows for adjustment of the pressure exerted by the spring 46.

At the end of the housing 12 beyond the intermediate wall 36 is a separate sealed, pressure-resistant, compartment 60. The pressure-resistant compartment 60 continues the generally-cylindrical shape of the housing 12 and has a far end plate 62. The pressure-resistant compartment 60 is thus of cylindrical shape, continuing the shape of the part of the housing surrounding the float/weight assembly 20. The intermediate wall 36 defines an end of the pressure-resistant compartment 60. At the centre of the intermediate wall 36 is a tubular portion 64 which is joined to the wall 36 and extends into the pressure-resistant compartment 60. The end 66 of the tubular portion 64 where it joins the intermediate wall 36 accommodates the bearing 35, that is the bearing is in an aperture at the centre of the wall 36 around which the end of the tubular portion is attached. The free end 68 of the tubular portion thus lies within the pressure-resistant compartment 60. The free end 68 of the tubular portion 64 is closed off. The interior of the tubular portion 64 is at the external pressure, that is the pressure of the fluid being measured, which can be very high. The interior of the pressure-resistant compartment 60, around the outside of the tubular portion 64, is at atmospheric pressure or is close to it. The longer axle 32 is relatively simply journalled in the jewelled bearing 35 and extends into the tubular portion 64. At its end within the tubular portion 64, the axle carries a magnet 70. The magnet lies transverse to the longitudinal axis 26 and is attached to the axle 32 so that rotational or pivoting movement of the float/weight assembly 20 is transmitted to the magnet 70.

The pressure-resistant compartment 60 contains two wheels 72 and 74 which are mounted for rotation on the outer surface of the tubular portion 63. The wheels 72 and 74 are preferably supported on the tubular portion by means of jewel bearings carried in respective bosses. The boss 73 for wheel 72 is shown in Figure 1. The wheel 72 is a slave or follower wheel and its boss 73 carries a small magnet 76 so as to be linked magnetically to the magnet 70 mounted on the longer axle 32 and hence to the weight/float assembly 20. That is, the wheel 72 rotates on the tubular portion 64 so as to follow the movement of the magnet 70 and hence of the axle 32. The second wheel 74 is a reference wheel and rotates freely on the tubular portion 64 without being magnetically linked. However, the second wheel 74 carries an eccentric or off-centred weight 78. As a result of the weight 78, the wheel 74 will always pivot so that the weight 78 hangs down under gravity, thereby causing the wheel 74 always to define the vertical direction. An angular measurement is made between the two wheels 72 and 74. Given that the wheel 74 always defines the vertical, and the wheel 72 follows the angular orientation of the float/weight assembly 20, the relative angular position of the wheel 72 relative to the wheel 74 is a measure of the inclination of the float/weight assembly 20 to the vertical, irrespective of the rotary orientation of the housing 12. As noted above, this angular orientation is determined by, and hence can be used to determine, the fluid density.

The angular displacement between the wheels 72 and 74 is detected electrically or electro-optically by means of a detector 80 and fed away through electrical leads (not shown) to a remote display. The detector is not subject to the high pressures of the fluid as it is within the pressure-resistant compartment.

A servo mechanism may be provided to ensure that the wheel 72 better follows the axle 32. In other arrangements, other means than the magnets illustrated may be used to couple the axle to the follower wheel 72. The detector 80 may take the form of an optical, capacitive, inductive or potentiometric detector as an alternative to the electrical/electro-optical detector mentioned above. In operation, the fluid density measurement device illustrated is lowered down an oil extraction pipeline, with the longitudinal axis 26 in line with the borehole. The device is lowered on a winch to which are attached the electrical leads which carry the measurement signal from the detector 80 to the display apparatus. The float/weight assembly 20 is mounted for rotational movement around the axis 26. The density of the fluid affects the amount of this rotation about the longitudinal axis. The degree of rotation of the axle 32, coupled to the float and weight, is measured relative to the vertical, which is defined by the eccentrically-weighted wheel 74. The device described provides an effective construction which detects the orientation of the float/weight assembly relative to the vertical when the housing is generally horizontal without the housing 12 having to be kept in any particular alignment. The wheel 74 acts as a reference pendulum in the low-pressure environment. The pressure-resistant compartment additionally provides protection for the detector mechanism such that it does not have to withstand the high pressure of the fluid. The arrangement allows full and unlimited rotation of the housing relative to the float/weight assembly, such as might occur when it tumbles down a pipe, without affecting the measurement ability of the device. The device is also capable of passing through apertures of relatively small diameter, while still making readings based on a reasonable displacement volume of fluid.

Though the term "horizontal" is used, being the normal attitude of the device as seen in Figure 1, the device is designed to operate over a range of values of inclination to the horizontal, that is, any essentially non-vertical orientation. Of course, it will not function when the axis 26 is vertical.

A second fluid density measurement device 86 embodying the invention will now be described with reference to Figure 4. In this arrangement, the housing 12 and the float/weight assembly 20 are similar to those of Figure 1, though in this case the float is shown as being of part- cylindrical or more particularly semi-cylindrical shape. The stub axle 30 is mounted in bearing 34 as before, and the longer axle 32 is likewise mounted in a bearing 88 at the end of the tubular portion 64 which replaces the bearing 35 in the intermediate wall 36. Similar reference numerals are used for similar parts, which will not be described again. In the Figure 4 embodiment, the magnet 70 and co-operating follower wheel 72 together with the reference wheel 74 are replaced by a different mechanism for detecting the angle of axle 32 relative to the vertical and transmitting this to a display. In Figure 4, an asymmetric magnetic core 90 is free for rotation by a bearing 84 on the longer axle 32. Also mounted on the axle 32, for rotation with the axle 32, is a rotary LVDT (linear variable differential transformer) coil assembly 92. The coil assembly may be a sealed unit so as to protect the bearing on which it is mounted from the fluid pressure. The core 90 hangs on the axle 32 at a fixed position by virtue of its weight. That is, the core 90 is mounted eccentrically on the axle 32 so that it always adopts an upright position under the action of gravity. In this way the core defines the vertical direction regardless of the rotational position of the housing 12 about the axis 26. In an alternative arrangement, the core could be fixed to the axle 32 and the coil assembly 92 mounted for free rotation.

The coil assembly 92 comprises two windings, the coupling between which varies in dependence upon the rotational position of these windings relative to the asymmetric magnetic core 90. The windings form a transformer and one winding receives an input signal while the other winding provides an output signal. The two windings of the coil assembly are connected by leads 94 which pass along the interior of the tubular portion 64 to a respective one of two spaced coils 96 and 98. The coils 96 and 98 are integral with the coil assembly 92 for rotation as a unit so as always to adopt a vertical orientation. The coils 96 and 98 are mounted on the axle 32 within the tubular portion 64 which extends into the pressure-resistant compartment 60. Around the outside of the tubular portion 64 in the region of the coils 96 and 98 respectively are two further coils 100 and 102. Coil 100 ιs an input coil and constitutes a primary winding to which the coil 96 on the axle 32 is a secondary winding. The coil 102 is an output coil and constitutes a secondary winding to wnich the coil 98 is a primary winding. The coupling between these respective pairs of coils is independent of their relative rotary positions.

Thus, if a signal is applied to the input primary winding 100, it is applied through the input secondary winding 96 to the input winding of the LVDT coil assembly 92. This induces a signal in the output winding of the

LVDT coil assembly, the magnitude of which depends upon the position of the core 90 and hence of the float/weight assembly relative to the vertical, as defined by the coil assembly 92. This output signal is applied to the output primary winding 98 and thence by transformer action to the output secondary winding 102. The input/output characteristic between coils 100 and 102 is thus dependent upon and therefore indicates the orientation of the float/weight assembly, which is itself dependent upon fluid density.

In this embodiment the construction is again relatively simple and yet the device can operate in a generally-horizontal orientation without the rotary position of the housing being known. The LVDT coil assembly, as described, has to withstand the full fluid pressure, though it could be kept clean in a fluid-filled or alternatively atmospheric chamber of its own. In principle it could be arranged within the pressure- resistant compartment 60 in an analogous manner to Figure 1. The pressure-resistant compartment m any event can contain the electronic components necessary to supply signals to the coils shown and to receive signals from the coils and transmit them to a remote display.

A third fluid density measurement device 110 embodying the invention will now be described with reference to

Figures 5 and 6. In this arrangement, the housing 12 and the float/weight assembly 20 are the same as in Figure 1. The stub axle 30 is mounted in bearing 34 as before, and the longer axle 32 is likewise mounted in a bearing 88 at the end of the tubular portion 64 which replaces the bearing 35 in the intermediate wall 36. Similar reference numerals are used for similar parts, which will not be described again.

Within the low-pressure compartment 60 there are now two independently co-axially mounted rotary systems. The first comprises an axle 116 which extends between a bearing 118 in the outer side of the end wall 68 of the tubular portion 64 opposed to the bearing 88 and a bearing 120 in the far end plate 62 being the end of the compartment 60. This axle 116 carries an eccentrically mounted weight 122 so that the axle always adopts a defined attitude to the vertical regardless of the orientation of the housing 12 about its longitudinal axis. The axle 116 carries four coils, two of which 96,98 correspond to the same coils in Figure 4 and are coupled with fixed windings 100,102 attached to the housing 12 so as to provide input and output transformers. The coils 96,98 are connected by wires along the axle to two further sets of coils 126,128. These coil sets are hollow 126,128 and are mounted with their axes tangential to an arc centred on the longitudinal axis of the axle 116, that is circumferentially relative to the pivot axis, as indicated in the detail of Figure 6, and are supported on the axle 116 by respective radial arms 130,132. Thus the axle is freely mounted in the bearings 118,120 so as to adopt a vertical position, with the coil sets 126,128 always at the same orientation to the vertical, and the coil sets 126,128 are coupled through the windings 96,98,100,102 to input and output electrical connections for the device.

The second rotary system mounted coaxially with the axle 116 is based on a short axle 140. This axle is accommodated within a generally cylindrical chamber 142 which actually lies within a section of the axle 116. At each end of the chamber 142 are respective jewel or needle bearings 144,146 which receive respective ends of the axle 140. As seen in Figure 5, the axle 140 carries two radial arms, namely an upper arm 148 and a lower arm 150. The arms extend respectively through an upper opening 152 and a lower opening 154 through the wall of the chamber 142 formed by the axle 116, but seen in Figure 6. Thus the only parts of the axle 116 which extend over the region of the chamber 142 are two side wall sections 156, also best seen in Figure 6. The upper and lower openings 152 and 154 extend over a sufficient circumferential extent to allow the axle 140 with arms 148,150 to pivot through an angle of about 90°. Each of the arms 148,150 terminates in a longitudinal rod 158,160 extending towards the wall 36 and passing over the tubular portion 64. In a location over the tubular portion 64, the forward ends of the rods 158,160 carry respective magnets 162,164. These magnets are outside the tubular portion 64 whereas the stub axle 32 is within the tubular portion 64. At a position opposed to the magnets 162,164, the stub axle 32 carries a pair of transverse aligned magnets 106,108. The magnets 162,164 are similarly transversely or radially aligned with their north and south poles (N and S) orientated as shown in Figure 5. The magnets attract through the wall of the tubular portion 64 so that the magnets 162,164, and thus the axle 140 to which they are mounted, adopt a rotary position which is determined by the rotary position of the float/weight assembly 20. Thus the axle 140 always adopts the same rotary position as the float/weight assembly.

At the other end of the lower rod 160 there is a curved metal core 170 in the shape of part of an annulus or ring, extending over about 90°. This moves about an arc with the axle 140 and hence the float/weight assembly 20 so as to move into and out of the hollow coil sets 126,128. The ends of the core arcuate 170, when in its central position, lie within the centre of the coil sets 126,128. As the float/weight assembly deviates from a central position the core 170 moves outwardly of one of the coil sets 126,128 and more into the other.

The coil sets 126,128 are connected to the coils 96,98 in such a way that this movement of the core can be detected. Various ways of connecting the coils are possible, using techniques known for linear variable displacement transformers. Preferably the electrical output of the central position is zero, with outputs of opposed phases and increasing magnitude resulting as the core moves to one or the other side of the central position. For example, the coil sets 126,128 can each comprise a primary winding and a secondary winding. The primary windings are connected in series to the coil 96 and the secondary windings in series to the coil 98. The windings are connected in opposition, so that a zero output results when the core 170 is in its centre position. This leads to high sensitivity.

Finally it should be noted that the rod 158 carries a counter balance weight 172 to ensure that the centre of gravity of the rotary system mounted between the bearings 118,120 is exactly on the rotary axis between them. In the embodiment of Figures 5 and 6, there are, as with Figure 1, two rotary members which are linked by the magnets and one rotary member which is weighted to stay vertical and provide a reference. The reference member and one of the others of these members are in the low-pressure compartment .

The fourth embodiment shown in Figures 7 and 8 consists of a density measurement device 200 which is suitable for use where there is less need for the mechanical components to be located within the compartment 60. In this arrangement the longer stub axle 32 extends into the tubular portion 64 and carries the coils 96 and 98, in similar manner to the second embodiment of Figure 4. Between the end plate 28 and the intermediate wall 36 is mounted an axle 210, in similar manner to the axle 140 of Figure 5 between bearings 144 and 146 in a chamber 142 in the stub axle 32. The axle 210 carries a downwardly- projecting arm 212 which passes through a single slot 214 in the wall of the chamber 142 at the underside of the axle as seen in the figures. The arm 212 carries a weight 216 and has a rearwardly extending portion 218 at the end of which is carried an arcuate core 220 similar to the core 170 of the third embodiment. The axle 210 and the core 220 carried by it are only influenced by gravity and always hang vertically as seen in Figures 7 and 8.

The stub axle 32 carries two arms 130,132 which in turn carry coil sets 126,128. The core 220 moves within the coil sets 126,128 in similar manner as previously described for the core 170 to provide a means for measuring relative rotary displacement between the axles 32 and 210. As compared with the third embodiment, a difference is that now it is the core 220 which defines the vertical and the coil sets 126,128 which move with the float/weight assembly 20 rather than vice versa. It will be noted that the arms 130,132 provide a contribution to the weight which has to be considered when establishing the correct value for the weight 24.

In both the third and fourth embodiments the construction is much simplified by having one axle pivoted on another.

Various embodiments have therefore been described but in all of them there is a float/weight assembly 20 which adopts an orientation about its longitudinal axis which is dependent upon the density of the fluid, usually liquid, in which the device is immersed, and a reference member mounted for free pivotal movement and eccentrically weighted so as to adopt a defined orientation relative to the vertical direction. The relative orientation of these two components is detected, and the different embodiments illustrate four different and preferred ways in which this can be done. The housing for the device has a compartment which is sealed against fluid pressure which accommodates at least part of the detection system.

It will be appreciated by those skilled in the art that many modifications may be made to the devices described by way of example. For example, the tubular portion 64 extending into the pressure-resistant compartment 60 may be removed and the axle 32 supported in a cage attached to the front of the intermediate wall. Alternative means can be provided for transmitting signals from the high-pressure fluid zone to the interior of the pressure-resistant compartment. Various features of the embodiments described can be used in combinations other than those shown.

Claims

1. A fluid density measurement device (10) for measuring the density of a fluid in which the device is immersed, the device comprising: a housing (12) having a chamber (13) in fluid communication with the exterior of the housing; a float/weight assembly (20) within the chamber (13) and including a first member (72) mounted for pivotal movement about a non-vertical axis and adapted to adopt an orientation about that axis which is dependent upon the density of the fluid in which the device is immersed; a second, reference member (74) within the housing and mounted for free pivotal movement and eccentrically weighted so as to adopt a defined orientation relative to the vertical direction; and electrical or electro-optical detection means (80) for detecting the relative orientation of the first and second members, and thereby to provide an output dependent upon the density of the fluid.

2. A fluid density measurement device according to claim

1, in which the housing (12) on the one hand and the float/weight assembly (20) and the reference member (72) on the other are fully relatively rotatable about the axis (26) .

3. A fluid density measurement device according to claim

2, in which the housing (12) is of generally-cylindrical shape .

4. A fluid density measurement device according to claim 2, in which the housing (12) is elongate in shape.

5. A fluid density measurement device according to claim 2, in which the housing includes apertures (18) to the chamber (13) to allow fluid to surround the float/weight assembly (20) .

6. A fluid density measurement device according to claim

I, further comprising a pressure-resistant compartment (60) forming part of the housing and accommodating at least part of the detection means (72,74,80,96,98,100,102).

7. A fluid density measurement device according to claim 1, in which the float/weight assembly (20) includes a prismatic-shaped float (22).

8. A fluid density measurement device according to claim 7, in which the float (22) and weight (24) are elongate in shape and are pivotable about a common longitudinal axis (26) .

9. A fluid density measurement device according to claim 7, in which the float (22) is cylindrical or part- cylindrical in shape.

10. A fluid density measurement device according to claim 8, in which the weight (24) is in the shape of a rod.

II. A fluid density measurement device according to claim

6, in which the reference member (74) is mounted in the pressure-resistant compartment (60).

12. A fluid density measurement device according to claim 1, in which the reference member (74,90) is mounted for free rotation in the fluid to be measured adjacent the float/weight assembly (20) .

13. A fluid density measurement device according to claim 1, in which the detection means comprises an electrical or electro-optical detector (80) .

14. A fluid density measurement device according to claim 1, in which the movement of the first member (32,92) is coupled to the detection means (74,80) by a magnetic follower arrangement (106,108,162,164).

15. A fluid density measurement device according to claim 1, in which the detection means comprises a linear variable differential transformer (LVDT) (92) .

16. A fluid density measurement device according to claim 15, in which the LVDT (92) is coupled to two windings (96,98) located to pass signals to and from two fixed windings (100,102) of the device.

17. A fluid density measurement device according to claim 1, in which second member (90) and either the first member (92) or a member movable with it (140), are mounted such that one of these two members is pivoted on the other.

18. A fluid density measurement device according to claim 17, in which the said one member (140) is pivotally mounted in a chamber (142) within the other member and has one or more portions (148,150) extending radially through corresponding apertures (152,154) in the wall of the chamber (142) .

19. A fluid density measurement device according to claim 18, in which the second member (116) pivotally carries a third member (140) within a pressure-resistant compartment (60), the third member (140) being coupled to move with the first member (32) which is outside the pressure-resistant compartment (60) .

20. A fluid density measurement device according to claim 18, in which the second member (116) is pivoted on the first member (32).

21. A fluid density measurement device according to claim 17, in which the said one member (140) comprises one or more hollow coils (126,128) arranged circumferentially relative to the pivot axis and the other of the said two members comprises a curved core (170,220) movable into and out of the coil or coils.

22. A fluid density measurement device according to claim

17, further comprising non-contacting means for communicating between the said member of the float/weight assembly (20) and the part of the detection means (80) which is within the second part of the housing through a wall portion of the second part of the housing.

23. A fluid density measurement device according to claim

18, in which the non-contacting means comprises a magnetic follower arrangement (106,108,162,164).

24. A fluid density measurement device according to claim 18, in which the non-contacting means comprises coupled inductive windings (96,98,100,102).