Fluid properties measurement device having a symmetric resonator

A fluid properties measurement device includes a symmetric resonant element having a first mass and a second mass, balanced to the first mass and coupled to the first mass by a torsional spring, having a nodal support between the first mass and the second mass. Also, a chamber having at least one opening accommodates the first mass, free of mechanical constraint and a driving and sensing assembly, is adapted to drive the first mass in torsion and sense resulting torsional movement of the first mass. The torsional spring passes through the opening which is sealed about the torsional spring at the nodal support and the second mass is free to be placed into a fluid, for fluid property measurements.

BACKGROUND

A viscometer based on the damping of a mechanical resonator can be very accurate in theory, but if installation into a fixed location introduces an unknown and immeasurable amount of intrinsic damping (that is, the amount of damping that the viscometer would experience in a vacuum) then that unknowable quantum of intrinsic damping limits accuracy. The essential problem is that of preventing the vibrations of the viscometer's resonator from leaking into the structure holding the viscometer, for example a pipe or the wall of a tank, thereby affecting the level of intrinsic damping.

Some prior art systems have relied on compliant elements such as elastomeric O-rings to isolate the vibrations to the viscometer structure. Unfortunately, the use of such O-rings limits the pressure and temperature range of viscometer usage, thereby limiting the environments in which such a viscometer can be used. Finally, even if everything about the environment (tank walls, pipe or other holding structure) were known, and O-rings that could accommodate a broad range of temperature and pressure were available, it is very difficult to make the installation process perfectly repeatable. Any time screw threads must be tightened, there is the possibility of variation that can introduce an immeasurable quantum of difference.

SUMMARY

In a first separate aspect, the present invention may take the form of a fluid properties measurement device that includes a symmetric resonant element having a first mass and a second mass, balanced to the first mass and coupled to the first mass by a torsional spring, having a nodal support between the first mass and the second mass. Also, a chamber having at least one opening accommodates the first mass, free of mechanical constraint and a driving and sensing assembly, is adapted to drive the first mass in torsion and sense resulting torsional movement of the first mass. The torsional spring passes through the opening which is sealed about the torsional spring at the nodal support and the second mass is free to be placed into a fluid, for fluid property measurements.

In a second separate aspect, the present invention may take the form of a fluid properties measurement device that includes a symmetric resonant element having a first mass and a second mass, balanced to the first mass and coupled to the first mass by a torsional spring, and having a nodal support between the first mass and the second mass. A driving and sensing assembly, adapted to drive the first mass in torsion and sense resulting torsional movement of the first mass. The symmetric resonant element defines a longitudinal passageway from near to the longitudinal end of the second mass to exit point from the first mass and electrical conductors pass through the passageway and out of the exit point. Finally, an electrical temperature measurement device is placed in the second mass and is connected to the electrical conductors, thereby providing an electrical signal reflective of a temperature through the exit point.

In a third separate aspect, the present invention may take the form of a fluid properties measurement device that has a resonator capable of resonating in a preferred anti-symmetric mode, having a first resonant frequency. The device drives the resonator to resonate in a first frequency band about the first resonant frequency. But the resonator may be caused to resonate in a symmetric mode, under some loading scenarios, the symmetric mode having a second resonant frequency that is significantly different from the first resonant frequency. The device detects frequencies within a second frequency band about the second resonant frequency, and stops and restarts the driving of the resonator when a frequency in the second frequency band is detected.

Exemplary embodiments are illustrated in referenced drawings. It is intended that the embodiments and figures disclosed herein are to be considered illustrative rather than restrictive.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT(S)

Referring toFIGS. 1 and 2, in a preferred embodiment a resonator assembly10, which forms the physical portion of a fluid properties measurement device, such as a viscometer, comprises a housing12defining a chamber14. A resonant element15is formed from an upper, enclosed mass16and a lower, exposed mass18, which are joined by a torsional spring20. Torsional spring20is made up of an upper torsional spring22, a nodal support24and a lower torsional spring26. Housing12terminates in a threaded element30and nodal support24is held in place on the interior surface of threaded element30by a pair of O-rings32. The upper mass16includes a magnet40, defining a polarization vector42(FIG. 1), which is driven torsionally by a pair of electromagnetic coils44defining polarization vector46(FIG. 1). The entire resonator assembly may be fitted into a threaded hole in a wall50(FIG. 2).

Assembly10has the advantage that the resonant element15may be removed by sliding it out, and another, similar member may then be installed. Some applications such as use in a corrosive or abrasive particle rich environment, wear down the exposed mass18, making replacement necessary.

ForFIGS. 3-6, all reference numbers for like elements are given the same reference number as inFIG. 2, but with 100, 200, 300 or 400 added, per the formula (FIG. #−2)×100. In a general discussion of the effects of design variations, the reference numbers ofFIG. 1will be taken to apply to reference numbers for all like elements in related embodiments. Resonator assembly110, shown inFIG. 3, is much the same as assembly10, but in this instance the resonant element115, is not removable from housing112. Accordingly, O-Rings32are replaced with a more robust seal132, made of an annulus of resiliently deformable material that is permanently affixed in place. Assembly10can be made with resonant element15machined as all one piece or with nodal24, upper mass16and lower mass18added to torsional spring20.

Referring toFIG. 4, a resonator assembly210is similar to assemblies10and110, except that the flexibility to permit torsion through a nodal support226that forms part of the seal of chamber214, is provided by physical design of resonant element215. An inner rod that serves as a torsion spring222is mounted in upper mass216and lower mass218. In turn, masses216and218are mounted into an outer tube224, which is sealed into nodal element226. The torsional flexibility of rod222and tube224permits the coupled torsional flexure of masses216and218; the tube224being rigidly affixed inside rigid nodal element226.

Referring toFIG. 5, resonator assembly310is much like assembly210, but instead of having a central rod222, assembly310has a central or inner tube322. Tube322tends to be more naturally flexible than rod222, and its characteristics can be chosen to achieve a desired effect. Also, exterior masses360and370can also be chosen to achieve a desired effect.

The rotational inertia of a cylinder is proportional to the fourth power of its radius. Accordingly, embodiments having radially expanded cylinders for masses16and18are dominated by these cylinders and the resonant frequency is determined by the spring constant of the torsional spring20and the rotational inertia of the end masses16and18. Such a system is referred to as a “lumped constant” system. The lumped constant systems10,110,310and410provide greater design flexibility and can be made to have a relatively low resonant frequency. Embodiment210, is a “distributed constant” system, and by contrast, must be made longer than a similar lumped constant system to have a comparably low resonant frequency. It is well known that the shearing of a fluid by a torsional resonator takes place in a boundary layer the thickness of which is inversely related to the frequency of vibrations. A thicker boundary layer can be advantageous for measuring properties of inhomogeneous fluids, such as emulsions and suspensions.

Resonator assembly410, shown inFIG. 6, is similar to assembly310, but having a longitudinal space450defined in magnet assembly442and mass416, and another space452defined in bottom mass418, thereby accommodating a twisted wire pair460, connected to an electrical temperature measurement device462. In one preferred embodiment device462is a platinum resistance thermometer, whereas in another preferred embodiment device462is a thermocouple welded into the end of mass418. A sealing element464permits wire pair460to exit, while keeping fluids out of the chamber414.

Outer tubes224and like elements that are exposed to the fluid being measured, are typically made of stainless steel, such as 316 stainless steel, to avoid damage from corrosion. Interior parts may be made of stainless steel, brass, ceramic, or any material with low and well-characterized intrinsic damping characteristics.

Assembly such as10preferably resonates in an anti-symmetric mode, in which second mass18vibrates in 180 degree opposite phase to first mass16. When in anti-symmetric mode, nodal support24is situated at the natural node of the resonator. There is a degenerate symmetric mode, however, in which first mass16and second mass18vibrate in phase with one another. In the symmetric mode, nodal support24is not at a natural node of the resonator and the connection to the housing acts to damp the resonant element15, leading to a false reading. Assembly10is carefully designed so that the frequency of the symmetric mode is sufficiently far from the frequency of the anti-symmetric mode, that the influence of the fluid is very unlikely to cause accidental excitation of the symmetric mode. To further protect the system, the frequency is checked regularly and if it enters a band defined around the symmetric mode, then system excitation is stopped and restarted, to bring resonant element15vibration back to the anti-symmetric mode.

Among the advantages of these embodiments10,110,210,310and410is that they provide a well contained resonant system, with little energy leakage through mounting threads30because of the balanced resonant element15. Accordingly, the details of installation make little difference to the operation, and therefore accuracy, of the resonant assembly.