Abstract:
A viscometer for sensing or characterizing the stress required to shear a fluid at a given rate includes a pair of members coaxially mounted for relative rotation. Between the members is an annular gap defining a flow path for the fluid. The flow path is configured such that during differential rotation of the members, fluid is caused to flow through the annular gap that is a function of the differential rotation and the viscosity of the fluid. A sensor measures the torque or torque equivalent required to achieve such differential rotation between the members.

Description:
FIELD OF THE INVENTION  
         [0001]    This invention relates to viscometers used to measure or characterize the stress needed to shear a fluid at a given rate. In particular, this invention relates to viscometers for continuously monitoring changes in the viscosity of fluids used in or produced by a device or process including low-viscosity fluids such as engine lubricants, by monitoring the torque required to achieve differential rotation between two elements defining a flow path for the fluid there between. Such viscometers may be used for example in on-board systems to maintain the quality of engine lubricants which is essential to the proper operation and long service life of internal combustion engines or other equipment.  
         BACKGROUND OF THE INVENTION  
         [0002]    One common form of viscometer comprises two coaxial cylinders (cylinder-in-cylinder) which are rotated relative to one another while measuring, either visually or electronically, the torque, or torque equivalent, required to achieve a differential rotation speed. The flow characteristics of the fluid can be determined by interposing the fluid in an annular gap between the cylinders and for a known differential rotational speed, measuring the torque, or torque equivalent. By factoring in the physical dimensions and the drag associated with bearings or seals of the viscometer that can affect torque measurement, the viscosity of the fluid can be calculated for a particular shear rate. Typically, a viscometer is driven at a single speed and the viscosity calculated at a single shear rate to allow relative comparison of fluids. However, if desired the viscometer can also be used to more fully characterize a fluid, by measuring torque over a range of differential rotational speeds.  
           [0003]    In certain applications, viscometers are used to continuously monitor a fluid used in or produced by a device or process. The fluids can be either totally liquid or a liquid containing particulate. One method for using known coaxial cylinder viscometers in these applications is to put the viscometers in line with the fluid flow. Problems with this method include the complexity of designing the viscometers into the flow circuit, the difficulty in replacing components of the viscometers should failure occur, and accuracy issues should the fluid flow past the viscometers vary from a constant rate.  
           [0004]    One way of overcoming some of the problems associated with mounting cylinder-in-cylinder viscometers in line with the flow path is to mount the viscometers outside the main flow path. In this arrangement, the outer cylinder of the viscometers is capped to form a cup-like structure with the inner cylinder or bob inside the cup. This allows the drive for the differential rotation to be mounted quite close to the rotating elements for a more compact design and also allows maintenance issues to be more easily addressed.  
           [0005]    A problem with prior bob-in-cup viscometers used to continuously monitor a fluid used in or produced by a device or process is that a pump or other hardware is needed to control the fluid flow through the viscometers, which adds to the cost and complexity to using the viscometers. Another problem with prior bob-in-cup viscometers is that, when used to accurately measure low viscosity fluids containing particulate, particulate settling can occur resulting in inaccurate viscosity calculation. Thus, careful placement of prior bob-in-cup viscometers is critical to proper operation. Also, such viscometers are potentially subject to a number of possible sources of error due to unwanted friction and/or drag effects.  
         SUMMARY OF THE INVENTION  
         [0006]    The present invention overcomes the above-noted and other shortcomings of prior bob-in-cup viscometers by providing a relatively simple way of continuously monitoring fluid viscosity without the cost and complexity of a pump or other hardware to maintain flow through the viscometers, and without the placement issues normally needed to prevent particulate settling when measuring particulate-containing low-viscosity fluids.  
           [0007]    In accordance with one aspect of the invention, the viscometers are self pumping in order to maintain controlled fluid flow through the sections of the viscometers where the fluid flow properties are measured, essentially independent of flow rate of the fluid through its primary flow path. The self-pumping character of the viscometers is also a benefit in preventing particulate settling when used to accurately measure relatively low viscosity (e.g., 1 to 100 cSt.) fluids that may contain finely, relatively well-dispersed solids.  
           [0008]    In accordance with another aspect of the invention, the viscometer bobs and cups are designed such that relative rotation between the two elements urges the fluid to flow through the viscometers due to a pressure differential caused by the rotation.  
           [0009]    In accordance with another aspect of the invention, the flow through the viscometers is both controlled and sufficient to minimize or eliminate clogging due to any particulate settling from the fluid.  
           [0010]    In accordance with another aspect of the invention, in one embodiment, the bob comprises a hollow cylinder closed at one end adjacent the closed end of the cup and open at the other end. Extending through the wall of the bob at a location near its closed end and facing a continuous wall of the cup are a plurality of discrete circumferentially spaced openings. Differential bob/cup rotation urges fluid from a volume outside the bob through the bob and out through the discrete openings in the bob wall for passage through an annular gap between the bob and cup and into a volume outside the cup.  
           [0011]    In accordance with another aspect of the invention, in another embodiment, a plurality of discrete circumferentially spaced openings are provided in the wall of the cup near the closed end of the cup facing a continuous wall of the bob. Differential bob/cup rotation urges fluid from the volume outside the bob, through the annular gap between the cup and bob and out through the discrete openings of the cup wall to a volume outside the cup.  
           [0012]    In accordance with another aspect of the invention, in another embodiment, the wall of the bob has discrete circumferentially spaced openings near one end facing a continuous wall of the cup, and the cup has discrete circumferentially spaced openings facing a continuous wall of the bob near the end of the bob that is opposite the end of the bob containing discrete openings. Differential bob/cup rotation urges fluid from a volume outside the bob through the discrete wall openings of the bob and annular gap between the bob and cup and out through the discrete openings in the cup to a volume outside the cup.  
           [0013]    In accordance with another aspect of the invention, in another embodiment, the bob is a cylinder of finite side wall thickness open at both ends. Also, one of the open ends is spaced from the closed end of the cup an axial distance of between one half to five times the radial separation between the bob and cup, whereby differential bob/cup rotation urges fluid from a volume outside the bob through the bob, the separation between the end of the bob and closed end of the cup, and the annular gap between the bob and cup and into a volume outside the cup.  
           [0014]    In accordance with another aspect of the invention, in another embodiment, a series of alternate coaxial cylinders of finite wall thickness are alternately supported by a pair of axially spaced end plates to provide alternate coaxial bobs and cups. One end plate has a central opening providing fluid communication between a volume outside the end plate and the center cylinder. Discrete circumferentially spaced openings are provided through the cylindrical wall of at least one bob/cup near its open end facing a continuous cylindrical wall of an adjacent cup/bob. Differential rotation of the end plates urges fluid from a volume outside the viscometer through the center cylinder and separations between the bob/cup cylinders and opposed end plates and through the circumferentially spaced openings in at least one bob/cup cylinder and the annular gaps between adjacent bob/cup cylinders and out through the annular gap between the last two bob/cup cylinders into the volume outside the viscometer.  
           [0015]    In accordance with another aspect of the invention, in another embodiment, the separation between the end of at least one of a plurality of coaxial bob/cup cylinders and the opposed end plate is between one half to five times the annular gap between adjacent bob/cup cylinders. Differential rotation of the end plates urges fluid from a volume outside the viscometer through the center bob and separations between the bob/cup cylinders and opposed end plates and through the annular gaps between adjacent bob/cup cylinders and out through the annular gap between the last two bob/cup cylinders into the volume outside the viscometer.  
           [0016]    In accordance with another aspect of the invention, in another embodiment, the bob and cup are axially symmetric but non-cylindrical. Also, the bob has a coaxial bore extending all the way through the bob, and an annular gap is provided between the bob and cup that either remains the same or increases as a function of radius from the common axis of the bob and cup. Differential bob/cup rotation urges fluid from a volume outside the bob through the coaxial bore of the bob, through the gap between the bob and cup and into the volume outside the cup.  
           [0017]    In accordance with another aspect of the invention, a magnetic drive coupling is provided between the rotating element of the viscometer and the viscometer drive motor.  
           [0018]    In accordance with another aspect of the invention, in one embodiment, the rotating element is the driven magnet of the magnetic drive coupling and is surrounded by the driving magnet, allowing the rotating element to self-locate centrally in the magnetic field of the driving magnet, thus eliminating the need for end thrust location of the rotating element, which is a possible source of error due to friction on the thrust faces.  
           [0019]    In accordance with another aspect of the invention, the rotating element is mounted on a hollow shaft which permits fluid from a volume outside the rotating element to pass through the rotating element into a separation between the end of the rotating element and the closed end of a relatively fixed cup. Rotation of the rotating element within the cup urges fluid from a volume outside the rotating element through the rotating element and separation between the end of the rotating element and closed end of the cup and through the annular gap between the cup and rotating element and into a volume outside the cup.  
           [0020]    In accordance with another aspect of the invention, the portion of the viscometer housing carrying bearing bushes for the rotating shaft is provided with radial holes to reduce the friction effect caused by fluid between the rotating shaft and housing in order to reduce unwanted drag effects.  
           [0021]    In accordance with another aspect of the invention, in another embodiment, the driven magnet is mounted on the bottom side of the viscometer bob and is polarized north and south from one side to the other for magnetic coupling with a driving magnet on the rotor shaft.  
           [0022]    In accordance with another aspect of the invention, a bob shaft is pressed into an axial hole in the bob and has radiuses at both ends slightly smaller than half ball radiuses in insert bearings in which the bob shaft ends are received to cause the bob to move like a gyro with little effort required.  
           [0023]    In accordance with another aspect of the invention, end play between the bob shaft and insert bearings is preferably no more than 0.010 inch, whereby the viscometer may operate in any position.  
           [0024]    In accordance with another aspect of the invention, a plurality of circumferentially spaced slots are provided in the side of the cup in line with the cup bottom to allow debris and sediment entering the annular gap between the cup and bob to exit the cup and allow free flow of fluid through such annular gap.  
           [0025]    To the accomplishment of the foregoing and related ends, the invention, then, comprises the features hereinafter fully described and particularly pointed out in the claims, the following description and the annexed drawings setting forth in detail certain illustrative embodiments of the invention, these being indicative, however, of but a few of the various ways in which the principles of the invention may be employed. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0026]    In the annexed drawings:  
         [0027]    [0027]FIG. 1 is a fragmentary longitudinal section through one form of viscometer according to this invention;  
         [0028]    [0028]FIG. 2 is a longitudinal section through another form of viscometer according to this invention; and  
         [0029]    [0029]FIGS. 3 through 10 are schematic longitudinal sections through differently configured relatively rotating viscometer elements according to this invention. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0030]    The viscometers of the present invention are generally of the “bob-in-cup” type and are designed such that relative rotation between the bob and cup causes fluid flow through the viscometers due to a pressure differential created during rotation. The fluid flow through the viscometers is both controlled and sufficient to minimize or eliminate clogging due to any particulate settling from the fluid. Such viscometers are designed to detect small changes in the viscosity of low viscosity fluids such as engine lubricants, by monitoring the load imposed on a suited drive motor which may be a precision motor or a suited air motor.  
         [0031]    [0031]FIG. 1 shows one such viscometer  1  in accordance with this invention which is constructed as a unit for in-line or diagnostic chamber mounting and includes two distinct sections, a sensing section  2  and a drive motor section  3 . The sensing section  2  includes a coaxial bob  4  and cup  5 , whereas the drive motor section  3  includes a suited drive motor  6  which in this case is a precision electric motor which preferably drives the bob  4  through a conventional magnetic drive coupling  7  as described hereafter.  
         [0032]    In the embodiment shown in FIG. 1, the drive motor  6  is mounted to a locating plate  8  with its motor shaft  9  extending through an opening  10  in the plate. Attached to the motor shaft  9  is the driving magnet  11  of the magnetic drive coupling  7 . Driving magnet  11  is cylindrical in shape and surrounds the cup  5  through which the fluid is continuously circulated during monitoring of the fluid.  
         [0033]    The mounting plate  8  may be attached to the viscometer housing  15  by suitable fasteners  16  which, when tightened, cause the plate  8  to be pressed against one end of a sleeve  17 . This forces the other end of the sleeve into engagement with a clamping ring  18  which in turn presses a ring seal  19  into sealing engagement with an outturned flange  20  on the cup received in an annular groove  21  in an end wall  22  of the viscometer housing to clamp and seal the cup to the viscometer housing.  
         [0034]    Extending axially outwardly from the housing end wall  22  is a concentric hub portion  23  containing a longitudinal bore  24  concentric with the cup  5 . Mounted within the bore  24  are spaced apart ball bearing  27  and bearing bush  25  for rotatably supporting one end of a hollow shaft  26  within the bore. The hollow shaft  26  extends into the cup  5  to provide a rotating support for the bob  4  in concentric spaced relation within the cup  5 .  
         [0035]    In the FIG. 1 embodiment, the bob  4  is the driven magnet  30  of the magnetic drive coupling  7  which is surrounded by the external driving magnet  11 . The magnetic field of the external driving magnet  11  acts through the cup  5 , causing the bob  4 , which has a finite side wall thickness, to self-locate centrally within the cup  5  with the inner end of the bob spaced from the closed end  31  of the cup an axial distance that is between one half to five times the radial separation between the bob and cup. Such self-location of the bob  4  centrally within the magnetic field permits the bob shaft  26  to be installed in the bearing bushes  25  with no end thrust location on the shaft, thus removing a possible source of error due to friction on the thrust faces.  
         [0036]    The hub portion  23  of the viscometer housing  15  carrying ball bearing  27  and bearing bush  25  has additional radial holes  32  communicating with an annular groove  33  in the wall of the bore  24  between the bearing and bush to reduce the friction effects caused by fluid between the rotating shaft  26  and hub  23  to reduce unwanted drag effects. This insures that the viscosity monitoring occurs along the annular gap between the rotating bob  4  and the fixed cup  5 .  
         [0037]    A series of circumferentially spaced holes  34  are provided in the end wall  22  of the viscometer housing  15  generally in line with the upper end of the rotating bob  4  to insure continuous flow of fluid through the viscometer.  
         [0038]    The sensing section  2  of the viscometer  1  including the hub portion  23  extends through an opening  40  in the wall  41  of a diagnostic chamber  42  and may be clamped and sealed against a seal ring  43  between the viscometer housing  15  and diagnostics chamber wall  41  around the opening by the same fasteners  16  used to mount the motor plate  8  to the outwardly protruding end of the viscometer housing. Fluid enters and exits the diagnostics chamber  42  through suitable inlet and exit ports  44  and  45  in the wall of the chamber. This completely immerses the sensing section  2  of the viscometer in the fluid during operation of the viscometer  1 , which occurs by energizing the drive motor  3  to drive the external cylindrical magnet  11 . As the external cylindrical magnet  11  rotates, the magnetic field acts through the cup  5  and drives the inner magnet  30  (which in this case is the bob  4 ) and associated bob shaft  26  which runs in the fluid. Such differential bob/cup rotation creates a pumping action urging fluid from the diagnostics chamber  42  outside the bob  4  through the bob shaft  26 , then through the separation between the inner end of the bob  4  and closed end of the cup  5 , then through the annular gap  46  between the bob  4  and cup  5  and out through the exit ports  34  in the end wall  22  of the viscometer housing into the volume of fluid within the diagnostics chamber outside the cup thus insuring continuous flow of fluid across the viscometer.  
         [0039]    Measurement of the resistance to rotation (drag) of the rotating element  30  (in this case the bob within the cup  5 ) caused by the presence of the fluid in the annular gap between the bob and cup enables the viscosity of the fluid to be continuously monitored.  
         [0040]    The effects of viscosity change of the fluid have a direct affect on the motor  3  loading due to the change in drag between the relatively rotating bob and cup. The load variations which are a function of changes in viscosity are translated into motor speed or current variations which are monitored by the electronics and the controller  47  for the motor. These variations are calibrated against known viscosities and can be programmed into the electronic control system to detect very small changes in viscosity over the viscosity measurement range expected. Also, current limiting may be used to prevent any damage to the motor or equipment in unusually high viscosity or load conditions.  
         [0041]    The viscometer housing  15  and locating plate  8  as well as the cup  5  are desirably made of stainless steel.  
         [0042]    One or more other ports  48  may also be provided in the wall  41  of the diagnostics chamber  42  for use in inserting other types of sensors including dielectric sensors, temperature sensors and/or pressure sensors and the like for sensing other parameters of the fluid.  
         [0043]    [0043]FIG. 2 shows another form of viscometer  50  in accordance with this invention for continuously monitoring fluid viscosity by monitoring the load imposed on an air motor  51  instead of a precision motor. In this embodiment, the fluid enters the viscometer through slots  52  in a cover or cap  53  on the outer end of a viscometer cup  54  and flows through an annular gap  55  between the bob  56  and cup  54  and out through a plurality of discrete circumferentially spaced openings  57  in the wall  58  of the cup near its closed end  59  that face a continuous wall  60  of the bob. During differential rotation of the bob and cup, these discrete openings  57  around the circumference of the cup coaxial to the rotating bob create a pressure differential causing fluid to be pumped through the viscometer that is a function of viscometer rotational speed and fluid viscosity.  
         [0044]    In the embodiment shown in FIG. 2, the discrete openings  57  in the cup side wall  58  are located closely adjacent the closed end wall  59  of the cup to allow any debris or sediment within the fluid flowing through the viscometer to exit the cup through the openings.  
         [0045]    The bob  56  is mounted for relative rotation within the cup  54  by means of a bob shaft  61  pressed into a coaxial bore  62  in a transverse wall  63  intermediate the ends of the bob. The bob shaft  61  extends beyond opposite ends of the bob into bronze inserts  64 ,  65  pressed into coaxial recesses in the cap  53  and cup end wall  59 . Each bronze insert has a close tolerance hole with a half ball radius at the bottom for seated engagement by the ends of the bob shaft which have radiuses that are slightly smaller than the radius of the bronze inserts, whereby the bob will move like a gyro within the inserts with little effort required. Preferably, the end play between the bob shaft  61  and bearing inserts  64 ,  65  is no more than 0.010 inch, thus allowing the viscometer to operate in virtually any position.  
         [0046]    At the inner end of the bob  56  is a cylindrical recess  70  containing the driven magnet  71  of a magnetic drive coupling  72  used to drive the bob by the air motor  51  as described hereafter. The driven magnet  71  is polarized north and south from one side to the other rather than the more typical top to bottom.  
         [0047]    The air motor unit  51  that drives the viscometer bob includes an air rotor  73  mounted for rotation within a motor housing  74 . At one end of the motor housing  74  is an internally threaded bore  75  for threaded engagement by an externally threaded inner end  76  of the viscometer cup  54 . Coaxially spaced from the internal threads  75  is a larger diameter counterbore  77  in the motor housing containing a ring seal  78  for sealing engagement with a larger diameter cylindrical surface  79  on the viscometer cup  54 .  
         [0048]    Threadedly attached to the other end of the motor housing  74  is an end cap  80  containing a coaxial bore  81 . Pressed into the bore  81  is a roller bearing  82  through which the rotor shaft  83  extends to stabilize the rotor  73 . A second roller bearing  84  is pressed into a coaxial bore  85  in the motor housing  74  coaxially spaced from the end cap  80  to provide a slip fit for the rotor shaft.  
         [0049]    Threadedly connected to the inner end of the rotor shaft  83  is a magnetic driver  86  containing a pocket  87  for receipt of the driving magnet  88  of the magnetic drive coupling  72 . The pocket portion  87  of the magnetic driver  86  containing the driving magnet  88  extends into an annular recess  90  in the innermost end of the viscometer cup  54 . The axial distance between the driving magnet  88  and driven magnet  71  of the magnetic drive coupling  72  is set by locating the rotor shaft  83  within the motor housing  74  and an internal shoulder  91  on the motor housing that the viscometer cup  54  locks against.  
         [0050]    The motor housing  74  and end cap  80  as well as the rotor shaft  83  and magnetic driver  86  are desirably made of stainless steel, whereas the rotor  73  is desirably made of aluminum. Rotor  73  is provided with a plurality of circumferentially spaced panels  94 . Regulated air pressure is directed through inlet and outlet ports  95  and  96  in the motor housing  74  in alignment with the rotor for driving the rotor and thus the viscometer bob  56  magnetically coupled thereto. Attached to the outer end of the rotor shaft  83  is a hub  97  with gear teeth for reading the RPMs of the rotor.  
         [0051]    At the inner end of the rotor housing  74  are external threads  98  for threaded engagement within an opening  99  in the wall  41  of the diagnostics chamber  42  with the wet side of the viscometer  50  including the viscometer cup  54  and bob  56  extending into the fluid within the chamber. Differential bob/cup rotation urges fluid from the volume within the diagnostics chamber through slots  52  in the cup cap  53 , then through the annular gap  55  between the bob and cup and out through the discrete openings  57  in the cup wall to the volume within the diagnostics chamber outside the cup.  
         [0052]    Variations in fluid viscosity affect loading of the air motor  51  due to the change in drag between the relatively rotating bob  56  and cup  54 . However, in this case, the changes in viscosity are translated into regulated air pressure and RPMs of the rotor  73  which are monitored by the electronics in the controller  100 . Here again, these variations can be calibrated against known viscosities and can be programmed into the electronic control system  100  to make it possible to detect very small changes in viscosity.  
         [0053]    If desired, the discrete openings used to create a pressure differential and cause fluid to be pumped through the viscometer during relative bob/cup rotation may be provided in the bob in lieu of the cup or in both the bob and cup. FIG. 3 schematically shows a bob  105  and cup  106  arrangement in which the bob  105  is closed at one end  107  adjacent to the closed end  108  of the cup and is open at the other end  109 , and has discrete circumferentially spaced openings  110  through the wall  111  of the bob near the closed end  107  and facing a continuous wall  112  of the cup. In this embodiment, differential bob/cup rotation urges fluid from a volume outside the bob (e.g., within the diagnostics chamber  42 ) through the bob  105  and out through the discrete openings  110  in the bob wall  111  and then through the annular gap  113  between the bob and cup and back into the volume outside the cup.  
         [0054]    [0054]FIGS. 4 and 5 schematically show other bob/cup embodiments which are similar to the bob/cup arrangement shown in FIG. 3 except that in both FIGS. 4 and 5 the bob  115  and cup  116  have discrete circumferentially spaced radial openings  117 ,  118  through their respective walls  119 ,  120  in axially spaced apart relation from each other and facing a continuous wall  121 ,  122  of the other. In FIG. 4 the discrete openings  117  in the bob  115  are adjacent the closed end  123  of the bob and the discrete openings  118  in the cup  116  are near the open end  124  of the bob, whereas in FIG. 5 the discrete openings  117  in the bob are near the open end  124  of the bob and the discrete openings  118  in the cup are near the closed end  123  of the bob. In either case, differential bob/cup rotation urges fluid from a volume outside the bob through the bob, then through the discrete openings  117  in the wall of the bob and through the annular gap  125  between the bob and cup and out through the discrete openings  118  in the wall of the cup and return to the volume outside the cup.  
         [0055]    [0055]FIGS. 6 and 7 show two other bob/cup embodiments in accordance with this invention in which a series of alternate coaxial cylinders  130 - 135  of finite wall thickness are alternately supported by a pair of axially spaced end plates  136 ,  137 . In this arrangement, cylinder  130  is the bob in the cup formed by the cylinder  131  and end plate  137 . Cylinder  131  is the bob in the cup formed by cylinder  132  and end plate  136 . Cylinder  132  is the bob in the cup formed by cylinder  133  and end plate  137 . Cylinder  133  is the bob in the cup formed by cylinder  134  and end plate  136 . Cylinder  134  is the bob in the cup formed by cylinder  135  and end plate  137 . End plate  136  has a center opening  144  providing fluid communication between a volume of fluid outside end plate  136  (e.g., the diagnostics chamber  42  shown in FIGS. 1 and 2) and cylinder  130 . In the FIG. 6 embodiment, discrete circumferentially spaced radial openings  145  are provided through the wall of at least one cylinder near its open end facing a continuous wall of the adjacent cylinder or cylinders. For example, if the openings  145  are through either of the inner or outermost cylinders  130  or  135 , the openings  145  face only one adjacent cylindrical wall  131  or  134  whereas if the openings  145  are through any of the intermediate cylinders  131 ,  132  and  133 , the openings  145  face two adjacent cylindrical walls. In the FIG. 7 embodiment, the cylinder of at least one bob/cup is located near the end plate of the adjacent cup/bob such that the separation  146  between the end of the one bob/cup cylinder and the end plate of the adjacent cup/bob is between one half to five times the gap  147  between adjacent cylinders.  
         [0056]    In both embodiments shown in FIG. 6 and  7 , differential rotation of the end plates  136 ,  137  and thus the associated cylinders  130 - 135  urges fluid from a volume outside the end plate  136  (e.g., the diagnostics chamber  42  shown in FIGS. 1 and 2) through the center cylinder  130  and separations  146  between the bob/cup cylinders and opposed end plates (and in the case of the FIG. 6 embodiment through the circumferentially spaced openings  145  in the cylindrical wall of at least one bob/cup), then through the annular gaps  147  between adjacent bob/cup cylinders and out through the annular gap between the last of the bob/cup cylinders into the volume outside the outermost cylinder  135 .  
         [0057]    [0057]FIGS. 8 through 10 schematically show still other bob/cup embodiments in which the bobs  150  and cups  151  are axially symmetric but non-cylindrical. In each case the bobs  150  have a coaxial bore  152  all the way through the bobs. Also, the relative shapes of the bobs and cups are such that the gaps  153  there between remain the same as shown in FIG. 8 or increase as a function of radius from the common axis of the bobs and cups as shown in FIGS. 9 and 10 to facilitate pumping and removal of particulate. Differential bob/cup rotation urges fluid from a volume outside the bobs through the coaxial openings in the bobs and gaps between the bobs and cups and out into a volume outside the cups.  
         [0058]    From the foregoing, it will be apparent that the various viscometers of the present invention include novel bob/cup configurations having discrete circumferentially spaced radial openings in the wall of one or both of the bob/cup facing a cylindrical surface on the other cup/bob or in which both ends of the bob are open and the axial separation between the end of the bob and adjacent cup bottom is between one half to five times the annular gap between the bob/cup to create a pressure differential during differential bob/cup rotation to cause fluid to be pumped through the viscometer. Thus, the viscometers are capable of maintaining a fluid flow through the viscometers that is a function of viscometer rotational speed and fluid viscosity, independent of any sources used to produce a fluid pressure differential. This self-pumping feature is also important when measuring low viscosity fluids that contain particulate, in that the pumping action keeps the particulate in suspension during normal use, and redisperses particulate should settling occur during shut down.  
         [0059]    Although the invention has been shown and described with respect to certain preferred embodiments, it is obvious that equivalent alterations and modifications will occur to others skilled in the art upon the reading and understanding of the specification. The present invention includes all such equivalent alterations and modifications, and is limited only by the scope of the claims.