Abstract:
Viscometer ( 80 ) with a closed bottom rotor assembly ( 51 ) rotatable by a coupling magnet ( 34 ) and a driving magnet ( 38 ) to shear a tested fluid thus imparting torque to a bob ( 30 ) mounted on a bob shaft ( 24 ) supported via bearings ( 22  and  18 ) inside rotor assembly ( 51 ). An upper chamber ( 96 ) located in the upper portion of rotor assembly ( 51 ) is at least partially filled with sample and communicates pressure with lower portion of rotor assembly ( 51 ) and rotor top via small gap ( 106 ) and small gap ( 110 ). A spiral spring ( 70 ) restricts the rotation of bob shaft ( 24 ). Magnetometer ( 10 ) measures the angular position of a top magnet ( 72 ) connected to the top of bob shaft ( 24 ). This angular position information is further converted to the viscosity of the tested fluid.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     Not applicable. 
     BACKGROUND 
     1. Field of Invention 
     The present invention relates to a low maintenance high pressure viscometer configured for the study of a cement of the inorganic, organic or mixed type. 
     2. Description of Prior Art 
     Drilling oil and geothermal wells requires the use of well cements. In wells, particularly petroleum wells, it is necessary to inject a liquid cement between a metal casing and the ground formation bordering the borehole. The setting of the liquid cement isolates the various layers of the ground formation around the borehole and holds the casing in place. For successful cementing it is important to use a liquid cement having a clearly determined rheological profile in order to determine true “pumpability time” (or setting time). Too short a time would result in premature clogging, and too long a time would needlessly delay resumption of work after cementing. Thus, for development and testing of well cements, pressure vessels are required to simulate downhole conditions accurately and repeatably. A rheometer configuration to enable cement testing of this kind makes it possible to follow the rheological changes over time of a progressively hardening material under conditions very close to the real conditions likely to be encountered downhole. 
     However, when testing cement samples, standard rheometer hardware (such as that as shown in U.S. Pat. No. 7,412,877) is prone to undesirably long test setup times, as well as extended cleanup and maintenance times, and can even become damaged when testing fluids such as cements as the cement sets or becomes solid. U.S. Pat. No. 4,653,313 describes a container for testing cement sample under pressure. However, this configuration would not provide sensitive and accurate measurement because of the friction between the central shaft and the seal around it. As a matter of fact, any seal directly in contact with the moving torque measurement parts would cause a considerable number of measurement errors due to friction. 
     Therefore, it is an object of this invention to provide a viscometer configuration allowing the rheological testing of cements under conditions closely simulating downhole conditions while avoiding contamination of cement samples with pressurization fluid. 
     It is another object of this invention to provide a viscometer configuration that requires substantially less maintenance work than would normally be necessary when testing cements, yet meets industry standards of accuracy, reliability, durability, dependability, repeatability, and ease of cleaning. 
     SUMMARY 
     A viscometer configuration in accord with the present invention comprises a pressure vessel inside which a rotor assembly is mounted on a pivot while a magnetic coupling for rotating the rotor is mounted outside the vessel. The interior of said rotor assembly is largely isolated from said pressure cell except for small gaps in the rotor assembly. Suspended within the rotor assembly is a bob capable of angular motion about the longitudinal axis of the rotor. The device is constructed so that the bob is immersed in a cement sample, the rheological changes of which are to be measured. The bob is suspended within the rotor by a bob shaft and is functionally protected from contamination with pressurization fluid. A spiral spring permits limited angular motion of the bob shaft. A magnet is secured on top of the bob shaft. A magnetometer located on the top of the pressure vessel senses the rotation of the magnet. 
     The apparatus and method of the present invention provide a way to measure the shear stress property of cement samples under shear conditions. 
    
    
     
       DRAWING FIGURES 
       Other objects, features and advantages will be apparent from the following detailed description of preferred embodiments taken in conjunction with accompanying drawings in which: 
         FIG. 1  is a cross-section view of a preferred embodiment of the invention. 
         FIG. 2  is an alternative embodiment with jewel bearings and three-piece pressure vessel configuration. 
     
    
    
     REFERENCE NUMERALS IN DRAWINGS 
     
       
         
               
               
               
               
             
           
               
                   
               
             
             
               
                  10 
                 magnetometer 
                  10A 
                 magnetometer 
               
               
                  12 
                 inlet 
                  12A 
                 inlet 
               
               
                  13A 
                 top jewel bearing 
                  14 
                 set screw 
               
               
                  14A 
                 set screw 
                  15A 
                 top sleeve 
               
               
                  16 
                 spring holder 
                  16A 
                 spring holder 
               
               
                  18 
                 bob shaft bearing 
                  19 
                 small gap 
               
               
                  20 
                 bearing spacer 
                  21A 
                 sample injection hole 
               
               
                  22 
                 bob shaft bearing 
                  24 
                 bob shaft 
               
               
                  24A 
                 bob shaft 
                  26 
                 o-ring 
               
               
                  26A  
                 o-ring 
                  30 
                 bob 
               
               
                  30A 
                 bob 
                  31 
                 sample 
               
               
                  31A 
                 sample 
                  32 
                 bearing 
               
               
                  32A 
                 jewel bearing 
                  33 
                 thread 
               
               
                  33A 
                 thread 
                  34 
                 coupling magnet 
               
               
                  34A 
                 coupling magnet 
                  35 
                 cell wall 
               
               
                  35A 
                 cell wall 
                  38 
                 driving magnet 
               
               
                  38A 
                 driving magnet 
                  39 
                 thermal couple 
               
               
                  39A 
                 thermal couple 
                  40 
                 magnet mount 
               
               
                  40A 
                 magnet mount 
                  41 
                 straight bore 
               
               
                  41A 
                 straight bore 
                  42 
                 bearing 
               
               
                  42A 
                 bearing 
                  43 
                 conical surface 
               
               
                  43A 
                 conical surface 
                  44 
                 bearing 
               
               
                  44A 
                 bearing 
                  46 
                 lock nut 
               
               
                  46A 
                 lock nut 
                  47 
                 cell bottom 
               
               
                  47A 
                 cell bottom 
                  51 
                 rotor assembly 
               
               
                  51A 
                 rotor assembly 
                  52 
                 heater 
               
               
                  52A 
                 heater 
                  54 
                 pivot 
               
               
                  54A 
                 pivot 
                  56 
                 rotor wall 
               
               
                  56A 
                 rotor wall 
                  57A 
                 jewel bearing 
               
               
                  59 
                 conical surface 
                  61 
                 venting hole 
               
               
                  63 
                 screw thread 
                  63A 
                 screw thread 
               
               
                  64 
                 snap ring 
                  66 
                 bearing holder 
               
               
                  66A 
                 bearing holder 
                  67 
                 flat 
               
               
                  67A 
                 flat 
                  68 
                 snap ring 
               
               
                  70 
                 spiral spring 
                  70A 
                 spiral spring 
               
               
                  72 
                 top magnet 
                  72A 
                 top magnet 
               
               
                  74 
                 outlet 
                  74A 
                 outlet 
               
               
                  76 
                 cell cap 
                  76A 
                 cell cap 
               
               
                  78 
                 thread 
                  78A 
                 thread 
               
               
                  80 
                 viscometer 
                  80A 
                 viscometer 
               
               
                  90A 
                 pin 
                  94 
                 rotor cover 
               
               
                  94A 
                 rotor cover 
                  96 
                 chamber 
               
               
                  96A 
                 chamber 
                  98 
                 rubber diaphragm 
               
               
                  98A 
                 rubber diaphragm 
                 100 
                 rotor bottom 
               
               
                 100A 
                 rotor bottom 
                 102 
                 o-ring 
               
               
                 102A 
                 o-ring 
                 104 
                 collar 
               
               
                 104A 
                 collar 
                 106 
                 small gap 
               
               
                 106A 
                 gap 
                 107 
                 thread 
               
               
                 107A 
                 thread 
                 108 
                 lock ring 
               
               
                 108A 
                 lock ring 
                 110 
                 small gap 
               
               
                 110A 
                 small gap 
                 112 
                 set screw 
               
               
                 112A 
                 set screw 
               
               
                   
               
             
          
         
       
     
     DESCRIPTION 
     FIG.  1 —Preferred Embodiment 
       FIG. 1  is a cross-section view of a viscometer  80  with a cylindrical cell wall  35  and a cylindrical cell cap  76 . Cell wall  35  is detachable from cell cap  76  via a screw thread  63 . An o-ring  26  assures against the escape of fluid through screw thread  63 . Inside of cell wall  35  and below screw thread  63  is a conical surface  59  with reduced diameter, below which cell wall  35  extends downward to a cell bottom  47 . A tapered hole with a conical surface  43  and a straight bore  41  is located in the center of cell bottom  47 . A pivot  54 , which is secured to cell bottom  47  by a lock nut  46  through a thread  78 , is seated into said tapered hole on conical surface  43 . Lock nut  46  is tightened to provide initial seal on straight bore  41  between cell bottom  47  and pivot  54 . A thermal couple  39  is inserted into the center of pivot  54 . 
     A bearing  32  and a coupling magnet  34  are positioned inside a rotor bottom  100 . Rotor bottom  100  is attached to a rotor wall  56  via a thread  33 , forming the outer structure of rotor assembly  51 . An o-ring  102  ensures against fluid leakage from the junction of a rotor wall  56  and rotor bottom  100 . Rotor assembly  51  is placed onto pivot  54  so that it rests on top of bearing  32 , so that bearing  32  provides vertical support of rotor assembly  51  and enables it to rotate freely on the same axis as pivot  54 . 
     A bob  30  is placed inside rotor wall  56 . A collar  104  is placed radially around the shaft of bob  30  so that it rests inside rotor wall  56 . A rubber diaphragm  98  is placed on top of the collar  104 . Rubber diaphragm  98  is shaped to allow a small gap  106  between the interior edge of rubber diaphragm  98  and bob  30 . A rotor cover  94  is placed on top of the rubber diaphragm  98 . A lock ring  108  is attached to the top of rotor wall  56  via a thread  107 . 
     A bearing holder  66  consists of a conical section and two different outside diameter sections. The outer surface of the conical section of bearing holder  66  mates inside conical surface  59  of cell wall  35 . An o-ring  26  provides a liquid-tight seal on conical surface  59 . A bob shaft  24  passes through the center of bearing holder  66  and is rotationally supported by a bob shaft bearing  18 , a bob shaft bearing  22 , a bearing spacer  20 , a snap ring  68  and a snap ring  64 . 
     A machined flat  67  is provided on the top of bearing holder  66 . Mating and resting on flat  67  is a spring holder  16 . A spiral spring  70  is placed in the center of spring holder  16  so that the outside lead of spiral spring  70  is fixed to the inside counter bore of spring holder  16  and the inside lead of spiral spring  70  is fixed to bob shaft  24  with any conventional means. A horseshoe-type top magnet  72  is fixed to the top of bob shaft  24  with a set screw  14 . Additionally, a small gap  110  channels from the top of bearing holder  66  to a chamber  96 . A sample  31  is injected into chamber  96  and fills all of the space inside rotor wall  56 . A venting hole  61  connects the outer surface of bearing holder  66  to a small gap  19  between bob shaft  24  and bearing holder  66 . A set screw  112  mounted inside the shaft of bob  30  can be tightened to attach the bob shaft  24  to the bob  30 . 
     An inlet  12  and an outlet  74  provide ports for applying and releasing pressure. A magnetometer  10  located on the top of cell cap  76  can measure the rotational displacement of top magnet  72 . 
     A magnet mount  40  is rotationally supported on the outside of cell wall  35  by a bearing  42  and a bearing  44 . Magnet mount  40  can be rotated by any conventional means such as a gear box or motor. A pair of driving magnet  38  is mounted on magnet mount  40  at considerably the same level where coupling magnet  34  is mounted inside of the cell wall  35 . Heat is provided by a heater  52 . 
     OPERATION 
     FIG.  1 —Preferred Embodiment 
     Begin assembly of viscometer  80  by inserting pivot  54  into cell bottom  47  through the conical hole with straight bore  41  and conical surface  43 . Secure pivot  54  to cell bottom  47  by screwing lock nut  46  onto thread  78 . Pivot  54  and cell bottom  47  can be cleaned together with cell wall  35 . Insert thermal couple  39  up into pivot  54 . 
     Install bearing  32  and coupling magnet  34  into the rotor bottom  100 . Install o-ring  102  into the lower end of rotor wall  56 , then attach rotor bottom  100  to rotor wall  55  via thread  33 , thus forming the outer structure of rotor assembly  51 . Holding bob  30  by the stem, lower it inside the top of rotor assembly  51  so that it fits just inside rotor wall  56 , then place collar  104  and rubber diaphragm  98  inside rotor assembly  51 , on top of bob  30 . Pour sample  31  into rotor assembly  51  until it submerges rubber diaphragm  98 . Put rotor cover  94  inside rotor assembly  51 , on top of bob  30  and use a syringe to inject sample  31  into chamber  96  via small gap  110  until chamber  96  is full. Screw lock ring  108  onto rotor assembly  51  via thread  107 . 
     Holding bob shaft  24  in hand, install bob shaft bearing  18 , bearing spacer  20 , bob shaft bearing  22 , snap ring  68  and snap ring  64  onto bob shaft  24 . Then vertically insert this subassembly into bearing holder  66 . Next, install spring holder  16  and spiral spring  70  onto the top of bearing holder  66 , resting spring holder  16  on flat  67  at the top of bearing holder  66 . Secure top magnet  72  to the top of bob shaft  24  via set screw  14 . Attach bob  30  onto bob shaft  24  bottom via set screw  112 . Then vertically push this bob shaft holder assembly down into cell wall  35  slowly, so that the bearing  32  inside of rotor bottom  100  rests on top of the pivot  54  and the whole assembly is able rotate on top of the pivot freely. 
     Using screw thread  63 , screw down cell cap  76  with o-ring  26  in place. Pump pressurization fluid from inlet  12  until all air inside of pressure vessel is expelled out through outlet  74 . Sample testing pressure can be raised by pumping more pressurization fluid into pressure vessel or releasing some pressurization fluid from pressure vessel. 
     It is very important to have rotor assembly  51  and bob  30  concentrically aligned. Conical surface  59  is machined with high precision to ensure bob  30  is concentrically aligned with rotor assembly  51 . This conical surface  59  also significantly simplifies the installation process since no addition adjustment or screw turning is required to ensure the good concentricity between rotor assembly  51  and bob  30 . 
     A motor or gearbox drives magnet mount  40  to rotate on bearing  42  and bearing  44 , carrying driving magnet  38 . A heater  52  heats up cell wall  35  while thermal couple  39  provides temperature feedback for temperature control. Due to the magnetic coupling between driving magnet  38  and coupling magnet  34 , rotor assembly  51  rotates at the same revolving speed as magnet mount  40  does. Because of the viscosity of the tested sample, a torque is generated on bob  30 , causing it to rotate. Because of spiral spring  70 , the rotation angle of bob shaft  24  is roughly proportional to the torque applied on bob  30 . Magnetometer  10  picks up the rotation angle of top magnet  72  which rotates with bob shaft  24 . The rotation angle in turn can be used to calculate the viscosity of tested sample. 
     One of the drawbacks of most liquid pressurized viscometers is the mixing between tested sample and pressurization fluid. If a seal is provided between pressurization fluid and tested sample, the seal will induce friction errors causing inaccurate measurement. If pressurization fluid is allowed to contact tested sample directly, pressurization fluid will mix with tested sample because of stirring and the compressibility of tested sample. 
     In the current invention, when pressurization fluid is introduced, the sample fluid level is pushed down due to the compressibility of tested sample. Thus, some of the pressurization fluid goes down through small gap  19  and enters chamber  96  through small gap  110 . However, chamber  96  is large enough so that at maximum rated pressure, chamber  96  is still at least half filled with sample fluid. This ensures the accuracy of the measurement because the measurement zone below collar  104  is always totally filled with sample fluid. 
     Additionally, because collar  104  separates lower measurement zone and chamber  96 , fluid inside of chamber  96  is relatively static. Thus no stirring could cause mixing between pressurization fluid and tested sample if the interface between pressurization fluid and tested sample is inside of chamber  96 . 
     The pressurization fluid should be chosen carefully. This pressurization fluid should not spontaneously dissolve into or mix with the tested sample, and should have a specific gravity lower than the specific gravity of the sample. Pressurization fluid communicates pressure with sample fluid through venting hole  61  and small gap  110  and small gap  106  while keeping bob shaft bearing  18  and bob shaft bearing  22  submerged. Because pressurization fluid is generally a clean, nonabrasive liquid, this ensures bob shaft bearing  18  and bob shaft bearing  22  rotate freely and have a long working life span. If conventional type of bearings, such as roller bearings, ball bearings or spherical bearings are used in a comparative viscometer without a mechanism preventing sample mixing with pressurization fluid, those bearings will quickly stop working properly, normally with excessive drag, because tested sample is normally filled with a lot of fine solid contents. 
     If a cement sample is left to set inside the rotor assembly  51 , said assembly can be completely disassembled so that the cement sample can be pushed out of the rotor wall  56  quickly and the rotor assembly  51  components can be cleaned easily. 
     DESCRIPTION 
     FIG.  2 —An Alternative Embodiment with Jewel Bearings and Three-Piece Pressure Vessel Configuration 
       FIG. 2  is a cross-section view of a viscometer  80 A with a cell wall  35 A, a bearing holder  66 A and a cell cap  76 A. Cell wall  35 A is detachable from bearing holder  66 A via a screw thread  63 A and cell cap  76 A is screwed on top of bearing holder  66 A. An o-ring  26 A assures against escape of fluid through screw thread  63 A. 
     Cell wall  35 A extends downward to a cell bottom  47 A, where a tapered hole with a conical surface  43 A and a straight bore  41 A is located in the center of cell bottom  47 A. A pivot  54 A, which is secured to cell bottom  47 A by a lock nut  46 A through a thread  78 A, is seated into said tapered hole through straight bore  41 A. Lock nut  46 A is tightened to provide initial seal on conical surface  43 A between cell bottom  47 A and pivot  54 A. A thermal couple  39 A is inserted into the center of pivot  54 A. Radially outward of the outer surface of pivot  54 A is a coupling magnet  34 A. 
     A jewel bearing  32 A is fitted into a rotor bottom  100 A. When rotor bottom  100 A is placed on top of pivot  54 A, jewel bearing  32 A allows it to rotate freely on the same central axis of pivot  54 A. 
     A rotor wall  56 A is attached to rotor bottom  100 A via a thread  33 A, forming the outer structure of rotor assembly  51 A. An o-ring  102 A assures against escape of fluid from between rotor wall  56 A and rotor bottom  100 A. These components can rotate freely on the same central axis of pivot  54 A. 
     A bob shaft  24 A passes through the center of bearing holder  66 A while not in contact with its inside bore directly. A machined flat  67 A is provided on the top of bearing holder  66 A. Mating and resting on flat  67 A is a spring holder  16 A. A spiral spring  70 A is placed in the center of spring holder  16 A so that the outside lead of spiral spring  70 A is fixed to the inside counter bore of spring holder  16 A and the inside lead of spiral spring  70 A is fixed to bob shaft  24 A with any conventional means. 
     Resting on top of spring holder  16 A is a top sleeve  15 A, on which a top jewel bearing  13 A is mounted. The tip of bob shaft  24 A is in contact with top jewel bearing  13 A. Bob shaft  24 A is secured to a bob  30 A by a set screw  112 A. 
     Bob  30 A is positioned inside rotor wall  56 A. Directly above bob  30 A, inside rotor wall  56 A, is a collar  104 A. A rubber diaphragm  98 A is placed above collar  104 A, and a rotor cover  94 A is placed above rubber diaphragm  98 A. A small gap  106 A allows fluid to flow between the area around bob  30 A and a chamber  96 A, which is inside rotor cover  94 A. Fluid can also flow into chamber  96 A through small gap  110 A and into the lower assembly surrounding the bob  30 A through a small gap  106 A. A lock ring  108 A is fixed to the top of rotor assembly  51 A via a thread  107 A, above rotor cover  94 A. A pin  90 A is set into the center of the top of rotor bottom  100 A. Pin  90 A fits into a jewel bearing  57 A which is set into the center of the bottom of bob  30 A. Jewel bearing  57 A and top jewel bearing  13 A support the bob shaft  24 A and allow it to rotate. 
     A horseshoe type top magnet  72 A is also fixed to the top of bob shaft  24 A with a set screw  14 A. A sample  31 A is injected into a sample injection hole  21 A and fills the area above the rotor assembly  51 A. 
     An inlet  12 A and an outlet  74 A provide ports for applying and releasing pressure. A magnetometer  10 A located on the top of cell cap  76 A can measure the rotational displacement of top magnet  72 A. 
     A magnet mount  40 A is rotationally supported on the outside of cell wall  35 A by a bearing  42 A and a bearing  44 A. Magnet mount  40 A can be rotated by any conventionally means such as gear box or motor. A pair of driving magnet  38 A is mounted on magnet mount  40 A at considerably the same level where coupling magnet  34 A is mounted inside of the cell wall  35 A. Heat is provided by a heater  52 A. 
     OPERATION 
     FIG.  2 —An Alternative Embodiment with a Jewel Bearing and Three-Piece Pressure Vessel Configuration 
     Begin assembly of viscometer  80 A by inserting pivot  54 A into cell bottom  47 A through the conical hole with straight bore  41 A and conical surface  43 A. Secure pivot  54 A to cell bottom  47 A by screwing lock nut  46 A onto thread  78 A. Pivot  54 A and cell bottom  47 A can be cleaned together with cell wall  35 A. Insert thermal couple  39 A up into pivot  54 A. Place coupling magnet  34 A onto pivot  54 A so that it is positioned radially outward from pivot  54 A. 
     Install o-ring  102 A into rotor wall  56 A, then attach rotor wall  56 A to rotor bottom  100 A via thread  33 A, thus forming the outer structure of rotor assembly  51 A. Install jewel bearing  32 A into rotor bottom  100 A. Insert pin  90 A into the top of rotor bottom  100 A. 
     Install jewel bearing  57 A into the bottom of bob  30 A. Drop bob  30 A into rotor assembly  51 A so that it fits inside rotor wall  56 A and the jewel bearing  32 A rests on the pin  90 A. Drop collar  104 A into rotor wall  56 A, and drop rubber diaphragm  98 A on top of collar  104 A. Drop rotor cover  94 A on top of diaphragm  98 A, and fasten lock ring  108 A onto the top of rotor wall  56 A via thread  107 A. 
     Insert bob shaft  24 A into bearing holder  66 A. Attach spring holder  16 A and spiral spring  70 A onto the top of bearing holder  66 A, resting on the flat  67 A. Install the top sleeve  15 A onto the top of bearing holder  66 A, so that bob shaft  24 A is centered by top jewel bearing  13 A. Attach top magnet  72 A to the top of bob shaft  24 A using set screw  14 A. Fasten bob  30 A onto the bottom of bob shaft  24 A with set screw  112 A. Pour test sample  31 A into cell wall  35 A so that the sample surface just submerges the top of pivot  54 A. 
     Screw bearing holder  66 A, with attached bob shaft assembly, onto cell wall  35 A using thread  63 A. O-ring  26 A ensures against fluid leakage here. Rotor bottom  100 A should be rotationally supported by jewel bearing  32 A mounted on top of pivot  54 A. As it is lowered, bob  30 A will sink into the sample fluid, causing the sample fluid level to rise. A syringe is used to inject additional sample fluid through small injection hole  21 A to bring sample fluid level up so that sample fluid totally fills the area above the rotor assembly  51 A. 
     Finally, screw down cell cap  76 A onto bearing holder  66 A. Pump pressurization fluid from inlet  12 A. Sample testing pressure can be raised by pumping more pressurization fluid into pressure vessel through inlet  12  or releasing some pressurization fluid from pressure vessel through outlet  74 A. 
     A motor or gearbox mounted on bearing  42 A and bearing  44 A drives magnet mount  40 A to rotate carrying driving magnet  38 A. A heater  52 A heats up cell wall  35 A while thermal couple  39 A provides temperature feedback for temperature control. Due to the magnetic coupling between driving magnet  38 A and coupling magnet  34 A, rotor assembly  51 A rotates at the same revolving speed as magnet mount  40 A does. Because of the viscosity of tested sample, a torque is generated on bob  30 A causing it to rotate. Because of spiral spring  70 A, the rotation angle of bob shaft  24 A is roughly proportional to the torque applied on bob  30 A. Magnetometer  10 A picks up the rotation angle of top magnet  72 A which rotates with bob shaft  24 A. The rotation angle in turn can be used to calculate the viscosity of tested sample. 
     In this embodiment, when pressurization fluid is applied, the sample fluid level is pushed down due to the compressibility of tested sample. Thus initial sample fluid goes down to the area above the rotor assembly  51 A through inlet  12 A, and into chamber  96 A through small gap  110 A and into the rotor assembly  51 A through small gap  106 A. However, chamber  96 A is large enough so that at maximum rated pressure, chamber  96 A is still at least half filled with sample fluid. This ensures the accuracy of the measurement because measurement zone below collar  104 A is always totally filled with sample fluid. Additionally, because collar  104 A separates lower measurement zone and chamber  96 A, fluid inside of chamber  96 A is relatively static. Thus no stirring could cause the mixing between pressurization fluid and tested sample if the interface between pressurization fluid and tested sample is inside of chamber  96 A. 
     RAMIFICATIONS 
     Bob  30  does not have to be cylindrical shape. It could be a blade, frame or any geometry shape. 
     Bob shaft bearing  18  and bob shaft bearing  22  could be combined as one needle bearing or equivalent bearing with low friction. 
     Rotor assembly  51  does not have to be driven with a magnetic coupling across cell wall  35 . Rotor assembly  51  could be driven to rotate with any means such as directly driven at the bottom of the cell body with dynamic seal, etc. 
     Spiral spring  70  could be helical spring or other types of equivalent resilient mechanism. 
     There are many other ways to measure the angular displacement of bob shaft  24 . For example, in preferred embodiment viscometer  80 , top magnet  72  and magnetometer  10  can be replaced with a pair of concentrically mounted electrical stator and rotor to measure the rotation of bob shaft  24 . Additionally, top magnet  72  and magnetometer  10  can be replaced with an encoder to measure the rotation of bob shaft  24 . A potentiometer and a brush attached to bob shaft  24  could measure the rotation as well. 
     Alternatively, a metal arm or wiper which rotates with bob shaft  24 , and a wire-wound conductance transducer which is mounted directly or indirectly on bearing holder  66  or cell cap  76 , can also be used to measure the rotation of bob shaft  24  by measuring the conductance change in the wire-wound coil. 
     Rotor cover  94  and rubber diaphragm  98  could be attached from bob shaft  24  and rotate together with bob shaft  24 . In this case, their outside edge will be separated from rotor assembly  51  by small gaps to reduce pressurization fluid mixing with tested sample using the same working principle described in this invention. 
     Pressurization media used to pressurize viscometer  80  can be any high pressure gas or any liquid fluids that do not dissolve into tested sample and have a density smaller than tested sample. 
     CONCLUSION, AND SCOPE 
     Accordingly, the reader will see that this invention can be used to construct a high pressure viscometer for accurate and repeatable measurement of cement rheology. The configuration enables simple operation procedure and low maintenance. 
     OBJECTS AND ADVANTAGES 
     From the description above, a number of advantages of my viscometer become evident:
         (a) Due to conventional low friction ball bearings design, current invention substantially reduces operation cost comparing to jewel bearing designs. It had been unsuccessful to use conventional ball bearings in liquid pressurized high pressure viscometer because any fine solids in tested sample would cause low friction ball bearings to fail. Also because ball bearings are much durable than jewel bearings, maintenance task is reduced significantly.   (b) Totally eliminate the measurement error because of sample mixing with pressurization fluid in a comparative viscometer.   (c) Very conveniently isolate all electrical component from pressurized zone thus reducing maintenance work.   (d) Very compact design by using small size spiral spring.   (e) Extremely simple installation and disassembly procedures due to conical bearing holder design while maintaining high concentricity between bob and rotor.
 
Further objects and advantages of my invention will become apparent from a consideration of the drawings and ensuing description.