GB2188162A - Viscometer - Google Patents

Abstract

The viscometer 200 comprises a cup 14 and a plug 16 which define a sample space 12 between them. To effect measurement, the cup 14 is rotated. Fluid in the space 12 provides a viscous coupling to the plug 16 which rotates to a position in which a torsion mounting 26 provides a counterbalancing torque. The size of the counterbalancing torque and hence the position of the plug in balance, depend on the viscosity of the fluid. When measuring viscosity of fluid flowing through chamber 18 a collar 56 is allowed by ratchet mechanism 60 to rotate with the cup 14, but when the rotation of the cup is reversed to recharge the sample space 12 the collar is held stationary by the ratchet mechanism and piston pumps 40 rotating with the cup are reciprocated by cooperating cams 66 on the collar 56 to recharge the sample space 12 with liquid representative of that currently flowing through chamber 12. A centrifugal pump actuated by high speed rotation of the cup may be used instead of the piston pumps. <IMAGE>

Description

SPECIFICATION Viscometer The present invention relates to viscometers, and is more particularly but not exclusively concerned with viscometers ofthe type used to measure the viscosity, elasticity or otherflow- orshear-related parameters of non-Newtonian fluids over a range of flow- orshear-rates. Viscometers ofthistype are often referred to as rheometers, and the term "viscometer" as used herein is to be understood as including rheometers.

It is well known to measure the viscosity of a fluid by placing a sample ofthe fluid in a sample space between two surfaces, before moving the surfaces relative to one another. The fluid sample provides a viscous coupling between the surfaces, so that the force ortorque needed to maintain the movement, or transmitted between the surfaces, is dependent on the viscosity ofthe sample. The force ortorque and the relative speed of the surfaces can be measured and the viscosity found by calculation.

Viscosity is an important parameter of many manufactured fluids, such as paint. Where this is the case, the viscosity of fluid being manufactured is monitored by means of a viscometer, either continuously or by regularsampling. The outputof the viscometer can be used to provide a feedback signal to control the manufacturing process so that fluid ofthe desired viscosity is produced. A viscometer used to provide feedback control is known as an on-line viscometer.

Figure 1 shows, schematically, a known on-line viscometer 10. The viscometer 10 has an annular sample space 12 defined between the innerwall of a cup 14 and the surface of a plug 16 suspended in and coaxial with the cup. In use, the cup 14orthe plug 16 is continuously rotated. This rotation tends to rotate the plug 16 or the cup 14, respectively, by virtue of the viscosity of the fluid with in the annular sample space 12. A restoring torque is applied to counteract this tendency. The restoring torque required and the rotation speed are measured and the viscosity of the sample can be calculated from these measurements.

Thecup 14and plug 16arelocated inachamber 18. Fluid manufactured in a manufacturing process (by apparatus not shown) enters the chamber 18 through an inlet port 20 and leaves through an outlet port22. Asthefluidflowsthrough the chamber 18, fluid in the sample space 12 is displaced by fresh fluid, so that the viscosity measured by the viscometer lOwill tend to changeto reflect changes in the viscosity offluid entering the chamber 18. The top ofthe cup 14 is open and openings23 are provided in the bottom of the cup to faci I itate movement of fresh fluid into the sample space 12.

It isfound in practice that the fluid inthesample space 12 is replaced by fresh fluid only very slowly.

Consequently, the viscosity measurement given by the viscometer does not closely follow the viscosity ofthefluid entering thechamberl8.

It is an object ofthe present invention to provide an improved viscometer which can be used on-line and in which the sample under test is more closely representative of the surrounding fluid.

According to the present invention there is provided a viscometer comprising two members having surfaces which define between them a samplespaceforfilling with a sample of fluid to provide viscous coupling between the members, drive means for providing relative motion at a known speed between the members,and meansfor measuring torque or force applied by the drive means ortransmitted between the members, the defining surfaces being immersed, in use, in fluid which may enterthe sample space, and the viscometerfurther comprising at least one pump operableto drawfluidfromthesample space orthe surrounding fluid, and to discharge into the surrounding fluid orthe sample space, respectively.

The pump is preferably operated periodically to recharge the sample space so thatthefluid within the sample space is a true sample, at least periodically, ofthefluid surrounding theviscometer.

Preferred features of the invention are defined in the subsidiary claims set out below, to which reference should now be made.

Two embodiments ofthe invention will now be described in more detail, byway of example, and with reference to the remaining accompanying drawings in which: Figure2 is a sectional view of a first embodiment ofthe invention installed for operation as an on-line viscometerand incorporating a centrifugal pump; Figure 3 is a similar view of a second embodiment ofthe invention likewise installed, and incorporating piston pumps; and Figure is a perspective view of the cam member of the embodiment shown in Figure3.

Someofthefeaturesshown in Figures2and3 correspond to features shown in Figure 1. The same numerals are used in each Figureforcorresponding features.

Figure 2 shows a viscometer 100 comprising two members, which are a cup 14and a plug 16 and which define between them an annularsample space 12 forfilling with a sample of fluid to provide viscous coupling between the cup 14 and the plug 16. The sample space 12 is about 1 mm wide. Relative motion between the cup 14andthe plug 16is provided by a rotating drive shaft 24 which causes the cup to rotate. Atorsion device 26 enables torque transmitted to the plug 16fromthe rotating cup 14to be measured, as will be described. The viscosity of the fluid in the sample space 12 can be calculated from this measurement and a measurement of the speed of the shaft 24. The viscometer 100 is located in a chamber 18 so that, in use, the surfaces defining the sample space 12 are immersed in fluid which may enterthe sample space 12 through the mouth 28 of the cup 14. The viscometerfurther comprises a centrifugal pump 30 operable to drawfluid from the sample space 12 and to discharge into the surrounding fluid in the chamber 18.

The centrifugal pump 30 comprises a manifold 32 communicating through the floor ofthe cup 14with the interior ofthe cup 14. Fluid flow passages 34 communicate with the manifold 32 and with the chamber 18 and extend generally radially of the axis of rotation of the shaft 24. The outer ends ofthe passages 34 are normally closed by caps 35, retained in position by springs 36.

The torsion device 26 which supports the plug 16 in the cup 14 provides a restoring torque to counteract rotation of the plug 16 about its central, vertical axis. The restoring torque is provided by a hollowtorquetube 37. Thetorquetube 37 is attached to the housing 38 at its upper end and to the plug 16 at its lowerend,thereby mounting the plug 16 on the housing 38. The lower end ofthetorque tube is closed to preventfluid escaping. The torque tube 37 may be attached directly to the plug 16, or, as shown, by means of a spring 39 which assists in centering the plug 16 in the cup 14. Athin indicator rod37a is attached at its lower end to the lower end ofthe torquetube 37 and extends upthetube37 and out of the housing 38.The rod 37a carries a perpendicular indicator arm 37b at its upper end.

The torque tu be 37 allows the plug 16to twist under the influence of an applied torque, until the applied torque is balanced by the restoring torque provided bytwisting the torquetube. The rod 37a rotates with the lower end of the torque tube 37 as the torque tube twists, so that the amount of twist andhencethetorqueappliedtotheplug 16 can be measured by measuring the deflection of the indicator arm 37b relative to the housing 38. A scale may be provided adjacentthearm37bformeasuring its deflection, or a transducer, such as an inductive displacement transducer may be provided to generate a signal indicative ofthe magnitude of any deflection.

When the device is in use, fluid enters the chamber 18 through the inlet port 20 and leaves from the outlet port 22. When a viscosity measurement is required, the drive shaft 24 is rotated, at a known speed,to rotate the cup 14. Fluid in the sample space 12 provides a viscous coupling to the plug 16 and the plug 16 rotates until coming to rest when the torque communicated from the cup 14 is balanced bythe restoring torque provided bythe torquetube 37. The corresponding deflection of the arm 37b is measured. If the fluid in the sample space 12is non-Newtonian, measurements ofthetorque exerted on the plug 16 will be made for a plurality of different rotation speeds of the shaft 24. Typically, these speeds will be in the range 1 r.p.m. to 20 r.p.m.

These measurements enable a curve to be plotted of measured viscosity against the rotation speed ofthe shaft 24. The curve can then be compared with a standard curve drawn from readings taken when the sample space is occupied by a control sample known to have the desired properties. Any discrepancy between the curves would indicate thatthe current contents ofthe sample space do not have the desired properties, and appropriate parameters ofthe manufacturing process can be changed to correct the discrepancies.

As manufactured fluid continues to enter and leave the chamber 18,the contents ofthe sample space 12 will cease to be an accurate sample ofthe chamber contents. Consequently, the pump 30 is periodically operated, to change the sample in the sample space, as follows. The rotation speed ofthe shaft 24 is increased to a Figure much greaterthan the Figures at which measurements are taken.

Eventually, a speed is reached at which the centrifugal force exerted by thefluid in the passages 34 on the caps 35 exceeds the force provided by the retaining springs 36. The caps 35 open, and the centrifugal forces pump the contents of the sample space 12 out into the chamber 18. Fresh fluid is drawn intothe sample space 12through the mouth ofthe cup. After sufficient time has elapsed to allow for complete recharging of the contents ofthe sample space 12, the speed of the shaft 24 is reduced, whereupon the caps 35 close, and measurement of the fresh sample can beg in. A measurement truly representative of the contents of the chamber 18is only provided immediately after recharging.

However, recharging can be effected as often as necessary to make the readings useful.

The pumping operation just described is also used when the viscometer is used forthefirsttime, in order to fill the sample space before taking the first readings. It may be convenient to distinguish betweenthemeasurementandpumping phases of operation not only by significant differences in rotation speed, but by a reversal of the sense of rotation of the shaft 24.

This embodiment has been found to be suited for use with fluids of low viscosity, for instance 2 Pascal seconds or less.

A second embodiment of the invention is shown in Figure 3. This embodiment differs from the embodiment of Figure 2 in that piston pumps instead of a centrifugal pump are provided for recharging the sample space 1 2. Consequently, only the pumps will be described with particular reference to Figure 3.

Two piston pumps 40 are mounted on the outside wall ofthe cup 14. Each pump 40 comprises a cylinder 42 having an in let port 44 and an outlet port 46. A non-return ball valve comprising a ball 48 and a seating 50 is provided at the inlet port 44. Each outlet port 46 communicates with the sample space 12 by way of an opening 51 through and approximately mid-wayupthewallofthecup 14.

A reciprocable piston 52 is provided in each cylinder 42. A compression spring 54 acts in each cylinder between the lowerface of the piston 52 and the bottom of the cylinder42,to urge the piston in an upwardly direction.

The lower end of each spring 54 is located around a perforated cage which limits movementofthe ball 48 away from its seating 50.

AcollarS6,shownalonein Figure4, is provided around the cup 14, above the pumps 40. The collar 56 is free to rotate relative to the cup 14, but is prevented from vertical movement relative to the cup 14. A ratchet mechanism 60 comprises a circumferential ratchet 62 on the collar 56 and cooperating pawls 64 mounted on intermediate members which are, in turn, fixed to the wall ofthe chamber 18. The ratchet mechanism 60 allows the collarto rotate in only one sense about the axis ofthe shaft 24. The lower surface of the collar 56 is shaped to form cams 66 which may cooperate with the upper surfaces of the pistons 52, as will be described, to drive the pistons down against the influence of the springs 54 and to allow the pistons 52 to rise under that influence.

During normal operation ofthe viscometer 200, the drive shaft 24 is rotated in the sense in which the ratchet mechanism 60 allows the collar 56 to rotate, sothatthe drive shaft drives the cup 14 and the collar 56, by virtue of engagement between the pistons 52 and the cams 66. While the shaft 24 is rotating in this sense, viscosity measurement can be carried out as described above in relation to Figure 2.

If the sense of rotation of the shaft 24 is reversed, the ratchet mechanism 60 engages to prevent rotation of the col lar 56. According Iy, as the cu p rotates, relative rotation occurs between the collar 56 and the pumps 40. The cams 66 bear on the pistons 52 to reciprocate the pistons 52. On their up-stroke, thepistonsdrawfluid intothe cylinders 42. On their down-stroke, the ball valves 48,50 close, and the pistons 52 discharge the contents of the cylinders 42 into the sample space 12, thereby recharging the sample space with fresh fluid. The displaced contents of the sample space leave through the mouth ofthe cup 14,ortheopenings23.

Since the sample space 12 is recharged intermittently, by reversing the drive shaft 24, the measurement made by the viscometer 200 is a true measurement of the viscosity offluid in the chamber 18 only immediately after recharging. However,the frequency of recharging may be chosen to be sufficiently high that this source of error is insignificant.

In both ofthe embodiments described above, the torquetransferred to the plug 16 is measured and the viscosity is calculated from this value and the speed of the drive shaft 24, taking into account, as necessary, other factors such as temperature and the dimensions of the apparatus. Conveniently, both measurements can be made by transducers which supply signals two data processing apparatus for performing the calculations, including comparing measured data with stored data taken when a control sample offluid occupied the sample space 12 and chamber 18. The data processing apparatus can also be arranged to automatically generate feedback signals for controlling the manufacturing process in response to the results of the measurements.

Claims (29)

1. A viscometer comprising two members having surfaces which define between them a sample spaceforfilling with a sample offluidto provide viscous coupling between the members, drive means for providing relative motion at a known speed between the members, and means for measuring torque orforce applied bythedrive means or transmitted between the members, the defining surfaces being immersed, in use, in fluid which may enter the sample space, and the viscometerfurthercomprising at least one pump operable to draw fluid from the sample space orthe surrounding fluid, and to discharge into the surrounding fluid or the sample space, respectively.

2. Aviscometeraccording to claim 1,whereinthe drive means provides rotational relative motion between the members.

3. A viscometer according to claim 2, wherein the orat least one pump is a centrifugal pumpwhich rotates with a rotating one of the members.

4. Aviscometeraccordingto claim3,whereinthe centrifugal pump comprises a fluid flow passage communicating with the sample space and the surrounding fluid and extending in a generally radial direction with respect two the rotational motion.

5. Aviscometer according to claim 4, wherein the centrifugal pump comprises a plurality offluid flow passage communicating with the sample space through a manifold and extending in respective generally radial directions.

6. Aviscometeraccordingtoclaim4or5, wherein the or each fluid flow passage is provided with a closure member resiliently biassed to close the passage, whereby pumping can occur only when the rotational motion is sufficiently fast for centrifugal forces to overcome the resilient biassing and open the passage.

7. Aviscometer according to claim 2, wherein the or each pump is operable when the drive means provides relative motion between the members in one but not the other rotational sense.

8. Aviscometer according to claim 7, wherein viscosity measurement takes place when the relative motion is in the said other rotational sense.

9. Aviscometeraccordingtoclaim2,7or8 wherein the or each pump is a reciprocatory pump.

10. Aviscometer according to claim 9, wherein the or each reciprocatory pump is caused to reciprocate by a cam or cams, the pump and the cam or cams being driven relative to oneanother,when the pump is in use, by the drive means.

11. A viscometer according to claim 10, wherein relative motion is provided between the or each reciprocatorypumpandthecam orcamswhenthe drive means provides relative motion between the members in one but not the other rotational sense.

12. Aviscometeraccording to claim 10 or 11, wherein the or each pump is mounted for rotation with a rotatable one of the members.

13. Aviscometeraccording to claim 10,11 or 12, wherein the cam or cams are provided on a cam member which rotates with a rotatable one of the members when the rotatable member is rotating in a first sense only, the cam member being prevented from rotation in the opposite sense.

14. Aviscometeraccordingtoclaim 13,wherein the cam member is prevented from rotation in the said opposite sense by a ratchet and pawl mechanism, one ofthe ratchet and pawl being carried by the cam member and the other being fixed against rotation.

15. Aviscometeraccording to claim 13 or 14, wherein the cam member is a collar located around the rotatable member.

16. A viscometer according to any of claims 7 to 15, wherein the or each pump draws from the surrounding fluid and discharges into the sample space.

17. Aviscometeraccording to any of claims 2to 16, wherein the drive means applies a rotational drive to a first of the members, the second member being held against rotation.

18. Aviscometeraccording to claim 17,wherein the second member is held against rotation by a torsion device which allows deflection of the second memberwhen the second member is driven by the first member by virtue of the viscous coupling between the members.

19. Aviscometer according to claim 18, comprising a sensor four measuring deflection ofthe second member.

20. Aviscometer according to any of claims 17 to 18, comprising a second sensorfor measuring the speed of rotation of the first member.

21. Aviscometer according to claims 19 and 20, comprising data processing means receiving signals indicating values measured by the first and second sensors and operable to use the values to calculate the viscosity ofthe fluid in the sample space.

22. Aviscometer according to claim 21, wherein the data processing means compares the values with previously stored values measured while the sample space is filled by fluid of known viscosity.

23. Aviscometer according to any of claims 19 to 22, wherein the drive means is operableto provide relative motion between the members at a range of speeds, to enable velocity measurements to be made over the range of speeds.

24. Aviscometer according to claims 23 and 22, wherein the data processing means compares values measured over a range of speeds with previously stored values measured over the same range of speeds.

25. Aviscometer according to any of claims 2to 24, wherein one member is a cup and the other member is a plug inside the cup, the defining surfaces being the outer surface ofthe plug and the innerwall ofthe cup.

26. A viscometer according to claim 25, wherein the outer surface and the innerwall are cylindrical, whereby the sample space is annular in cross-section.

27. A viscometer according to claim 25 or 26, wherein rotational drive is applied to the cup bythe drive means.

28. Aviscometer according to claim 25,26 or 27, wherein the pump is mounted for rotation with the cup.

29. Aviscometersubstantiallyas described above with reference to Figures 2 or 3 of the accompanying drawings.