GB2120793A - Viscometer - Google Patents

Abstract

In a piston and cylinder viscometer suitable for investigating the rheological properties of Newtonian fluids and Bingham plastics, an undersized piston (18) is accelerated from rest down through a hollow cylinder (6) in a cylinder block (1) containing a fluid (7) under investigation, by the action of a piston rod driven from above by a piston driving means (50). Both the instantaneous pressure ahead of the piston (18) and instantaneous velocity of the piston rod are measured and recorded in phase to determine the relationship between shear stress and shear strain induced in the fluid (7) throughout the piston stroke. Pressure is measured by peizo electric transducer 16 and velocity by a magnet 33 whose poles 34, 35 move in respective opposed series windings 37, 38 which produce an e.m.f. proportional to piston velocity. By providing a conductive sleeve on piston 18 and applying a voltage gradient between it and the cylinder block, the viscometer may be used with an electroviscous fluid. <IMAGE>

Description

SPECIFICATiON Viscometer This invention relates to a viscometer which operates on the piston and cylinder principle offluid viscosity measurement (hereinafter known as a piston and cylinderviscometer), which viscometer may be applied to the measurement of the rheological properties ofvarioustypes offluids, in particular but not exclusively Bingham plastics.

Various types of piston and cylinderviscometers are known whereby in operation an undersized piston falls vertically at a constant, terminal velocity through a sample of test fluid disposed within a cylinder and into a blanked-off cylinder head, which terminal velocity of the piston is the maximum velocity it achieves when its free vertical fall under gravity is balanced by this viscous drag ofthe fluid exerted on the piston. This piston is usually coaxially guided withinthe cylinderto provide a constant annular gap between the piston and the walls ofthe cylinder, and thus as the piston advances through the cylinder it displaces fluid beneath it and forces the fluid up through the gap at a uniform rate symmetrically about the piston.

As with all other types of viscometer, in orderthat fluid viscosity may be measured a relationship must be obtained between the shear stress applied to the fluid undertest and the corresponding shear rate induced. In piston and cylinderviscometersthe shear stress applied across an annularvolume offluid flowing between the piston and the cylinderwalls is derived from the vertical force the piston exerts on the cylinderwalls through the fluid. Afree falling piston thus exerts a force equal to its own weight. The corresponding shear rate induced is a function of, and thus may be determined from, the piston velocity.

Piston and cylinderviscometers have several advantages overothertypes ofviscometerinthatthey are simple to operate, they are generally rugged in construction and they usually only require a small sample offluid to effect measurement. In addition, problems associated with a sample being heated by the input of mechanical energy (which may result in an inaccurate result, particularly where a fluid is sensitive to viscosity change over small temperature rises) are minimised as measurements are made over a single vertical stroke ofthe piston. Aviscometerwhich relies on continuous or cyclical shear stressing of the sample would necessitate sample cooling to prevent excessive temperature rise.

The pre-requisitethatthe piston reaches a max imum, terminal velocity through the fluid, does however give rise to a number of disadvantages in a piston and cylinderviscometer. Unless some ofthe fluid rheological properties are already known, it may not be possible to ensure that the piston will reach terminal velocity beforevelocity measurements are made, which is a particular disadvantagewhere the velocity measurement consists of timing the fall of the piston between two fixed points.More importantly, such a viscometercan only produce one pair of readings of shear stress and corresponding shear strain once piston terminal velocity has been reached, hence to study fluids whose rheological properties are any more complexthan those of a Newtonian fluid, a whole succession of piston strokes must be made each under different conditions of loading and/or piston geometry, and the technique is therefore unduly time-consuming.

The present invention relates to a piston and cylinderviscometerwhereinthese disadvantages are overcome or at least mitigated.

According to the present invention, a piston and cylinder viscometer having a hollow cylinder with a closed first end and open second end, an undersized piston coaxiallyslideablewithin the cylinder, and a piston driving means capable in use of driving the piston through the cylindertowardsthe closed first end is characterised in that there is included a pressure sensing element located within the cylinder adjacent the closed first end capable of providing an output signal indicative ofthe instantaneous pressure therein and a piston velocity providing an output signal indicative of the instantaneous velocity ofthe piston.

Theviscometeris best suited forthe measurement of non-gaseousfluid viscosities. In use, a sample of a fluid to be tested is located within the cylinder, which cylinder is preferably disposed vertically with the open second end facing upwards.

The piston driving means is preferably a freefalling body whose mass is mechanically linked to the piston through the open second end of the cylinder by a coaxial piston rod. The mass ofthe free falling body is advantageouslyofa sufficientmagnitudeto ensure thatfor a given fluid the piston in use accelerates over a substantial portion of the piston stroke before reaching terminal velocity.

The coaxial guidance of the piston in the cylinder is conveniently achieved by two axially spaced sets of ball bearings, each set coaxially disposed about the piston rod within a housing mounted upon the cylinder adjacentthe open second end.

The pressure sensing element is preferably a piezoelectric transducer electrically connected to a pressure recorder. The pressure recorder is conve- niently calibrated such that the magnitude of an electrical signal transmitted by the transducer is automatically converted to a corresponding gauge pressure reading on the recorder, the term "gauge pressure" being used in this specification to mean pressure in excess of surrounding ambient air pressure.

The piston velocity sensing element preferably comprises two helical coils coaxially disposed about a bar magnet firmly attached to the piston rod and connected to provide that a voltage induced in the coils by axial motion in the bar magnet is directly proportional to the magnitude of piston velocity. The induced voltage is fed to a velocity recorder which is conveniently calibrated such that the magnitude of the induced voltage received from the coils is automati cally converted to piston velocity reading on the velocity recorder.

Embodiments of the present invention will now be described byway of example only, with reference to the accompanying drawings of which: Figure 1 is a sectional view of a viscometer containing an undersized piston located in a sample of liquid priortothe performance of a viscosity measure ment test.

Figure 2 is a simplified elevated view of the same viscometerdisposed beneath a piston driving means, Figure 3 is a diagrammatic representation of electrical and instrument equipment connected to the same viscometer so as to effect viscosity measurements, and, Figure 4 is a sectional view of an alternative piston suitable for use in the viscometer illustrated in Figure 1 when the sample fluid is an electro-rheological fluid.

The viscometer 40 illustrated in Figure 1 comprises a rigid hollow cylinder block 1 symmetrically disposed about a vertical axis AA' and bolted to a cylinder head 2, the cylinder block 1 and cylinder head 2 being pivotally mounted by a pin 3 upon a base block4 which in turn rests upon a base unit 5 also illustrated in Figure 2.

Within the cylinder block 1 iscoaxiallydisposeda cylinder6 having a closed first end 6a and an open second end 6b filled with a non-gaseous fluid 7 whose viscosity isto be measured. A gasket 8 interposed the cylinder block 1 and the cylinder head 2 assists in preventing leakage of fluid 7 from the cylinder 6. The portion ofthe cylinder block 1 surrounding the cylinder 6 is externally recessed to hold an electrical heating coil 9 and a platinum coil resistancethermometer element 10. The cylinder head 2 has a gasket opposable face 11 provided with an annular recess 12 coaxial with respect to the axis AA' and open to the cylinder 6.The head 2 is provided with a first radial port 13 opening to the annular recess 12 which port is sealed by a screwed bleed nipple 14, and a second radial port 15 also open to the recess 12 in which is disposed a piezoelectric pressure transducer 16 exposed to pressurewithin the fluid 7 and compressibly held in place by a hollowed screw 17.

An undersized piston 18 attached to one end of a piston rod 19 is fully submerged in the fluid 7 within the upper portion ofthe cylinder 6. The piston rod 19 pivotally suspends the piston 18 about a pin 20 attached to a piston driving means 50 which includes a mass 42. The pin 20 passes through a cylindrical body 21 into which the other end of the piston rod 19 is screwed, which body 21 holds the pin 20 in parallel alignment with the pin 3transverse to the axis AA'.

The piston rod 19 has an accurately-machined tapered portion 39 at its lower end which co-operatively engages the piston 18 and is held in position by a screw 22 and a washer 23.

In conducting a viscosity measu rementtest, the piston 18 is forced by the piston driving means 50 to acceleratethrough a downward piston stroke into the fluid 7 from the position illustrated in Figure 1 until it strikes the gasket-opposable face 11. It is important to ensure that the fluid 7 flows symmetrically about the piston 18 through the downward pistonstroke by accurately maintaining the piston coaxiallywithin the cylinder 6 and thus axially aligned with the axisAA'.

This is achieved by a first set 24 and a second set 25 of ball bearings which are symmetrically disposed about the piston rod 19 and are mounted coaxiaily with respect to the axis AA' in a truncated conical housing 26. The conical housing 26 rests within an upper funnelled portion 27 ofthe cylinder block 1. Atubular spacer 28 is interposed the first set 24 and second set 25 of bearings, and an upper plate 29 and a lower plate 30 are screwed onto the upper and lower ends respectively of the truncated conical housing 26 to hold the bearings and spacer in place. The piston rod 19 is free to slide through the conical housing 26 guided bythe bearings 24 and 25, which bearings thus maintain both the piston rod 19 and the piston 18 in axial alignment with the axis AA'.

Atransverse metal arm 31 is attached at one end to the piston rod 19, and to the other is bolted a metal bar 32 from which suspended a bar magnet33 with a first pole 34 and a second pole 35. The transverse arm 31 and metal bar32actto hold the barmagnet33 rigidly in parallel alignment with the piston rod 19 and hence the axis AA'. The bar magnet 33 is slideably located within a plastictube 36 clamped to the cylinder block 1 in parallel aiignmentto the axisAA' byatube clamp (not shown). Afirst helical coil 37 and a second helical 38 of insulated wire are wound aboutthe plastic tube 36 each an identical number oftimes but in an opposite direction to the other.The helical coils 37 and 38 are located on the plastic tube such that in use the first pole 34 always remains well within the first helical coil 37, and the second pole 35 always remains well withinthesecondhelical coil 38, andtheythus comprise, when electrically connected in series, a pistonvelocitvtransducer 100 whose combined electro motive force (emf) output is proportional to the vertical velocity ofthe bar magnet 33 and hencethe piston 18 over a limited vertical displacement.

The piston 18 may be replaced by other pistons (not shown) of different dimensions as required, which is an advantage in that it increases the range offluid viscosities measureable using the viscometer 40. By removing the pin 20, the cylindrical body 21 may be disengaged from the piston driving means 50 and the piston 1 8togetherwith the piston rod 19, truncated conical housing 26, and bar magnet 33 may be verticallywithdrawn from the remainder of the viscometer40. The attachment ofthe piston 18 to the tapered portion 39 facilitates easy removal of the piston '18 from the piston rod 19, and its replacement by other pistons.

Figure 2 illustrates the piston driving means 50 and theviscometer40 mounted conjointly on the base unit 5. The piston driving means 50 comprises a lever arm 51 horizontally pivoted at one end about a pin 53 which passes th rough a vertical post 54, and weights 56 are freely suspended from the other end of the lever arm 51 in a tray 57. The lever arm 51 is attached to the viscometer40 bya clamp 58, which pivotally engages the cylindrical body 21 through the pin 20. The clamp 58 is slideable along the lever arm 51. Both the pin 3 and the pin 20 are illustrated in end view and are of necessity in parallel alignmentwith the pin 53.The lever arm 51 which is illustrated at a raised position 59, is suspended from an electromagnet 60 independent ly supported from above and activated to engage a steel bar 61 pivotally attached to the lever arm by a clamp 62.

In operation, the magnitude of the vertical force required to be exerted by the piston driving means 50 is selected by suitably adjusting the horizontal distance of the viscometer 40 from the vertical post 54 by moving the clamp 58 and the base block 4, and/or adjusting the number of weights 56. Once the viscometer 40 is vertically aligned, the viscosity measurementtest is commenced by de-activating the electromagnet 60 which releases the lever arm 51 and forces the piston rod 19 down into the cylinder block 1.

The measurementtestisterminatedwhenthe piston 18 of Figure 1 completes its downward stroke, which corresponds to the lever arm 51 falling to a second lower position 63, indicated by broken lines.

Figure 3 is a diagrammatic representation of the heating coil 9, the thermometer element 10, the pressure transducer 16, and the helical coils 37 and 38 ofthe viscometer 40 of Figure 1, and the electromagnet 60 of Figure 2, illustrating their interconnection with a gauge pressure recorded 70, a piston velocity recorder 71, a temperature indicator 72, and electrics power supply units 73 and 74. The velocity recorder 71 is electrically connected across the series-connected helical coils 37 and 38, and is calibrated to convert a voltage induced across them directly into a velocity reading. Similarly, the gauge pressure recorder 70 is electrically connected to the piezoelectric pressure transducer 16 and is calibrated to convert an electrical signal received from the transducer into a gauge pressure reading.The power supply unit 73 is electrically connected to the heating coil 9 illustrated in Figure 1, and the power supply unit 74 is electrically connected to the electromagnet 60 by a circuit 75 including a switch 76. A recorder activating connection 77 runs from the electromagnet 60 to both the velocity recorder 71 and the gauge pressure recorder 70. The temperature of the viscometer 40 (which may be controlled using the heating coil 9) is indicated on the temperature indicator 72 electrically connected to the thermometer element 10.

To perform a viscosity measurementthe electromagnet 60 is de-activated by opening the switch 76 and breaking the circuit 75. The steel bar 61 and hence the lever arm 51 both illustrated in Figure 2 are thereby released, and at the same instant an electrical impulse originating from the electromagnet60 is transmitted along the conneetion 77 to simultaneously activate the gauge pressure recorder 70 and the velocity recorder71. Gauge pressure and corresponding velocity readings are therefore recorded in phase along a common time base. The recorders 70 and 71 may be switched of at will after completion of the test measurement.

Wherethefluid 7 to be measured is a Newtonian fluid, only one pair of corresponding recorded readings for piston velocity and fluid gauge pressure need be taken, which for example may be taken at approximately the mid-point ofthe piston downward stroke through the cylinder.The viscosity ofthe fluid at the indicated temperature maythen be calculated from the following equation

where/U N = Fluid viscosity at the indicated tempera- ture P = Fluid gauge pressure beneath the piston V = Instantaneous piston velocity at Fluid pressure P Ro = Cylinder radius Ri = Piston radius L= Piston length Where the fluid to be measured is a Bingham plastic in which plastic viscosity is constant (ie shear stress and shear rate are linearly related only above an initial yield stress value), the piston remains stationary until the pressure beneath the piston reaches a yield pressure value, after which the piston accelerates from rest through the fluid.At least three recorded readings must be taken in orderto obtain a yield stress value and plastic viscosity for such a fluid; the fluid gauge pressure P at which position motion just begins, and at least one pairofsubse- quent readings for piston velocityV and fluid gauge pressure P taken,for example, at the mid-point ofthe piston downward strokethroughthe cylinder 6.

The yield stress b ofthe Bingham plastic atthe indicated temperature maythen be calculated from the following equation ll: (Ro-Ri) ra = pai II 2L This viscosity/UBP of the Bingham plastic at the indicated temperature may be calculated from the following equation lil from which all insignificant factors have been excluded.

where Ras, the outer radius of the non-shearing region within the annular gap between the piston when in motion and the cylinder may be solved by iteration from the following cubic equation IV which has only one rootforavalue of RB between that of Ro and Ri:

If several pairs of readings of V and Pare taken as the piston is accelerating, any deviation from linearity overthat range of fluid flow rates will be detectable after making measurements on only one piston stroke.

An alternative piston 85 permitting measurement of the viscosity of an electro-rheological fluid in the presence of an applied electrostaticfield is illustrated in Figure 4. This piston 85 and piston rod 86 are used in place of the piston 18 and piston rod 19 of Figure 1.

The piston 85 includes a cylindrical position core 87 of mild steel coaxially engaged with a tapered lower end 88 ofthe piston rod 86. An electrically conducting outer sleeve 89 of copper tube, and an electrically insulating,flanged inner sleeve 90 of polytetrafluoroethylene (PTFE) surround the piston core 87.

The core 87 is urged against the piston rod 86 by a coaxial screw 91 countersunk into a PTFE washer 92, which washer 92 also axially urges the outer sìeeve 89 againstthe flanged inner sleeve 90, and the inner sleeve 90 against a flanged portion 93 of the core 87.

The assembled piston 85 is an undersized fit in the cylinder 1 and provides an annular gap 96 which is filled, in use, with the electro-rheological fluid.

An insulated wire 94connected at one end a power supply unit 95 runs th rough a substantial portion of the piston rod 86 (which piston rod is in every other respect identical to the piston rod 19 of Figure 1) and into the piston 85to connect with the outer sleeve 89.

In use, a voltage is applied between the outer sleeve 89 and the cylinder block 1 by the power supply unit so asto set up a voltage gradient across the gap 96 containing the electro-rheological fluid. Viscosity measurements are then conducted in a similarmannerto thatalready described with reference to Figures 1,2 and 3.

Claims (6)

1. Apistonandcylinderviscometerhaving a hollow cylinder (6) with a closed first end (6a) and an open second end (6b), an undersized piston (18) coaxially slideable within the cylinder (6), and a piston driving means (50) capable in use of driving the piston (18) through the cylinder (6) towards the closed first end (6a) in which there is included a pressuresensingelement(16) located within the cylinder (6) adjacentthe closed first end (6a) capable of providing an output signal indicative ofthe instantaneous pressure therein and a piston velocity sensing element (100) capable of providing an output signal indicative of the instantaneous velocity ofthe piston (18).

2. Aviscometeras claimed in claim 1 in which the piston driving means (50) includes mass (42) mecha nically linked to the piston (18) through the open second end (6b) of the cylinder (6), the piston (18) being drivablethrough the cylinder (6) by the gravitational force acting on the mass (42).

3. An viscometer as claimed in claim 2 in which the mass (56) is mechanically linked to the piston (18) so asto applythe gravitational force acting on the mass (56) to the piston (18)with a mechanical advantage.

4. Aviscometeras claimed in claim 1 in whichthe piston (85) has an electricallyconducting outer surface (89) confronting the cylinder (6) which surface (89) is insulated from the piston (85) and the cylinder (6) and arranged for connection to a voltage source (95) so as to permit a voltage differential to be applied between the surface (89) and the cylinder (6).

5. A method of measuring the viscosity of a fluid (7) using a viscometer as claimed in claim 1 which includes the steps of- a. connecting thevelocity sensing element (100) to a piston velocity recording means (71); b. connecting the pressure sensing element (16) to a pressure recording means (70); c. charging the cylinder (6) with a sample of the fluid (7); d. initiating the piston velocity recording means (71) and the pressure recording means (70); e. initiating the piston drive means (50) thereby driving the piston (18) through the cylinder (6) towards the first closed end (6a); and f. calculating the viscosity ofthe fluid from the recorded values offluid pressure and piston velocity.

6. A method of measuring the viscosity of an electro-rheological fluid in the presence of an elec trostaticfield using a viscometer as claimed in claim 4 which includes the steps of- a. connectingthevelocitysensing element(100) toapistonvelocityrecording means (71); b. connecting the pressure sensing element (16) to a pressure recording means (70); c. chargingthe cylinder (6) with a sample ofthe fluid (7); d. applying a voltage differential between the surface (89) and the cylinder (6); e. initiating the piston velocity recording means (71) and the pressure recording means (70); and f. calculating the viscosity ofthefluid from the recorded value of fluid pressure and piston viscosity.