VISCOMETER

Abstract: 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 (19) 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 (19) 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.

Description: TITLE: VISCOMETER Technical Field This invention relates to a viscometer which operates on the piston and cylinder principle of fluid viscosity measurement (herein¬ after known as a piston and cylinder viscometer), which viscometer may be applied to the measurement of the rheological properties of various types of fluids, in particular but not exclusively Bingham plastics. Background Art Various types of piston and cylinder viscometers 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 of the fluid exerted on the piston. This piston is usually coaxially guided within the cylinder to provide a constant annular gap between the piston and the walls of the 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 order that fluid viscosity may be measured a relationship must be obtained between the shear stress applied to the fluid under test and the corresonding shear rate induced. In piston and cylinder viscometers the shear stress applied across an annular volune of fluid flowing between the piston and the cylinder walls is derived from the vertical force the piston exerts on the cylinder walls through the fluid. A free 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 cylinder viscometers have several advantages over other types of viscometer in that they are simple to operate, they are generally rugged in construction and they usually only require a small sample of fluid 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 of the piston. A viscometer which relies on continuous or cyclical shear stressing of the sample would necessitate sample cooling to prevent excessive temperature rise. The pre-requisite that the piston reaches a maximum, terminal velocity through the fluid, does however give rise to a number of disadvantages in a piston and cylinder viscometer. Unless some of the fluid rheological properties are already known, it may not be possible to ensure that the piston will reach terminal velocity before velocity measurements are made, which is a particular disadvantage where the velocity measurement consists of timing the fall of the piston between two fixed points. More importantly, such a viscometer can 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 complex than 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 cylinder viscometer wherein these disadvantages are overcome or at least mitigated. According to the present invention, a piston and cylinder visco- meter having a hollow cylinder with a closed first end and open second end, an undersized piston coaxially slideable within the cylinder, and a piston driving means capable in use of driving the piston through the cylinder towards the closed first end includes a pressure sensing element located within the cylinder adjacent the closed first end capable of providing an output signal indicative of the instantaneous pressure therein and a piston velocity sensing element capable of OMPI s y , vmo RN A \o sensing element capable of providing an output signal Indicative of the instantaneous velocity of the piston. The viscometer is best suited for the measurement of non-gaseous fluid viscosities. In use, a sample of a fluid to_bd 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 free falling body whose ' mass is mechanically linked to the piston through the open second end of the cylinder by a coaxial piston rod. The mass of the free falling body is advantageously of a sufficient magnitude to ensure that for 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 co- axially disposed about the piston rod within a housing mounted upon the cylinder.adjacent the open second end. The pressure sensing element is preferably a piezoelectric transducer electrically connected to a pressure recorder. The pressure recorder is conveniently 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 coax±ally ' 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 automatically converted to piston velocity reading on the velocity recorder. * Modes of Carrying Out the Invention Embodiments of the present invention will now be described by way 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 prior to the performance of a viscosity measurement test, OMPI Figure 2 Is a simplified elevated view of the same viscometer disposed 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 bloc 4 which in turn rests upon a base unit 5 also illustrated in Fiure 2. Within the cylinder block 1 Is coaxially disposed a cylinder 6 having a closed first end 6a and an open second end 6b filled with a non-gaseous fluid 7 whose viscosity is to 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 of the cylinder block 1 surrounding the cylinder 6 is externally recessed to hold an electrical heating coil 9 and a platinum coil resistance thermometer 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 pressure within 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 of the 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 3 transverse to the axis AA 1 . 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 OMPI s , VIPO position by a screw 22 and a washer 23. In conducting a viscosity measurement test, the piston 18 is forced by the piston driving means 50 to accelerate through 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 pistons roke by accurately maintaining the piston co- axially within the cylinder 6 and thus axially aligned with the axis AA' . 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 coaxially with respect to the axis AA * in a truncated conical housing 26. The conical housing 26 rests within an upper funnelled portion 27 of the cylinder block 1. A tubular 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 by the bearings 24 and 25, which bearings thus maintain both the piston rod 19 and the piston 18 in axial align- ment with the axis AA' . A transverse 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 magnet 33 with a first pole 34 and a second pole 35. The trans¬ verse arm 31 and metal bar 32 act to hold the bar magnet 33 rigidly in parallel alignment with the piston rod 19 and hence the axis AA' . The bar magnet 33 is slideably located within a plastic tube 36 clamped to the cylinder .block 1 In parallel alignment to the axis AA' by a tube clamp (not shown). A first helical coil 37 and a second helical 38 of insulated wire are wound about the plastic tube 36 each an identical number of times 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 within the second helical coil 38, and they thus comprise, when electrically connected in series, a piston velocity transducer 100 whose combined electro motive force (e f) output is proportional to the vertical velocity of the bar magnet 33 and hence the piston 18 over a limited vertical displacement. OMPI v sAAV , V/IPO 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 of fluid viscosities measureable using the visco- meter 40. By removing the pin 20, the cylindrical body 21 may be disengaged from the piston driving means 50 and the piston 18 together with the piston rod 19, truncated conical housing 26, and bar magnet 33 may be vertically withdrawn from the remainder of the viscometer 40. The attachment of the piston 18 to the tapered portion 39 facilitates easy removal of the piston 18 from the piston rod 19, and its replace- ment by other pistons. Figure 2 illustrates the piston driving means 50 and the visco¬ meter 40 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 through 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 viscometer 40 by a clamp 58, which pivotally engages the cylindrical body 21 through the pin 20. The clamp 58 Is slldeable along the- lever arm 51. Both the pin 3 and the pin 20 are illustrated in end view and are of necessity in parallel alignment with the pin 53. The lever arm 51 which is illustrated at a raised position 59, is suspended from an electromagnet 60 independently 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 adjust¬ ing 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 measurement test 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 measurement test is termin¬ ated when the 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 of the viscometer 40 of Figure 1, and the electromagnet 60 of Figure 2 , illus trat ing thei r interconnection wi th a gauge pressure recorder 70, a piston velocity recorder 71 , a temperature indicator 72 , and electrical power supply uni ts 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 measurement the 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 electromagnet 60 is transmitted. long the connec¬ tion 77 to simultaneously activate the gauge pressure recorder 70 and the velocity recorder 71. 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. Where the fluid 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 approx¬ ima tely the mid-point of the piston downward s troke through the cylinder. The viscosity of the fluid at the indicated temperature may then be calculated from the following equation I: P [ (Ro 2 4- Ri 2 ) loge (Ro/Ri) - (Ro 2 - Ri 2 ) ] I / U N = 4LV where / / U N = Fluid viscosity at the Indicated temperature 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 order to 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 pair of subsequent readings for piston velocity V and fluid gauge pressure P taken., for example, at the mid-point of the piston downward stroke through the cylinder 6. The yield stress /6 of the Bingham plastic at the Indicated temperature may then be calculated from the following equation II: 2L * This viscosity /U__ of the Bingham plastic at the indicated temperature may be calculated from the following equation III from which all insignificant factors have been excluded. PR 2 HS - P5(Ro-Ri)R β ^ PR^ - Pj5(Ro-Ri)) 2L P where R , 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 root for a value of R_, between that of Ro and Ri: -gtjREAt> OMPI - R 2 3(P 2 (Ro+Ri)+2PS 2 (Ro-Ri)) 3R_(Ro-Ri) (P 2 (Ro+Ri)+P/S 2 (Ro-Ri)) 4- + - 4P/JP 4P' 3 3 P/6 3 (Ro-Ri) 3 3P(Ro 4 -Ri 4 ) Ro +RI I 8P 3 8P<S(Ro-Ri) -2 If several pairs of readings of V and P are taken as the piston is accelerating, any deviation from linearity over that 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 electro¬ static field 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 of the' piston rod 86. An electrically conducting outer sleeve ' 89 of copper tube, and an elec¬ trically 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 sleeve 89 against the 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 94 connected at one end to a power supply unit 95 runs through 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 85 to 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 95 so as to set up a voltage gradient acorss the gap 96 containing the electro-rheological fluid. Viscosity measurements are then conducted in a similar manner to that already described with reference to Figures 1, 2 and 3.

Claims: CLAIMS 1. A piston and cylinder viscometer having 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) characterised in that there is included a pressure sensing element (16) located within the cylinder (6) adjacent the closed first end (6a) capable of providing an output signal indicative of the instantaneous pressure therein and a piston velocity sensing element (100) capable of providing an output signal Indicative of the instantaneous velocity of the piston (18). 2. A viscometer as claimed in claim 1 characterised in that the piston driving means (50) includes mass (42) mechanically linked to the piston (18) through the open second end (6b) of the cylinder (6), the piston (18) being drivable through the cylinder (6) by the gravita¬ tional force acting on the mass (42). 3. An viscometer as claimed in claim 2 characterised in that the mass (56) is mechanically linked to the piston (18) so as to apply the gravitational force acting on the mass (56) to the piston (18) with a mechanical advantage. 4. A viscometer as claimed in claim 1 characterised in. hat the piston (85) has an electrically conducting outer surface (89) con¬ fronting 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 visco¬ meter as claimed In claim 1 characterised by Including the steps of - a. connecting the velocity 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); - lHEXl OMPI ^JVAT10 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 of the fluid from the recorded values of fluid pressure and piston velocity. 6. A method of measuring the viscosity of an electro-rheological fluid in the presence of an electrostatic field using a viscometer as claimed In claim 4 characterised by including the steps of — a. connecting the velocity 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. 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. alculatlng the viscosity of the. fluid from the recorded value of fluid pressure and piston viscosity.

Inventors: STANGROOM JAMES EDWARD, STANGROOM, JAMES, EDWARD
Applicants: SECR DEFENCE BRIT, THE SECRETARY OF STATE FOR DEFENCE IN HER BRITANNI
Application year: 1983
Main classification: G01N11
IPC codes: G01N11