WO1990004161A1

WO1990004161A1 - Rheometer calibration

Info

Publication number: WO1990004161A1
Application number: PCT/GB1989/001206
Authority: WO
Inventors: David Chung-Hsun Cheng
Original assignee: The Secretary Of State For Trade And Industry In Her Britannic Majesty's Government Of The United Kingdom Of Great Britain And Northern Ireland
Priority date: 1988-10-13
Filing date: 1989-10-12
Publication date: 1990-04-19
Also published as: GB2242983A; GB9109004D0; GB8823987D0; GB2242983B

Abstract

A rheometer calibration device includes first (57) and second (62) attachment means whereby the device can be attached to a rheometer in a manner corresponding to a test item. Relative movement of the attachment means (57, 62) is resisted by a combination of at least two of a spring means (53, 71, 72) of known characteristics, a drag means (52, 50, 51) of known characteristics, and a frictional means (80, 81) of known characteristics.

Description

RHEOMETER CALIBRATION

The present invention relates to devices and methods for calibrating rheometers of the rotational and tensile-compressional types.

Rheometry, the study of the visco-elastic properties of materials, is becoming of increasing importance. Plastic materials are being increasingly used for many purposes and their rheological properties, and the way these properties change with reference to, for example, temperature, are frequently of vital importance for design purposes. Similarly the rheological properties of fluids, frequently a mixture of solid, gas and liquid', such as obtained from oil wells are of importance in designing transport and processing facilities. Yet another technology in which rheological properties are important is the food industry, in which these properties affect subjective estimates of the consistency of various products. There are many rheological instruments or rheometers on the market, there being 3 main types; namely rotational, tensile- compressional, and pressure flow (with the last of which this invention is not concerned). Ideally a particular instrument should be capable of giving results in terms of viscosity and elasticity for a particular test item. Rotational and tensile-compressional instruments are calibrated by the manufacturer through basic testing of torque/force/pressure and displacement/speed/flow rate character¬ istics and measurements of the dimensions of the measuring elements. The instruments can be calibrated by a user, in terms of viscosity characteristics, by using a standard viscosity liquid supplied by a national standards institution or by a commercial organisation.

Commonly available standard liquids are Newtonian in nature, whereas many substances to be tested, particularly fluids such as those output from oil wells,and food stuffs, are non-Newtonian. One method of calibrating an instrument for use with such a non- Newtonian fluid would be to carry out simultaneous tests on some suitable fluid using a standard rheometer, that has been properly calibrated by basic calibration, and the .rheometer to be calibrated. Any such standard rheometer would have to be maintained by, for example, a standards institution, and such a process could therefore be expected to be expensive and troublesome.

Liquids and fluids are usually tested using rotational instruments (or pressure devices) whilst solid materials are more usually tested on tensile-compressional instruments.

There is therefore a need for a device by means of which a user of a rheological instrument can quickly and easily calibrate the instrument.

According to the present invention a rheometer calibration device includes first and second attachment means whereby the device can be secured to a rheometer in a position corresponding to that of a test item, the attachment means being moveable relative to one another, characterised in that movement is resisted by a combination of at least two of a spring means of known characteristics, a drag means of known characteristics, and a frictional means of known characteristics. The drag means may include a dash-pot or may include electro¬ magnetic means. The first and second attachment means may be moveable relative to one another either in a rotational sense or in an axial sense, and the combination of at least two of the spring means the drag means and the frictional means may act either in series or in parallel.

According to another aspect of the invention a method of cali¬ brating a rheometer is characterised in that it includes the steps of securing to the rheometer, in place of a test item, a calibration device including first and second attachment means moveable relative to one another against resistance provided by a combination of at least two of a spring means of known characteristics, a drag means of known characteristics and a frictional means of known characteristics, and of operating the rheometer.

Rheometers with which the calibration device will be used are expected to incorporate computer software that will reduce testresults into parameters relating to the characteristics of the calibration devices. These parameters are known by calculation or otherwise for each device. If the rheometer reproduces these values the instrument is then judged to be correctly adjusted. Devices are known wherein relative movement of two components is opposed by single means which are used to measure the force causing the relative movement. For example in UK Patents 652,104 and 640,238 relative rotational movement between a driven shaft and a drive shaft is resisted by spring means. In 652,104 the relative rotation is con¬ verted into movement of measuring apparatus in an axial direction ^« relative to the shafts, and this movement is measured to give an in-

5 dication of torque. In 640,238 relative rotation results in the move- * ment of an iron core relative to an electromagnet, the electric current in a coil of the electromagnet giving a reading calibrated in terms of torque. Movement of the iron core is damped. However the damping is inteneded to remove the effects of small irregularities and pulsations 10 in the torque by damping movement of the iron core, and is not intended to affect the relative movement of the shafts.

In order that the invention be better understood some types of rheometers with which the invention can be used, and some devices according to the invention, will now be described, by way of example 15 only, with reference to the accompanying diagrammatic drawings, of which: Figure 1 is an elevation of a rheometer of the type known as a controlled stress viscometer,

Figure 2 is an elevation of a rheometer of a type known as controlled speed , 20 Figure 3 is an elevation of a tensile testing machine,

Figure 3a is a detail corresponding to an aspect of Figure 3 for a compression testing machine,

Figure 4A is an elevation, in Section along line II-II of Figure 4b of a device according to the invention, 25 Figure 4B is a plan view, in section along line I-I of Figure 4A of the device shown in Figure 4A,

Figure 5 is an elevation in section, corresponding to the section of Figure 4A, of another device according to the invention, and Figures 6a to 6g illustrate diagrammaticall frictional means 30 which can be used with the invention.

A controlled stress rheometer (Figure 1) has a controlled torque Unit 10 which drives a driven plate 11. A test piece 12 is placed between the driven plate 11 and a fixed plate 13 which is secured to A a base 14. In use the test material 12 is maintained at a desired known

35 temperature and pressure, the driven plate 11 is subjected to a controlled torque and the resultant speed of rotation of the driven plate 11 is measured. The rotational displacement or speed is transmitted to a processing and display Unit 15 where it is combined with the known torque and the calibration of the instrument to provide details of elasticity, viscosity and the relation there-between of the test specimen 12. In a controlled speed rheometer (Figure 2) a controlled speed drive box 20, controlled by a control unit 21, and a torque measurement Unit 22 are mounted on a stand 23. A test piece 24 is mounted between a driven plate 25 driven by the controlled speed drive 20 and a torque plate 26 mounted on the torque measuring Unit 22. In use details of

the torque Unit 22 and the control Unit 21 are passed to the process¬ ing and display Unit 15 where they are allied with the calibration details of the instrument to provide a display or print-out of the rheological properties of the test piece 24. In a tensile testing device (Figure 3) a test item 30 is placed between sample holders 31 mounted respectively on a fixed mounting 32 and a powered mounting 33 through which a controlled tensile force or displacement is applied to the specimen 30. Details of the tensile force or displacement and of the rate of relative movement of the chucks 31 are passed td a processing and display unit (not shown). As an alternative to the tensile device of Figure 3, a compression device has a similar design but with the holders 31 replaced by Pistons 40 (Figure 3a) between which is placed a test specimen 41. A device according to the invention for calibrating rheometers (Figure 4) has a hollow cylindrical drum 50 having a cavity 51 filled with a standard viscosity liquid and containing a cylindrical (preferably hollow) body 52 coaxially mounted in the drum 50. The body 52 is connected to one end of a spring 53, to the other end of which is secured a shaft 54 which extends along the axis of the body 52 and drum 50 through an internally threaded socket 55, into which is screwed a plug 56, to be secured to a first plate 57. An annular flange 58 secured to the plate 57 moveably encloses a boss 59 of the plug 56 and an elastomeric collar 60 sealingly contacts the shaft 54 and the inner surface of the socket 55 to prevent spillage of the calibrated liquid from the cavity 51. A spacer 61, which terminates in a jewelled bearing, locates the body 52 in the member 50, and the member 50 is secured to a second plate 62. The plates 57, 62 are parallel and normal to the axis of the drum member 50. In use the plug 56 is unscrewed from the socket 55, allowing the elastomeric collar to release the shaft 54. To test a controlled torque instrument such as that illustrated in Figure 1, the device is positioned in place of the test piece 12 by securing the plate 57 to the driven plate 11 of the instrument and the plate 62 to the fixed plate 13 of the instrument. The instrument is then operated in the normal fashion, the known characteristics of the device having been fed into the processor 15. The equation governing the motion of the device is

^{T +} I ⁼ θ (1) Where T equals torque, Cs is the rigidity factor of the spring, Cdp is the viscosity factor of the dashpot and θ is the angular displacement. The instrument can thus be calibrated.

The controlled speed instrument of Figure 2 can be calibrated in an identical fashion by securing the device in place of the test piece 24, securing Plate 57 to Plate 26 and Plate 62 to Plate 25. It will be realised that, in this device, the Spring 53 and dashpot 50,51 2 act, effectively, in Series. In a similar device (Figure 5) wherein like items are denoted by like numbers, the spring and dashpot operate, in effect, in parallel. This device differs from the device of Figure 4 in having, instead of Shaft 54, a Shaft 70 secured to the body 52 and to the Plate 57 and, instead of the spring 53 extending between the body 52 and shaft 54, a spring 71 coaxial with the axis of the drum 50 and shaft 70 having one end secured to the drum 50 and the other end secured to the body 52. This device might retain the spacer 61 or, alternatively and as illustrated, have this replaced by a second spring 72 also secured at one end to the drum 50 and at the other end to the dashpot 52.

Use of this device in calibrating instruments is idential with use of the device of Figures 4, the equation of motion in this case being T»Cs θ + Cdp θ (2)

The devices of 4 and 5 can be adapted to calibrate push-pull instruments such as those shown in Figures 3 and 3a. The devices are inherently capable of allowing for relative linear movement of the plates 57, 62. When used for linear measurement purposes the spacer 61 becomes redundant. In practice it will usually be unrealistic for a device to be usable both as a rotational and linear calibrator. For maximum efficiency the relative dimensions of the various items will need to be optimised depending on the particular use of a particular instrument. When used as a linear calibrator a device will have the plates 57, 62 secured to the chucks 31 or pressure plates 40 to replace, respectively, test piece 30 or test piece 41.

When used for linear testing the equation of motion for a device as illustrated in Figure 4 is:

£ ⁺ £ = X, Cs Cdp where F is Force and (3)

X is linear displacement, and

For the device as illustrated in Figure 5 the equation is F = Cs X + Cdp X (4)

For testing some materials where yield or rigid-plastic behaviour is important, such as those which do not deform until a critical yield stress is exceeded, a frictional element may be added to the devices described above, or substituted for the spring means 53, 71, 72 or drag means 50, 51, 52. The frictional element might be, for example, a friction pad 80 (Figure 6a) secured to a shaft 54 or 70, co-acting with a fixed friction pad 81. The frictional effect might be variable by means of, for example, a thumbscrew as illustrated diagrammatically in Figure 6g. The frictional element might be additional to the spring and drag means in the devices of Figures 4

(as illustrated diagrammatically in Figure 6e) and of Figures 5

(as illustrated diagrammatically in Figure 6f), or might be substituted for one of these, (as illustrated diagrammatically in

Figures 6b, c and d). It will be realised that many alternative constructional arrangements for the device, other than those described above, are possible within the scope of the invention. For example whilst helical springs have been illustrated other springs, such as torsion bars or leaf springs can be used, depending on the torque level desired. Also alternatives to the dashpot 52 are, for example, a disc or plurality of discs, a paddle, a cone and plate or double cones.

Standard viscosity liquids usually have viscosities dependent on temperature, and this will have to be taken into account during the calibration process. Temperature measuring means, such as, for example, a thermocouple, (not shown) are therefore preferably provided for recording liquid temperature. It may in some cases be necessary to have the provision of temperature control. The nature of the devices is preferably such that, as illustrated, a standard liquid can easily be replaced, to allow liquids of differing viscosities to be used as required by various calibration purposes, and to allow for deterioration of the liquid as a standard.

In some embodiments of the invention it may be preferable to use, as a way of overcoming possible deterioration of standard liquids, and to avoid temperature effects, magnetic means to provide drag. Such means are well known and their incorporation in place of the dashpot 50/51/52 and liquid arrangements of Figures 4 and 5 will be readilycomprehensible to those skilled in the art and will therefore not be described in detail here. Likewise processing and calibration methods using modern micro-computers are well known, do not per se form part of the present invention and will also not be described in detail here.

Claims

8 CLAIMSWhat is claimed is:-

1. A rheometer calibration device including first (57) and second (62) attachment means

device can be secured to a rheometer in a position corresponding to that of a test item, the attachment means being moveable relative to one another, characterised in that movement is resisted by a combination of at least two of a spring means (53, 71, 72) of know characteristics a drag means (52, 50, 51) of known characteristics and a frictional means (80, 81) of known characteristics.

2. A device as claimed, in Claim 1 characterised in that the combination consists of a spring means and a drag means.

3. A device as claimed in Claim 1 characterised in that the combination consists of a spring means, a drag means and a frictional means.

4. A device as claimed in any one of Claims 1 to 3 characterised in that the first and second attachment means are moveable relative to one another in a rotational sense.

5. A device as claimed in any one of Claims 1 to 3 characterised in that the first and second attachments means are moveable relative to one another in an axial sense.

6. A device as claimed in any one of Claims 2 to 5 characterised in that the drag means includes a dashpot.

7. A device as claimed in any one of Claims 2 to 5 characterised in that the drag means includes electro-magnetic means.

8. Amethod of calibrating a rheometer characterised in including the steps of securing to the rheometer, in place of a test item, a calibration device including first (57) and second (62) attachment means moveable relative to one another against a resistance provided by a combination of at least two of a spring means (53, 71, 72) of known characteristics, a drag means (52, 50, 51) of known characteristics and africtional means (80, 81) of known characteristics and of operating the rheometer.

? 9. A method as claimed in Claim 8 characterised in that the resistance is provided by a spring means of known characteristics and a drag means of known characteristics.

10. A method as claimed in Claim 8 characterised in that the resistance is provided by a spring means of known characteristics, a drag means of known characteristics and a frictional means of known characteristics.

11. A rheometer calibration device substantially as herein described with reference to Figures 4 to Figures 5 of the accompanying drawings.

12. A rheometer calibration device substantially as herein described.

13. A method of calibrating a rheometer substantially as herein described with reference to the accompanying drawings.

14. A method of calibrating a rheometer substantially as herein described.

15. A rheometer calibrated by the method described in any one of Claims 8 to 10, 13 or 14.