WO2004070359A1

WO2004070359A1 - Yield test method and apparatus

Info

Publication number: WO2004070359A1
Application number: PCT/US2004/003339
Authority: WO
Inventors: David A. Brookfield
Original assignee: Brookfield Engineering Laboratories, Inc.
Priority date: 2003-02-04
Filing date: 2004-02-04
Publication date: 2004-08-19

Abstract

Yield test for fluids utilizing viscometer apparatus through an elastic range of torque-time characteristics to drive a shearing member in contrast with the fluid to a yield state of non-linear torque-time characteristic and measuring, displaying and storing the data of the test through the linear state and yield state, the test being a process and apparatus that can also present the data in stress-strain mode and/or effect conventional viscosity measurement after yield.

Description

YIELD TEST METHOD AND APPARATUS

Field and Background of the Invention

The present invention relates to rheology measurement or rheology. The field involves characterization of shear stress and viscosity of fluids tested under well known conditions for laboratory research purposes and industrial process quality control purposes.

It is an object of the present invention to provide a test of yield point - the critical shear stress applied to a fluid sample material at which the material begins to flow as a liquid.

It is a further object of the invention to provide such a test applicable over orders of the magnitude of yield points for diverse materials including solvents, paints, food stuff (e.g. puddings, ketchup, dough), emulsions, gels/lubricants, industrial chemicals, inks, etc. Summary of the Invention

The objects of the invention are met by a new yield test process and apparatus utilizing a driven shear member such as a round spindle, vaned or ribbed spindle, disk, cone, paddle or other form rotating in or against a sample of the fluid to be tested. A rotating vaned cruciform spindle is preferred. The spindle for the shear member is driven by a motor via an intermediate calibrated spring illustrated schematically in FIG. 1. As the shear member meets increasing resistance over time, greatly increasing torque ultimately to a point where the fluid yields and torque increases at a lesser rate, reaches a maximum point corresponding to maximum torque and then decreases as shown in the torque-time plots of FIG. 2 (no yield) and FIG. 3 (yield). The maximum torque can be converted to or from a shear stress value, taking account of size and shape of the shear member (the portion of it in contact with the fluid), and the full scale torque range of the calibrated spring (in dyne-cm or 10^"3 Newton-m with various springs available over a range of several orders of magnitude). The calibration can be carried out by a hard wired logic circuit or in a programmed digital computer or a hybrid processor.

The test can be applied with customized variations for specific fluids corresponding to suitability for specific service conditions desired to be evaluated. The fluid can, optionally, be preconditioned by pre-shearing before doing a final shearing test with yield evaluation. The pre-shearing can be done externally or in the instrument of the invention, which should be re-zeroed after pre-shearing. The drive speed, and hence shear member run speed, can be varied for different types of torque-time curves. Generally a higher run speed will make a test fluid appear stiffer (steeper slope of torque-time curve) and increase the apparent yield stress.

Signal pick up of torque is made by a transducer associated with the drive assembly. The transducer can be a pair of coupled coils, magnetic or optical rotor part(s) on the drive shaft or spring plus optical detector or magnetic coil pickup on fixed structure of the instrument, Hall Effect device, rf pickup, acoustic detector and other transducers known in the art.

The readout from the transducer for display and/or processing signal purposes can be continuous or discontinuous.

Examples of common viscometer structures usable in drives, fluid immersion, detection are given in prior U.S. patents of myself and/or the assignee of this application, nos. 3,886,789 (6/75); 4,175,425 (11/79); 4,448,061 (5/84); 5,167,143 (12/92); 5,503,003 (4/96); 5,531,102 (7/96); 5,535,619 (7/96); and UK 205,341 (4/81), the content of said patents being incorporated herein by reference as though spelled out at length herein. Readings are taken at appropriate intervals.

In processing numerous fluid samples high and/or low alarm limits can be imposed to signal departure from an acceptable range.

Other objects, features and advantages will be apparent from the following detailed description of preferred embodiments taken in conjunction with the accompanying drawings in which:

385EIEF BESCKIPTΪON OF THE BEAWTOGS FIG. 1 is a schematically illustrated yield test apparatus; and FIGS. 2, 3 are torque vs. time curves as described above; FIG. 4 is an isometric view of a preferred form of spindle to implement the yield test; and

FIGS. 5A-5C are state diagram of software or firmward used for converting torque (shear stress-psi or Pa equivalent or percent of torque); '

FIGS. 6A and 6B illustrates a sample computer display for a tested product (a hair gel) showing torque vs. time and converted stress-strain (apparent strain) for the product. BRIEF DESCRIPTION OF PREFERRED EMBODIMENTS

FIGS. 1-3 were described above.

FIG. 4 shows a preferred form of vaned spindle 3 used in the viscometer device. It has a shaft Sh terminating in a cruciform vaned end with vanes V- 1 , N-2, N-3, V-4 establishing quadrants of fluid in a beaker or other test vessel trapped between orthogonal pairs of vanes and forming a virtual cylinder NC-1 (though in practice rotational shear effects occur further out at a second virtual cylinder indicated at NC-2). The dimensions of latter can be calculated and its diameter used in conversion of torque-time data and curve presentations to equivalent stress-strain presentations. This permits further analysis of fluid parameters including elasticity and viscoelasticity.

The viscometer is of a continuous read-out type as described in principle in my prior U.S. patent 4,448,061 granted May 15, 1984 but rather than using a synchronous motor, and gear box as shown in the '061 patent, the present invention employs a stepping motor and pulse feed controller, well known in the art per se, to establish pulse count groups and timing. Torque is the shear stress as measured by a spring coupling yielding in response to stress encountered at the spindle to establish a relative angular deflection continuously measurable by various forms of electrical, magnetic, optical, acoustic pick-ups well known in the art per se and convertible to a digital signal.

In accordance with the present invention the torque-time relation can be measured in two ways: either set a time base or torque base.

The time base corresponds to a certain increment of torque in a linear or near linear range of the relationship and indicates lesser torque increases as the fluid yields (moves from elastic through viscous) and the torque time selection.

The torque base is standard increment of torque that can correspond to an expected increment of time in a linear range and departure from linear corresponds to yielding. For many fluids, yield can be a curve reflecting gradually increasing time required to reach a given torque increment (or lesser torque change per lime increment) until the fluid finally reaches an ultimate break point, where torque can fall with additional time. See FIG. 3.

A calibrated rigid line of torque-time can be established by clamping the viscometer shaft as shown in FIG. 1. The real fluid response with the shaft unchanged will be a departure from the ideal rigid line response as indicated in FIG. The tests thus established can perform yield measurements (and subsequent viscosity tests after viscous mode is wholly or fully achieved) using a modified viscometer of under $5,000 cost with a high correlation (about 95%) to yield test results obtainable on sophisticated instruments costing well upwards of $30,000 (in some cases about $90,000). It was not apparent or otherwise obvious prior to the present invention that such reliable yield data could be so obtained economically. Testing has established validity of the present method and implementing apparatus.

FIGS. 5A-5D show state flow diagrams of the program for implementing the test. The basic operating functions are as follows: PRESET VALUES: Certain values will be entered by the operator before a yield test is performed. These are, spindle selection, optional pre-shear RPM and time, optional Zero RPM, optional Wait time, Base increment in milliseconds (100 to 2000), torque reduction value in percent (10% to 120%), and yield run RPM (up to 2 RPM) in the present configuration. CALIBRATION: A calibration mode can be added wherein the instrument will run it's motor with the sensing shaft locked rigid in order to determine the rigid torque displacement against a fixed amount of shaft movement. The instrument can pass over the initial startup of motor movement in order to ignore any mechanical inconsistencies. This may be either a fixed period of time, or delta torque/delta time which can be measured to determine where straight-line response begins. The motor then continues to turn for a fixed amount of time and the resulting torque displacement is saved, with the elapsed time, in a programmable ROM.

PRE-SHEAR: The operator is presented with the option of adding a pre-shear mode. The operator enters a motor speed and a time. Upon starting the yield test, the motor is run at the specified speed for the specified time, after which it will stop.

ZERO: This is an optional operation in which the motor shaft is rotated at a low speed in the reverse torque direction (or maintained at 0 RPM) until 0% torque is reached. The operator can select from 0 RPM or 0.1 RPM as motor speeds for zeroing. WAIT: The Wait mode is an optional period of time, entered by the operator, during which the motor shaft is held at 0 RPM. At the end of this time, the yield test is automatically continued. YIELD RUN: The motor begins running at a pre-entered yield run RPM. In the preferred embodiment , the instrument reads the torque at the end of every time increment (base increment value) and continues to do so until the change in torque during a single time increment is equal to, or less than, the torque reduction value. It may be necessary to throw out a first one or two base increments in order to by-pass inconsistencies in starting up the motor, or possibly to require a delta torque greater than the torque reduction value within the first 10% before testing for a "yield done" conditions (see Yield Done below). The torque reduction value is determined by reducing the calculated rigid torque displacement (for the selected base increment) by the pre-entered torque reduction percentage.

YIELD DONE At the end of the yield test an appropriate screen will be displayed having the yield value shown as "Pa" (shear stress in Pascals). This is the last torque value multiplied by the selected spindle factor (see "misc." below for existing multipliers). Over-range and under-range messages may be applied for ending torques less than 10% or greater than 100%, e.g.,. the test automatically ending with an "over-range" when the absolute torque exceeds 100%. A possible adjunct to this is to look for the delta torque to exceed the 'torque reduction value' within the first 10% of absolute torque, and to exit as "underrange' if this is achieved. There can be multiple selectable yield spindles 'and multipliers, e.g. a set of 0. 198, 0.806, 3.087, 14.58 multipliers established by appropriate spindles.

FIGS. 6A and 6B show graphic display (as torque-time in 6A and stress-strain in 6B) for a test made on hair gel in its shallow jar commercial container using a small vaned spindle that could be immersed. The test using the apparatus and software of FIGS. 1, 4, 5A-5D discussed above, produced a clean yield display with mid-range torque at 44.5% of yield reportable as 44.38 Pa. The internal point-to-point of measurement until yield was 1 second per increment of torque increase. Run speed for the driven spindle was 0.1 rpm. The yield torque and yield shear stress can also be preserved and/or presented in data table form.

The alternative program provides a delta (difference) torque value as a base for timing steps. That is, if the delta is put at 1% torque increase, then the program uses a counter means to count time for a 1% increase (successive increments advancing in an essentially linear progression per the slope) of the torque-time curve until yield break point when the progression expands non-linearly). The user sets a level of increase to show yield - typically 5x where x is a base increment of the % time increase in the linear portion of the scale or a selected single increment from within the linear portion.

This can also be set in successive increments to do a count in each increment slightly larger than the previous one per the linear slope, e.g. 1.05x prior point count (time to reach delta torque) with yield to be recognized only when the much larger time increment (e.g. 6x) is recognized.

The user creates test programs in software provided with the instrument. The instrument has multiple memory locations (e.g. ten) each for holding a particular test to capture all torque-time data of a test, not just the yield point.

The user enters test parameters of spindle immersion, zero spin, wait time, run speed (rpm), bar intercepts of interval between data points, torque reduction and low/high limits of the yield.

The embodiment can be provided in different sizes/configurations/resistance of spring and vane for different measurements. Spring torques for different range and vane shear stress range can be, e.g.

The pre-shear rotational speeds can be .01 to 200 rpm, with zeroing at .01-0.5 rpm and yield testing at .01 to 5 rpm. Temperature sensory and adjustment is provided at a -100 to 300°C range. The instrument can provide an analog output of 0 to 1 volt DC for 0 to 100% torque and a 0 to 4 volt reading for a -100 to 300°C temperature. Torque measurement has ± 1% of full scale accuracy and ±10.2% repeatability.

It will now be apparent to those skilled in the art that other embodiments, improvements, details, and uses can be made consistent with the letter and spirit of the foregoing disclosure and within the scope of this patent, which is limited only by the following claims, construed in accordance with the patent law, including the doctrine of equivalents.

What is claimed is:

Claims

1. A rheological yield test process comprising stressing a fluid to be tested to establish a torque-time line initially linear and departing from linear to a defined degree at yield, the line being established in incremental measurements.

2. The test process of claim 1 using time base increments.

3. The test process of claim 1 using torque base increments.

4. The test process of any of claims 1-3 wherein torque-time presentation is converted to a stress - strain presentation at yield and throughout the test, thereby permitting further analysis of fluid parameters such as elasticity and/or viscoelasticity.

5. Apparatus for yield test of a fluid comprising means for stressing the fluid to be tested to establish a torque-time line initially linear and departing from linear to a defined degree at yield, the line being established in incremental measurements.

6. The apparatus of claim 5 constructed and arranged to use time base increments.

7. The apparatus of claim 5 constructed and arranged to use the torque time increments.

8. The apparatus of any of claims 5-7 constructed and arranged to convert torque-time presentation to a stress-strain presentation at yield and throughout the test, thereby permitting further analysis of fluid parameters such as elasticity and/or viscoelasticity.

9. The apparatus of claim 8 constructed and arranged to also conduct viscosity measurement of the fluid after yield.