DHR/AR Rheometer: Setting Up a Flow Sweep Test

In this topic
Overview
Test Setup

Overview

In a rate (stress sweep) experiment the equilibrium stress, i.e., rate upon a strain rate or stress step, is measured. The steady state in this experiment is determined based on time or using a steady-sensing algorithm. The averaged equilibrium stress, or rate and viscosity, are recorded as a function of the strain rate or stress. Opposite of the rate ramp, the step rate experiment provides equilibrium data, and the results are not dependent on the test time (ramp rate).

In a time-based approach equilibrium and averaging time zones are set. The steady state viscosity is determined in the averaging zone. At low rate, the time to reach steady state is much longer than at high rates. For this reason, the time zones can be scaled with the applied rate or stress. Averaging over two directions eliminates drifts of the torque transducer and reduces secondary flow effects. Steady state sensing is an automated algorithm to record the steady state viscosity.

The rate (stress sweep) experiment is the method of choice to determine the zero shear viscosity of materials with long relaxation times, such as HDPE. Oscillation frequency sweeps become unpractical when the representative material relaxation is in the order of 1000 seconds and more; a frequency below 10-3 rad/s needs to be applied to determine the zero shear viscosity.

Test Setup

To select a Flow Sweep test, see Using Experimental Procedures for detailed instructions.

When you perform a Flow Sweep test, the following parameters need to be chosen.

Environmental Control

Select the following environmental control parameters:

Temperature: The temperature selection to maintain throughout the test. Enter the desired test temperature, or select Inherit set point to maintain the previously specified temperature at the start of this step. The temperature range is dependent on the configuration of your instrument and the installed environment control system.
Wait for temperature: Select this option to wait until the entered temperature is reached before beginning data collection. If you wish to begin data collection, while achieving the temperature, disable this option.
Soak time: The amount of time to delay data acquisition at the start of the step, typically to allow for temperature equilibration. This time is measured from the start of the step if Wait for temperature is not selected, or from the point at which the measured temperature becomes stabilized at the specified Temperature, if Wait for temperature is selected. Because of the mass of the sample, test fixtures, and environmental control systems a "soak time" (i.e., time to equilibrate at temperature) is recommended, particularly when starting steps at different temperatures. A five-minute soak time is sufficient for most samples in cases where the change in temperature is not too large. This time is also used at each increment temperature.

Test Parameters

Three types of flow sweep tests can be run. Choose the desired method from the list below. The available range is dependent on your instrument type and configuration.
Logarithmic sweep: The logarithmic sweep uses the entered values as the starting and ending range of the test, with intermediate points spaced logarithmically. Logarithmic sweeps can be run in ascending or descending order.
Choose between Torque, Stress, Angular velocity, and Shear rate, then enter the desired range.
Enter the desired Points per decade. This sets the number of points collected in each decade, based on the initial value. The final value is always collected, regardless of whether it is part of the normal pattern.
Linear sweep :The linear sweep uses the entered values as the starting and ending range of the test, with intermediate points calculated by adding or subtracting the increment until the final value is reached.
Choose between Torque, Stress, Angular velocity, and Shear rate, then enter the desired range.
Choose between Increment or Number of points to be used when collecting test data.
Enter the desired Increment: The increment specifies the change in value between subsequent measurements. If the final value is greater than the initial value, then this value is added at each measurement. If the final value is less than the initial value, then this value is subtracted. The final value is always measured, regardless of whether it is generated by the increment or not.
Enter the desired Number of points: The number of data collected in the specified range. This includes both the initial and final value. This includes both the initial and final value.

Discrete sweep: The discrete sweep takes a measurement at each frequency in a list of frequency values, with up to 10 discrete frequencies being specified. The test frequencies in the list can be run in any order (i.e., do not have to be monotonic).
- Select between Torque, Stress, Angular velocity, and Shear rate, then enter the desired range..
- Enter the value into the field.
- Click Add to add another value row.
- Repeat steps 2-3 until the desired values are specified.

If you want to edit the values already displayed in the table, place the cursor in the table at the desired point and make your editing changes. The entries will be used in order until the first 0.00 entry is encountered, which is recognized as the end of the table.

Click Add to add a new value at the selected location, shifting all of the other table entries down one position.
Click Remove to remove the selected value, shifting all of the table values below the deleted entry up one position.
Click Reset to clear all of the table values, leaving only one entry.
Enter the Equilibration time.
Enter the Averaging time.
Check the Steady state sensing box if you want to determine the Equilibration time automatically. The measurement is started when a steady state condition is obtained or the equilibrium time is reached.
NOTE: When steady state sensing is selected, the Equilibrium time is the maximum delay desired before the measurement starts. The averaging time is the time over which the measured torque (stress) is averaged.
The % Tolerance default value is five percent. Tolerance defines the maximum deviation of consecutive measurements.
The Consecutive within default is 3 measurement points. The measurement of the retained data point starts only after the average value is within the tolerance limit for the selected numbers of consecutive measurements.
Maximum point time is the maximum time for recording a data point. When the maximum point time is reached, the last measurement is retained independent of steady state has been reached or not.
Check the Scaled time average box to scale the Equilibrium time and Averaging time with the applied rate or stress. Enter the Scale factor; 0 stands for no scaling and 1 scales the time inverse proportional to the rate or stress. Intermediate values allow you to adjust the scaling as desired.

Controlled Rate Advanced

This option is available only if Angular velocity or Shear rate is selected.

Select the desired Motor mode. Choose between Auto, Soft, Medium, and Stiff, depending on the sample stiffness. As a rule of thumb, leave this option set to Auto. Matching the mode to the stiffness of your sample may increase the quality of your data.

Controlled Stress Advanced

This option is available only if Torque or Stress is selected.

Check Integer rotation averaging if you want to average over a complete revolution of the motor axes. Select if it is necessary to average over a complete revolution of the test geometry. This is typically used when the applied torque is low. Note that averaging over one revolution may take a long time for high-viscosity materials.

Data Acquisition

There are additional data collection options that can be adjusted to control how data is obtained and what additional information is collected during the measurement.

Save image: Select to store images of the test within the data file for future recall when using the Camera accessory. An image is stored for each data point.

Step Termination

TRIOS Software allows you to define conditions in which a step is halted ahead of its normal termination conditions (Limit checking). You can use this to ensure that, for instance, the instrument does not over speed or apply excessive strains.

Rather than running a step for a certain amount of time, you may wish to run it until stable data is obtained. You can set an Equilibrium limit (such as the viscosity value becoming constant when running a single shear with time) that will stop the currently active test.