DHR/AR Rheometer: Setting Up Conditioning Steps

In this topic
Overview
Sample
Options
End of Test

Overview

Conditioning steps are usually added to a procedure to change settings including environmental conditions, axial force adjustment, and pre-shear options. They are typically used as the first or last step in a procedure but can also be inserted between steps to change settings. Conditioning steps are not required in most tests unless the current default conditions must be overwritten.

To select a Conditioning step, see Using Experimental Procedures for detailed instructions.

The following settings can be adjusted in the conditioning step:

Sample: Sample step enables delays, environmental settings, preshear, and sample equilibration.
Options: The Options step enables axial force and adjustment settings.
End of test: End of Test step enables you to set temperature settings at the end of the experiment.

Sample

The Sample conditioning block is used to adjust various settings that can be used to condition the sample, typically before starting a test. Sample conditioning includes environmental control, delays for normal force, preshearing, or equilibration. Note that during preshear, no data are sampled.

Environmental Control Option

Select the following environmental control parameters:

Temperature: Enter the desired temperature value to maintain throughout the test, 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 still achieving the temperature, disable (uncheck) this option. The criteria for reaching temperature are set in Options > Temperature.
Soak time: This value is the amount of time to delay data collection at the start of the step, typically to allow for temperature equilibration and/or sample relaxation. If Wait for temperature is not selected, the time is measured from the start of the step. If Wait for temperature is selected, the time is measured from the point at which the measured temperature becomes stabilized at the commanded temperature. Because of the mass of the sample, geometries, and environmental control systems, a “soak time” (i.e., time to equilibrate at temperature) is recommended, particularly when starting experiments at subambient temperatures or when the temperature is changed significantly between steps. In cases where the change in temperature is not too large, a five minute soak time is sufficient for most samples. This time is also used at each increment temperature.

Wait for Axial Force

Select Wait for axial force if would like to delay the start of the test until the axial force/normal stress has decayed below a user-defined maximum value.

Choose between Axial force and Normal stress.
Enter the desired maximum value.

Preshear Options

Select Perform preshear to homogenize or orient a sample, or to break the internal structure to remove sample loading memories. Note that no data are collected during preshear.

Check Perform preshear to activate preshear.
Choose between Angular velocity, Shear rate, Torque and Shear stress: Enter the desired value.

Select the Duration of the preshear and enter a positive value.
Select Advanced (only available when Angular velocity or Shear rate are selected). Choose from the available motor mode settings: Auto, Soft, Medium, Stiff. Auto is recommended for standard operation.

Equilibration

Check the Perform equilibration box to insert a delay, which allows the sample to equilibrate either by letting the temperature equilibrate or the letting torque/force relax. Specify a Duration, in seconds.

Options

The Options conditioning step is used to set the axial force and the auto strain adjustment.

Axial force adjustment allows the control of the axial force during a test. It is necessary to use this function when the sample changes its dimensions as a result of temperature changes or reaction processes. It is not necessary to use this function when the sample has a low viscosity and flows easily. In such cases, the axial force might squeeze the sample out of the gap when the conditions are not set correctly. The axial force can be controlled in either tension for torsion experiments, or compression when using parallel plates. Note that the axial force control parameters are different for torsion and linear testing.

Auto strain adjustment changes the strain amplitude in order to hold the stress within the operation range of the instrument. Auto strain is typically applied during tests that monitor the transition of a sample from a liquid to a solid or solid to a liquid, such as in gel point or glass transition measurements. Auto strain adjusts the strain only within the specified strain amplitude range. NOTE: Auto strain is not available for DMA geometries.

Axial Force Adjustment

Mode: Select one of the three choices. Depending on the option selected, other parameters may need to be specified.
Disabled: Axial force control is disabled in subsequent steps. Note that any current axial force present will not be removed and may change depending on the stiffness of the sample and temperature profile.
Active: Axial force control is enabled using the specific force value as the commanded axial force to maintain during subsequent steps.
Inherit: Use this when you desire for the current axial force and other parameters applied to the sample to be inherited based on the last setting (either from the Gap Control panel or previous Conditioning > Options step). Note that for linear testing only, the Proportional mode Constant can be inherited from the Gap Control panel.
Select between: Tension and Compression: Define the direction of axial force control by selecting the appropriate radio button. Select Tension to control axial force in a torsion experiment, and Compression when using parallel plates.
Axial force and Set initial value: Enter the value for the axial force. Check Set initial value if you want the axial force algorithm to start at the command value exactly. If not checked, the axial force algorithm will start only when the force is outside of the window defined by the sensitivity.
Sensitivity: The sensitivity defines the axial force window for adjustment. The Axial force adjustment is active only when the measured (current) force falls outside of this window.
Proportional force mode (DHR linear testing only): Two modes are possible: Constant and Force Tracking.
- The Constant mode is identical to the mode on the control panel used on the ARES-G2, RSA-G2, and the DHR. The axial force set is held constant.
- The Force Tracking mode adjusts the axial force to track the applied dynamic force on the RSA. Based on the current dynamic force value, the static axial force applied before the next acquisition step is calculated based on the following criteria: Axial force > Dynamic Force and Minimum axial force.
  - Axial Force > Dynamic Force: Defines in % how much the new static axial force has to be higher than the last dynamic force.
  - Minimum axial force: Value of the minimum axial force to be applied, independent of the Force Tracking criterion.
Gap change up and down: The maximum gap change upwards and downwards defines the active displacement window available to the axial force adjustment algorithm. As soon as the gap falls outside of this window, the axial force adjustment is deactivated independent of the value of the axial force.
Return to window or initial value (not available for linear testing [DMA mode]): When the axial force becomes active, the algorithm can be set to continue adjusting the force until the force is within the sensitivity window or until the set axial force is reached. Select the appropriate radio button.
Adjustment time out (only available for linear testing [DMA mode]): After a user-defined Adjustment time out, data sampling is resumed, whether the axial force adjustment is completed or not. Note that the axial force adjustment is stopped and the data acquisition for the next data point started as soon as the axial force is reached, without waiting for the adjustment time to end. Use a short adjustment time if the priority is on data sampling, Use a longer adjustment time if the priority is the force control. Note that the measured axial force may be outside of the desired sensitivity window if the adjustment time is too short.

Purge Gas

Purge gas only (no active cooling): Set this check box when liquid nitrogen is not used to cool the ETC. The ETC would be cooled using the purge gas alone.

End of Test

The End of Test conditioning block sets the conditions of the instrument after the experiment finishes.

Check the Set temperature to set an end of test temperature different from the test temperature.
Enter a value for the new temperature.
Check Set temperature system idle (available only if Axial Force Adjustment is active). This turns of the environmental system; no temperature control is active. This prevents contraction forces due to sample cooling, which could destroy the instrument.