| Overview |
| Instrument Inertia |
| Rotational Mapping (AR Series) |
| Oscillatory Mapping |
| Reference Position |
There are instrument-specific calibrations that are performed from the Instrument calibrations document view. To access this view, select Instrument tab > Calibrate > Instrument calibrations, or select Calibrations from the File Manager navigation panel. In addition, some more commonly-used calibrations (Rotational Mapping and Oscillatory Mapping) also have a direct link on the Instrument tab > Calibrate toolbar.
In an ideal world, whenever a stress is applied by a rheometer, it acts solely upon the loaded sample and nothing else. In practice, however, the non-zero moments of inertia of the rheometer spindle and measurement geometry mean that some of the applied torque is being used to accelerate or decelerate these mechanical components (until steady state is reached). A correction needs to be made to the stress value to reflect more accurately the conditions that the sample is undergoing. The amount of correction applied is based upon the calibrated values of the instrument and geometry inertia.
The total moment of inertia (instrument + geometry) is an important parameter in the calculation of oscillatory data, and correcting controlled stress flow ramps (oversampling off). It is also an important parameter for optimal controlled rate control and creep recovery breaking.
In the oscillation mode, the correction is made automatically. In the flow mode, however, the correction can be toggled on or off as necessary. Flow inertia correction is more likely to be needed when low-viscosity materials are being measured using fast ramps over a wide shear range.
This calibration should be performed on a regular basis – at least every 3 months. An increase in value over time can indicate bearing contamination.
To calibrate the instrument inertia, expand the Calibration area, follow the instructions, and click Calibrate. When complete, you can Accept or Cancel the new value.
Any bearing will have small variations in behavior around one revolution of the shaft. They are consistent over time unless changes occur in the bearing. By combining the absolute angular position data from the optical encoder with microprocessor control of the motor, these small variations can be mapped automatically and stored in memory.
These variations can then be allowed for automatically by the microprocessor, which is in effect carrying out a baseline correction of the torque. This results in a very wide operating range of the bearing.
There are three levels of rotational mapping – fast, standard, and precision. It is also possible to perform multiple mappings (iterations).
When should you use rotational mapping? The magnitude of the torque correction varies with the instrument type, but typically has an absolute maximum in the range 0.2 to 1 µN.m. With this in mind, you can determine if mapping will improve your measurements. For example, if the minimum torque during your procedure never drops below 100 µN.m, then the maximum error that an unmapped bearing will contribute is 1%. In this case, you may consider that mapping is not required or that a fast mapping is sufficient. As the torques in your procedure drop, the error from an unmapped bearing increases, so it may become necessary to perform precision mappings to keep these errors to a minimum.
For optimum performance at very low torques, multiple mappings can be performed, but there are diminishing returns. Generally, you will see little further improvement in performance after three consecutive mappings.
For maximum accuracy at the lower torque end of the rheometer, it is recommended that the mapping be performed each time a new measuring system is used, or if the current geometry is removed for cleaning.
To calibrate the Rotational Mapping, select Rotational Mapping from the Instrument tab, select the type of mapping and number of iterations, and click Calibrate.
Oscillatory mapping is used to get the best data at low torque, low displacement, and especially low torque and low displacement. It acts like a baseline subtraction. Data is first acquired without any sample present over the same range of frequencies or displacements as the desired test. Then when the test is performed, a correction is made to the measured data. As this is a very detailed map, it is impractical, time wise, to map the whole bearing. Therefore, the bearing is mapped around a known reference position and then measurements made around the same position. Up to ten maps can be stored, and whenever conditions are changed, the software applies the most appropriate map as long as the new conditions are a subset of the stored map. For example, if you map at 1% strain from 0.01 to 100 Hz, and then measure between 1 and 10 Hz at 1% strain, the map will be applied, but not if you run from 0.01 to 100 Hz at 5% strain.
To calibrate the Oscillatory Mapping, select Oscillatory Mapping from the Instrument tab. From the Oscillatory Mapping drop-down menu, choose to either perform mapping or go to the reference position. Select the test type, enter details that match or exceed your measurement ranges, and then click Calibrate. To reduce the calibration time, the number of points can be less than in your actual experiment, but should be at least 2 points per decade.
Once complete, the mapping is added to the list of Current mappings.
If an oscillatory mapping has been used for your test, the details can be accessed from the spreadsheet view of the step by selecting Show toolbar > Scalars from the Format menu.
The position at which the map is performed is called the reference position. This setting should be left at its default value unless other information is available.
To send the bearing to the Reference Position, select Instrument tab > Oscillatory Mapping > Go to Reference Position. This should be selected prior to sample loading to ensure that sample measurements are made with the bearing at the reference position.