In the previous post, I gave an overview of the first three steps for preventing and troubleshooting rheology data variances: test method congruency, measuring systems, and operator error. Here we continue the discussion with additional steps to ensure that two rheometers can output the same results.
Step 4: Check for Sample Aging
Even if test methods are identical in every way and operators are well-trained, the samples themselves may be the cause of data discrepancies. Two samples taken from the same batch should ideally be tested at the same time to avoid sample aging influencing the results. For example, two paste samples are taken from the same batch. One is tested in Lab A on Monday, and the other is shipped to Lab B. Lab B receives the sample on Tuesday and runs the sample on Wednesday. Data variance between the two results may be influenced by sample aging, or it could be another factor; the two labs need to run their samples on the same day to find out which aspect is causing that. Additionally, shipping samples can induce physical changes that influence rheometry data, and therefore this must be acknowledged when comparing data.
To help evaluate the source of alteration between the two rheometers and samples, consider testing a viscosity standard in both instruments. Most viscosity standards are silicone-based oils of varying consistencies that should be tested with a concentric cylinder measuring system. If the resulting viscosity curve is within 5% of the listed viscosity value for the given temperature, then the instrument is performing acceptably. However, note that if the samples in question for the comparison use a different measuring system than the concentric cylinder, there still may be an issue with the measuring system even though the rheometer itself is functioning accurately.
Step 5: Investigate Environmental Influences
When comparing data from two different labs, variance may arise from the environmental conditions rather than the rheometers or samples. Vibration and air flow are main aspects that can negatively impact rheology data. They can interfere through interfering with the sensors that detect sample response to applied shear stresses and strains. The footsteps of many people nearby or the rolling of a cart can distort results. Even more vibration arises from a nearby rumbling machine, ongoing construction in the building, or close proximity to train tracks. In a similar manner, air vents close to the rheometer can alter data if streams of air can reach the sample. To avoid these issues, the instrument may need to be moved to another lab space.
Step 6: Instrument(s) in Need of Maintenance
If the other five steps have been accounted for but do not result in matching data from two rheometers, then it is time to evaluate the condition of both instruments. Annual preventative maintenance is highly recommended by rheometer manufacturers to maintain the electronic components, ensure calibration factors are properly set, and identify any emerging issues. From my experience, motor alignment of the rotor (upper measuring system) to the stator (lower measuring system or plate) is a main contributor to inconsistent data between instruments of the same model. If the measuring system is tilted (even just a little, may not be noticeable to the human eye), then the sample does not experience the same shear as it would for a properly aligned system. This concentricity issue should be fixed during a rheometer check-up. Once both instruments have been evaluated and tuned-up, the data outputs should be very similar. See our post on Rheometer Upkeep for tips on how to keep a rheometer healthy to avoid needing repairs.
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