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Why are my rheology data different from the other lab?

March 1, 2018

 

 

Data from your lab not matching data from another lab: It’s an all too familiar scenario for many of us who run rheometry tests. From quality control to R&D, this problem plagues a wide range of departments and industries that rely on comparing results from different labs for the same product.  Establishing viscosity or shear moduli tolerances is essential for evaluating samples. Without an established output designated for “good” and one for “bad” samples, each individual processing plant and laboratory would need to establish their own standards for their products, which takes time and money. Therefore, it is worthwhile to ensure that rheometers at different sites output the same results for the same sample and same inputs. However, there are many factors that influence data congruency. Follow these basic guidelines to prevent or troubleshoot deviation between instruments.

 

 

Step 1: Check for Test Method Congruency

 

It is surprising how many rheometry methods do not get repeated with identical values

for each variable. The lead scientist may believe that the quality control group is following their method to a tee, but unless every step of sample handling, loading, and test conditions are stated in the SOP, there are likely to be differences.  For example, using a spoon to load a yogurt onto a parallel plate system gives different results from using a pipette.  Furthermore, temperature equilibration time should be stated to allow the sample to reach the test temperature prior to the start of the measurement.

 

Additionally, input variable settings need to be highly specific. The instruction, “ramp shear rate from 1 to 100 1/s at 25°C”, is not detailed enough. Data collection, time per data point and number of data points, should be given. Results from quick tests vary from those with more time per data point. Ramped shear rate, shear stress, or oscillatory shear strain can be collected either linearly or logarithmically. Inform lab workers on which format to view the data, such as a log-log viscosity vs. shear rate graph or in a chart to identify key transitions in the sample behavior.

 

 

Step 2: Look at the Rheometer Measuring Systems in Use

 

 

There are two general categories of measuring systems: absolute and relative. Absolute measuring systems include parallel plates, cones, concentric cylinders, and double-gap geometries. These each have their appropriate conditions for use, but overall their data is comparable to that of other measuring systems. However, for the best results, data comparisons for the same sample should use the same measuring system. This concept also applies to the specific dimensions and material of the measuring systems, such as a 25 mm diameter stainless steel plate or a 10 mL volume, 0.25 mm gap aluminum concentric cylinder.

 

Relative measuring systems produce data that cannot be accurately compared to neither other relative measuring systems nor other absolute measuring systems.  These geometries mostly comprise of stirrers (also called “vanes”), which include Krebs stirrers, Brookfield stirrers, and custom-made geometries. Use of a different stirrer results in output that cannot be quantitatively compared to data from a different stirrer. The data incongruency is typically even larger if the shear rheometers are different models.

 

 

Step 3: Investigate Potential Operator Error

 

 

Rheology training is essential to obtaining repeatable and accurate measurements. All lab staff who operate the rheometer or analyze data from the instrument should be fully trained in the operation of the instrument. Doing so will help them understand how sample loading, test method variables, and data presentation impact the results. Murray Rheology Consulting offers both online and in-person rheology training for scientists, engineers, and technicians with topics ranging from basics to advanced material.

 

Rheology has advanced to a state where rheometers are expected to output consistent data with little impact from operator error, so do not lower your standards for your instrument. Use SOPs with detailed instructions and allow time for new lab members to practice running samples after they finish their training. See our recommendations for practicing rheometry in our blog post on Practicing Rheometry.

 

The next post will continue the topic of how to avoid and prevent data variances.

For answers to your questions about data incongruencies or any other rheology topic, contact us. 

 

 

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