I have been using a rheometer to characterize my sample and recently we found out that our gap setting is very wide compared to the recommended gap of x3 the particle size.
the Dz particle size of the DLS result has found to be around 0.005 mm which 3x0.005mm should be our measuring gap but we used 1mm gap ever since the project started without DLS test results that time.
Should we change the settings accordingly and disregard previous rheo results?
Would like to know your thoughts. Thanks!
Hello John,
If you’re using a parallel plate, then the results will be proportionally wrong depending on the gap error. E.g. if the gap is 20% too large, the results will be 20% lower than they ought to be, so long as the sample still filled the gap throughout the test and flowed in a laminar way.
If you are using a cone and plate or a cup and Bob, other complications are added.
Most DIN cup, and Bob geometries do not considerably account for the gap below the Bob so the results should be pretty accurate. However, for a Mooney Ewart cell the gap is important.
For a cone, the angle dictates the gap at any radius, and this is used for the calculation of shear rate from rotational speed. If you use a cone at the wrong gap then the constitute of equations to calculate share rate are thrown off completely.
Hi Philip Rolfe ,
Thanks for the info. Appreciate you sharing this!
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