This article applies to: All Brookfield laboratory viscometers
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This article will reference tables and figures in Appendix A of More Solutions to Sticky Problems.
Newtonian fluids versus non-Newtonian fluids
Newtonian fluids are fluids whos whose viscosity is independent of the shear rate of the test. Comparing measurements from differing test methods or instruments is quite easy if the fluid is known to be perfectly or very closely Newtonian, since the viscosity should be the same in both test cases no matter the measurement method.
Non-Newtonian fluids are fluids whos whose apparent viscosity is dependent on the shear rate of the test. Some fluids can be very minorly non-Newtonian and only undergo very small viscosity changes, and others can undergo extreme changes in viscosity over a relatively small range of shear rates. When comparing viscosity results between dissimilar test methods carried out on non-Newtonian fluids, extra care must be taken to properly compare or equate the results. The rest of this article assumes that the test or tests are being carried out on non-Newtonian fluids.
Shear rate as a method of comparison
The easiest way to quantitatively compare viscosity readings taken with dissimilar instruments or measurement methods would be to look at the shear rate of the measurement. The basic unit of “test force” is measured by the shear rate, which is dependent on the geometry and test speed of any measurement, which is constant for any given spindle and speed combination.
For non-Newtonian fluids, the apparent viscosity changes as a function of shear rate, so using this variable to compare dissimilar measurement methods is the most convenient.
Brookfield spindles with known shear rate constants
Some, but not all, of Brookfield’s spindles or accessories have known well-defined shear rate constants that can be used to determine the shear rate of a test. Most notably, standard spindles in the shape of a disk that are supplied with Brookfield laboratory viscometers (s01 through s06, s62, s63) do not have a known well-defined shear rate constant. These disk shaped spindles cannot be used if the results need to be compared with results from a dissimilar test method.
Spindles that are cylindrical in shape, or , Wells-Brookfield cone/plate geometries, and spindles that are part of a concentric-cylinder geometry such as small-sample adapter kits or the ULA do have a known well-defined shear rate constant. Standard cylindrical spindles s61, s64, and s07 (and optional spindles LV-2C s66 and LV-3C s67) should only have their shear rate calculated when used without the guard leg. All of these spindles can be used to generate results that can be quantitatively compared with results from dissimilar test methods, given certain conditions.
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For a comprehensive list of spindles and accessories that have or do not have shear rate constants known, as well as a listing of said constants, see Appendix A of the document More Solutions to Sticky Problems.
Developing comparable measurement methods
Determining the target shear rate
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Since the DV2T instrument is only able to control at whole-number RPM’s above 10RPM, you need to round the required RPM’s from 36.9RPM to 37, but that represents an insignificant change in the overall shear rate of the measurement. The supplier’s test has a shear rate of 12.54 1/s, and this comparable test with an s27 spindle would have a shear rate of 12.58 1/s. There is not one single guideline for how close is “close enough” when it comes to matching shear rates like this, though. The acceptable range of “close” matched shear rates would depend heavily on the specific sample properties, and would need to be determined experimentally if a rigorous proof is required.
Important notes
Following this method does not guarantee an exact match between dissimilar test methods. This is simply a framework to develop approximately equivalent test methodologies for basic comparison. There are several factors that could result in discrepancies when measuring materials using dissimilar methods, none of which are easily explained or accounted for in this format. Feel free to reach out to technical support with additional questions.