This article applies to: All Brookfield laboratory viscometers
The first step to utilizing your Brookfield viscometer effectively in your process is to form a basic understanding of viscosity measurements, and how our instruments read and present data.
This article is meant as a high-level description of viscosity measurement and analysis. For a deeper dive into the subject, see our document More Solutions to Sticky Problems.
Different kinds of viscosity
There are two main kinds of viscosity used in sample analysis, Dynamic viscosity and Kinematic viscosity.
Dynamic viscosity is the measure of a fluid’s internal resistance to shear or motion. This is the kind of viscosity reported by our instruments and is very commonly used in fluid analysis.
Common unit - Poise (P)
SI unit - Pascal-Second (Pa·s)
Common conversion:
1.0 centipoise (cP) = 1.0 milliPascal-Second (mPa·s)
Kinematic viscosity is the ratio of Dynamic viscosity and Density. Our laboratory instruments cannot natively measure kinematic viscosity. They can, however, perform the appropriate calculation if the density of the fluid is known before measurement. Brookfield viscometers are not able to measure density.
Common unit - Stoke (St)
SI unit - (m2/s)
Common conversion:
1.0 centistoke (cSt) = 1.0 mm2/s
Different kinds of fluids
There are two basic sub-sets of fluids that you might come across while measuring samples. Newtonian fluids, and Non-Newtonian fluids.
Newtonian fluids - Their viscosity is independent of how they were are measured. Some everyday examples of Newtonian fluids are water, air, and airmotor oil. Their viscosity will remain constant no matter the testing method used to measure them.
Non-Newtonian fluids - Their apparent viscosity changes depending on the amount of shear applied, or how roughly or gently they are disturbed. Fluids can be very mildly non-Newtonian and barely change viscosity over a huge range of forces, or they can be severely non-Newtonian and change from free-flowing fluids to almost solid masses when mild forces are applied. The two most common behaviors exhibited by non-Newtonian fluids are shear-thinning, where the substance will decrease in viscosity as the shear forces on it increase, or shear-thickening where the viscosity increases as the shear forces on the fluid increase. There are many more complicated behaviors, but that is beyond the scope of this article.
When in doubt, it is always safer to assume your sample is non-Newtonian. Many of our spindles and measurement systems have known shear rate constants associated with them, which allow you to compare measurements of non-Newtonian fluids and characterize their behavior across a range of shear rates.
Temperature is important
Some fluids change viscosity based on the forces applied to them, but all fluids change in viscosity to some degree when the temperature of the fluid changes. This is similar to non-Newtonian fluids, where some fluids are affected much more or less than others. There are two ways to deal with temperature in your viscosity measurements. You can either measure and report the current temperature of the fluid during the test, or you can actively control the temperature of the fluid during the test. Whatever option you choose, it is important to note the temperature of the fluid along with the viscosity you’ve measured.
Reporting viscosity measurements
When reporting a viscosity measured using our instruments, it is important to include the appropriate information that may be relevant to your situation. In general, it is good practice to report the measured viscosity, the instrument that was used, the spindle, speed, and temperature. This allows for the most reproducible results, and adequately captures the exact method used to measure this viscosity.
If you have less stringent requirements, or don’t need the measurement to be reproducible elsewhere, a simplified reporting style can be implemented such as simply giving the recorded viscosity and the temperature. This could be used in cases where all measurements are using a controlled test method on the same instrument (such as in a QC test environment), or if the fluid is known and demonstrated to be perfectly Newtonian. In general, it can never hurt to include as much information as possible.
If you are using a spindle that has a known shear rate constant, you can report the shear rate of the test in lieu of the instrument info, spindle, and speed. For spindles without known shear rates, it is important to report the unit type and torque, spindle, and speed so that the test can be replicated elsewhere if required.
An example of good reporting technique when using a spindle that does not have a known shear rate constant:
Measured oil sample lot 0999a. Brookfield RV DV2T viscometer, RV-02 spindle at 15RPM.
Single point data collection taken at 2 minutes.
Measured viscosity: 557cP
Temperature: 25.1*C
Another example of a succinct viscosity report for a measurement system with a known shear rate:
Slurry lot 12568, measured on Jan 4th 2021. Single point data collection taken at 4 minutes.
Measured viscosity: 27,356cP at a shear rate of 4.4 1/s
Temperature: 49.8*C
Knowing the shear rate applied during the test makes reporting viscosity much easier. The above test can be reproduced exactly across a number of different instruments and measurement geometries.