Viscometer or Rheometer – which is best for me?

Viscometer or Rheometer – which is best for me?

Deciding whether to purchase either a viscometer or a rheometer is not always straightforward. This guide will take you through the differences and how a rheometer could be more suited for your needs.

Deciding whether to purchase either a viscometer or a rheometer is not always straightforward. This guide will take you through the differences and how a rheometer could be more suited for your needs.

What’s the difference? 

Typically a viscometer employs a mechanical bearing that limits the speed and torque capabilities of the instrument, whereas a rheometer uses a low friction air bearing. This means a viscometer can be a solution for material, process or production tests that require simple flow measurements on Newtonian materials (where viscosity is independent of shear rate), however the performance of a rheometer allows far greater characterization of flow, deformation and even the tackiness of a material (for Newtonian and non-Newtonian materials).
A viscometer can offer portability for field or remote testing. Rheometers, while generally more expensive than viscometers, are more versatile and have a much wider dynamic range of control and measurement parameters.

Measuring Viscosity

Measuring viscosity is the most common application required for a viscometer or rheometer. For most products, the viscosity is required to be high at low shear rates to prevent sedimentation or slumping, but to thin down at higher shear rates to facilitate application or processing. Hence a single viscosity measurement is not sufficient to describe the viscosity of such materials and the viscosity should be measured over a range of shear rates or stresses.
Typically, a viscometer can measure in the range of about 0.1 to 103 s-1 while a rheometer extends the measurement range from 10-6 to 105 s-1. A broader measurement range enables relevant data to be obtained by exposing the sample to conditions that are realistic to the conditions applied during product manufacture or use.
Processes such as sedimentation are best suited to analyze with a rheometer due to its low torque capabilities. High speed control of a rheometer also allows analysis of a very high shear rate process, such as spraying.

ProcessMinimum shear rate (s-1)Maximum shear rate (s-1)ViscometerRheometer
Reverse gravure105106 
Spraying104105
Blade coat103105
Mixing/stirring10103
Brushing10103
Pumping1103
Extrusion1102
Curtain coating1102
Levelling10-20.1
Sagging10-20.1
Sedimentation 10-610-2

Yield stress

After viscosity, yield stress is probably the more routinely measured rheological property because many consumer products gain value from having one.
Yield stress varies as a function of the temperature and timescale during which the stress is applied. Rheometers can provide more relevant yield stress data than a viscometer by enabling application of the broad range of these methods. Applying a stress ramp is looked upon as the easiest way with a rheometer.

Which is best for me?

The increased versatility and performance make rheometers an excellent tool for research, product and process development, as well as quality control testing. Both viscometers and rheometers are complementary, and it is not uncommon within a single organization to find viscometers used for QC testing on products that have been developed using a rheometer. However, with the versatility and power of the Kinexus software, both R&D and QC measurements (with pass fail criteria), including the option of 21 CFR (for the pharmaceutical industry) can be customized to suit sample type, application and operator.

Kinexus RangeRosand Range
Measurement Type
Viscometry
–          Flow Curves (Shear & Extensional Viscosity)
–          Yield Stress
–          Thixotropy
–          Single Shear
–          Die Swell
–          Haul Off (Melt Strength)
–          Stress Relaxation
–          PVT
–          Co-Extrusion
–          Creep
Oscillation
–          Amplitude Sweep
–          Frequency Sweep
–          Single Frequency (with and without temperature ramps and tables)
–          UV Curing
Tackiness & Adhesion
Temperature Range-40 – 350 °C5 – 500 °C
Shear Rate Range (dependent on sample)<<1 x 10-3 to > 50,000 s-1

 

<1 to > 1 million s-1

 

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2 thoughts on “Viscometer or Rheometer – which is best for me?”

    • Hi Richard, the Rosand capillary rheometer has a co-extrusion accessory that allows the extrusion of two materials (one from each barrel) to come together as one extrudate. This means you can determine compatibility of flow behaviour and monitor the interface at different extrusion rates.

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