Rheometers are necessary laboratory instruments for manufacturers and researchers of fluids and soft solids. With quality assessment, product development, and process control reliant upon the physical properties of these materials, a rheometer should be selected to stand up to demands and last for a decade. Identifying key features and checking the resolution of various models will help you obtain an instrument that can meet your needs while also minimizing cost.
Rheometer sensitivity is based on the torque range of the motor that rotates the measuring system. Torque range serves as a guide for how easily the instrument can run low viscosity and high viscosity samples at various shear rates. This range cannot be directly represented in terms of minimum viscosity and maximum viscosity because torque depends on the geometry of the measuring system in addition to the viscosity of the sample. Low torque levels correspond to low viscosity and slow shear, whereas high torque represents a high viscosity sample ran under high shear conditions.
The minimum torque value is an indication of how sensitive the instrument is to low viscosity liquids (i.e. water) at slow rotation or small strain. Labs seeking to measure the viscosities of dilute aqueous solutions and organic solvents at low shear rates (< 10 1/s) benefit from having a rheometer with a torque minimum in the nano-Newton-meter (nNm) range. If such a low torque spec instrument is not obtainable due to budget, then tests may be conducted with higher shear rates to obtain the viscosities if the application is suitable for high shear measurements. To avoid measurement error, low torque measurements of dilute fluids should always be carried out with a cup measuring system regardless of the torque specifications of the rheometer.
On the other hand, thick samples can be tested at higher shear rates if the rheometer has a large maximum torque. These types of materials include pastes, gels, and films, which can exert a great deal of resistance to flow. Rheometers with torque maximums in the milli-Newton-meter (mNm) range are common for gel research labs, but special high-torque instruments may be required for testing at high shear rates. For example, cement rheometers have extreme torque maximums to handle cements with ground rock included in their pastes. Additionally, solid-like samples should always be tested with a measuring system that is designed to minimize torque in order to expand the shear rate range capable for that sample. Film and fiber holders and solid torsion bar geometries should be utilized to achieve more test capabilities within the torque range of the rheometer.
Although torque range is a key factor for assessing the utility of a rheometer, there are other aspects that need to be considered as well. Next week we will take a look at normal force and how to evaluate it when selecting a rheometer.
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