In a rotational viscometer, a rotating cylinder creates a defined shear rate in the sample and measures resistance to flow by the torque generated. It produces a precise measurement of absolute viscosity for a wide range of viscosities. Because the shear rate can be varied, it is possible to plot curves of non-Newtonian fluids.
As a category Rotational viscometers are divided into types based on whether the instrument measures shear rate against a constant torque (the Stormer type) or as more commonly seen measures torque with a defined shear rate. The second type is subdivided as to whether the inner or the outer cylinder rotates to produce flow.
Searle type viscometers have an inner cylinder called a rotor or bob which rotates to produce flow, while the outer cylinder remains fixed while a sensing unit measures torque. The advantage of this system is that the temperature can be more precisely controlled via a jacket on the outer cylinder.
The Couette type (the most commonly used oilfield viscometer) has an outer cylinder or cup that rotates at a defined speed producing flow and creating torque on the inner cylinder which is where the sensing unit is located. The major advantage here is the absence of turbulent flow on low-viscosity fluids at high shear.
OFITE offers a complete line of couette viscometers:
- Hand Crank Rheometer - This model is driven by a hand-powered crank. It has two speeds and a setting for testing gel strength. It is idea for field applications where electricity is not available.
- Model 800 Viscometer - The Model 800 is a direct-indicating, dial viscometer. It features 8 preset speeds and a lighted dial for better viewing.
- Model 900 Viscometer - The Model 900 is a digital, automated viscometer. It has a variable speed motor, a keypad with buttons for preset speeds, and it can be connected to a computer for use with our powerful ORCADA® software.
- Model 1100 Viscometer - The Model 1100 is a portable pressurized viscometer. It has a maximum pressure of 2,500 PSI and a maximum temperature of 500°F (260°C).
- HTHP Viscometer - The OFITE HTHP Viscometer is a lab-model viscometer. It can be pressurized up to 30,000 PSI and heated up to 500°F (260°C)