Viscosity is a measure of a fluid's resistance to flow. There are various instruments designed to measure viscosity, each employing different principles and suitable for different types of fluids and measurement requirements. Here are some commonly used viscosity measuring instruments:

1. Viscometers:

   • Viscometers are devices specifically designed to measure viscosity. They operate based on the principle of applying a force to induce flow and measuring the resulting deformation or resistance.
   • Rotational Viscometers: These instruments measure the torque required to rotate a spindle or cylinder immersed in the fluid. The torque is directly proportional to viscosity.
     • Examples: Brookfield viscometer, Rheometer.
   • Capillary Viscometers: These viscometers measure the time taken for a fixed volume of fluid to flow through a capillary tube under the influence of gravity. The viscosity is calculated using the flow rate and tube dimensions.
     • Examples: Ostwald viscometer, Cannon-Fenske viscometer.
   • Vibrational Viscometers: These viscometers measure the vibration frequency or damping of a vibrating element immersed in the fluid. The viscosity is inversely proportional to the damping or frequency shift.
     • Examples: Vibro viscometer, Vibro viscometer.

2. Rheometers:

   • Rheometers are advanced viscometers capable of measuring complex flow behavior, including shear stress, shear rate, and viscoelastic properties.
   • They are widely used in research, quality control, and product development across industries such as cosmetics, food, pharmaceuticals, and polymers.

3. Torsional Oscillation Instruments:

   • These instruments measure the response of a fluid to torsional oscillation. By applying an oscillatory force and measuring the resulting strain or deformation, they can characterize the fluid's viscoelastic properties.
   • They are commonly used in materials science, polymer rheology, and fluid mechanics research.

4. Falling Ball Viscometers:

   • Falling ball viscometers measure the time taken for a ball or sphere to fall through a fluid under the influence of gravity. The viscosity is calculated based on the ball's terminal velocity and fluid properties.
   • They are often used for quick and simple viscosity measurements of Newtonian fluids.

5. In-line Viscometers:

   • These instruments are integrated into process pipelines or systems to provide real-time viscosity measurements. They offer continuous monitoring and control of viscosity during manufacturing processes.
   • They are commonly used in industries such as petroleum, chemical processing, and food production.

6. Ultrasonic Viscometers:

   • Ultrasonic viscometers measure the time taken for ultrasonic waves to propagate through a fluid. The viscosity is calculated based on the attenuation and speed of the ultrasonic waves.
   • They are suitable for non-invasive viscosity measurements of opaque fluids or slurries.

Each type of viscosity measuring instrument has its advantages and limitations, and the choice depends on factors such as the type of fluid, viscosity range, accuracy requirements, and application-specific considerations.