A Brief Guide on Viscometer

A viscometer is a very important component in the field of fluid dynamics. This instrument is used widely to measure the viscosity of a fluid. Viscosity has several implementations not just in the field of physics but in engineering and tribology fields as well. For example, a viscosity that is too low can cause wear, owing to the interfacial contact between the internal moving components. On the other hand, a high viscosity would make the machine work harder to overcome the lubricant’s internal flow barrier. As a result, it’s critical to understand not only the base oil’s viscosity, but also how it may change as operating or environmental conditions change.

As mentioned above, a viscometer is used for measuring the viscosity of a fluid. However, it is only used in the case of streamline flow. Streamline flow is defined as the type of flow in which the velocity is the same at every point of the flow. In other cases,  we use rheometers for the measurement of viscosity.

Difference Between Viscometer and Rheometer

Viscosity is the measurement of a fluid’s resistance to flow at a specific temperature (shear stress). When flow conditions vary, not all fluids keep the same viscosity. The viscosity of a non-Newtonian fluid changes depending on the flow conditions. The viscosity of these fluids is measured using rheometers. Viscometers are used to test the viscosity of Newtonian fluids.

Different Ways to Measure Viscosity by a Viscometer

Among the different viscometer approaches, there are two distinct ways to express viscosity: kinematic viscosity and absolute viscosity. The fundamental difference is that kinematic viscosity is evaluated by measuring the fluid’s resistance to flow under gravity, whereas absolute viscosity is measured by measuring the fluid’s resistance to flow under an external and controlled force, such as a capillary or a body moving through the fluid.

The unit of absolute viscosity is centipoise, while the unit of kinematic viscosity is centistokes. By dividing absolute viscosity by the fluid’s specific gravity, absolute viscosity is frequently converted to kinematic viscosity. 

Testing of Viscosity and its Several Types

There are many methods that have been used to test viscosity. Each method has its own benefits. In the below-mentioned paragraphs, we will understand all the methods in detail.  

1. Capillary viscometer

The main instrument used in a capillary viscometer test is a glass tube in the shape of a “U,” earning it the moniker “U-tube.” The tube must be submerged in a temperature-controlled bath (typically 40 to 100 degrees C). An accurate time measurement (in seconds) for the time it takes for a fixed volume of fluid to flow through the tube by suction or gravity from one designated spot to another is required for a U-tube process.

By multiplying the measured period by a constant (unique to the tube), the absolute viscosity (vacuum) or kinematic viscosity may be computed (force of gravity).

2. Rotational viscometer

A rotating viscometer has a spinning apparatus called the spindle that is submerged into the test fluid, which is an important characteristic. The torque on the spinning shaft is then used to determine the fluid’s resistance to flow.

The absolute viscosity of a fluid is estimated using a spinning viscometer since it is determined by the fluid’s internal shear stress rather than gravity. This type of viscometer is known as the Brookfield viscometer.

The rotating viscometer is replaced by the Stabinger viscometer, which is a more sophisticated version of the rotating viscometer. A modified freestanding floating spindle driven by electromagnetic forces creates rotation inside the fluid. This is advantageous since it eliminates the time-consuming task of bearing calculation.

3. Falling ball viscometer

Viscometers with a falling ball or a falling piston are less commonly used to measure viscosity. A ball or piston is placed into the liquid, and the time it takes to travel from one marked point to the next is monitored in these experiments. To calculate the viscosity using Stokes’ law, you’ll need to know the ball or piston’s known terminal velocity, size, and density.

4. Falling piston viscometer

The Norcross viscometer was named after its creator, Austin Norcross. The cornerstone of viscosity measurement in this sturdy and sensitive industrial device is a piston and cylinder combination. The material to be measured is pulled down through the clearance (gap) between the piston and the cylinder wall into the space generated below the piston as it rises, thanks to an airlifting mechanism. The assembly is generally kept aloft for a few seconds before descending by gravity, discharging the sample in the same manner as it entered, generating a shearing effect on the measured liquid, making this viscometer extremely sensitive and perfect for detecting certain thixotropic liquids. 

Conclusion

In the above-mentioned paragraphs, we discussed different types of viscometers and their construction. But it should be kept in mind that other than the popular ones, there are other types of viscometers that exist. For example, to test oil’s viscosity, we sometimes use the bubble method. In this method, the distance travelled by the bubble in a given period of time is measured.