Viscometer

What is a viscometer?

Viscosity is the resistance experienced by the liquid in motion. A high-viscosity (thick) liquid flows slower than a low-viscosity (thin) liquid. This can be seen by comparing the speed of oil flow with water at the same temperature. When dealing with fluids in manufacturing, understanding their viscosity is crucial for various reasons: quality control, possible overheating, premature wear of the parts, leakages, etc. maintaining the right viscosity is key for the successful operation of a machine.

A viscometer (also known as a viscosimeter) is a tool used to determine a substance’s viscosity. In the case of liquids with viscosities that differ according to the flow conditions, the instrument known as a Rheometer is typically used. This is why a Rheometer may be described as a distinct kind of viscometer. Viscometers are only able to measure in one flow state.

Why are Viscometers Important?

Viscometers can help ensure that the viscosity of your fluid complies with the specified tolerances. In doing this, they help guarantee consistent operating results. In the past, if viscosities were not monitored then that could encounter various problems. Viscosity, whether thicker or thinner, can impact the way equipment processes the fluids, if it is not uniform, there could be problems like:

• Color shifts and inconsistencies
• Quality imperfections
• Material (i.e. paint or ink)
• Excessive solvent usage
• In-process or rework work

It is not surprising that customers will not be satisfied with products that are affected by these problems. Hence it is essential to check the viscosity of any processes.

Be Aware of Fluid Viscosity.

Achieving the viscometer tolerances by periodic calibrations. Steps to be followed for gaining good viscosity readings:

• Make sure you have calibrated the viscometer periodically.
• Make sure you test the mixture of the liquid to check its proper consistency so that it does not change with time.
• Examine your equipment for cleanliness and regular maintenance.

Six types of viscometers

There are different kinds of viscometers based on their operating principles:

• Viscometers with orifices
• Capillary viscometers
• Falling piston viscometers
• Rotational Viscometers
• Viscometers for falling balls
• Vibrational viscometers

Viscometers with orifices

An orifice viscometer typically consists of a glass with a hole through which fluid flows. Viscosity is measured by measuring the time it takes the cup to empty. It is calculated in seconds of a cup. Viscometers for orifices are easy to operate manually because they are placed in the fluid to be used and they are a popular tool in the painting industry. They can be found in Zahn Cups, Ford Cups, and others.

Figure 1. Orifice viscometer.

Capillary viscometers

It is also called U-tube viscometers, capillary viscometers are of two types: the Ostwald and Ubbelohde variants. They are easy to operate, comprising the shape of a U-shaped glass tube, which has 2 bulbs (one higher than the other). The fluid flows from the upper bulb into the lower via a capillary. The viscosity is determined by determining the time for the fluid to move within the tube.

Figure 2. Capillary viscometers.

Falling piston viscometers

The viscometers for falling pistons were developed in the late 1800s by Austin Norcross, hence they are also known as Norcross viscometers. They draw the liquid being determined into the piston-cylinder when the piston is elevated; the time it takes for the piston to lower (time-of-fall seconds) because of the fluid’s resistance can be used in determining the viscosity. Viscometers for falling pistons are easy to operate, maintain, and provide longer service life.

Figure 3. Falling piston viscometers.

Viscometers that rotate

Viscometers that rotate measure viscosity by submerging a rotating spindle into the liquid to be tested. The force (torque) required to rotate the spindle determines the viscosity of the liquid as they do not rely on gravity for work; the fluid’s internal shear force determines their calculations.

Figure 4. Viscometer that rotates.

Viscometers for falling balls

The falling ball viscometer works the same way as the fall piston viscometer. When using this type of viscometer, a small ball is dropped into the fluid that is being studied. Dimensions of the ball are well-known, so the viscosity is determined by the amount of time it takes for the ball (again using time-of-fall second) to drop into the fluid through gravity.

Vibrational viscometers

Vibrational viscometers employ a powered vibrating rod for measuring viscosity. Certain fluids can be more receptive to vibrations based on the degree of viscosity. So by measuring the dampening effect of the vibration or by observing the speed at which the vibrometer frequency decreases, the viscosity is assessed. Vibrational viscometers are extremely popular due to their highest sensitivity without moving components.

Accuracy

The precision or accuracy depends not just on the instrument’s design but also on the temperature control. Viscosity standards can be used to test the precision of a viscometer. It is a crucial factor in determining conformity with the industry’s quality standards. Making sure the manufacturer’s standards meet our requirements is important. Brookfield Silicone and Mineral Oil Standards provide a viscosity of the fluid that is constant at 25 degrees Celsius. The company suggests replacing the standards annually.

Consideration before purchasing viscometers

The selection of a viscometer is an intricate process based on the sample type and viscosity, the opacity and quantity of sample available, the required throughput, and the degree of automation required. At the minimum, these factors must be taken into consideration:

• Viscosity range
• Accuracy
• Control of temperature
• Microvolume and sample volume capabilities
• Computer interfacing and automation
• Industry standards and regulatory requirements.