An Introduction to Viscometers

Viscometers are instruments that measure the fluid flow and viscosity of liquids. The viscosity of a liquid can affect its performance, whether this is pumping or piping it, or how it performs for dipping and coating.

Measuring viscosity, therefore, applies to a wide range of materials, including:

  • Oil
  • Plastics
  • Paints
  • Coatings
  • Adhesives
  • Wax
  • Asphalt

Measuring viscosity and flow properties also applies to the production of familiar consumer items such as food and drink, toothpaste, cosmetics and shampoo.

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What Does a Viscometer Do?

Viscometers measure the viscosity and flow properties of liquids, and have a wide variety of applications, measuring these properties in a different substances and materials.

Examples of these applications include:

  • Monitoring the viscosity of jams and syrups in the production of food condiments, to ensure batches are consistent
  • Analysing milk’s flow properties when designing piping systems for it
  • Measuring how motor oil flows when under different temperature conditions
  • Testing the viscosity of inks for inkjet printers.

Viscosity is important in the lubrication of machines. If it is too low, there will be too much contact, and therefore wear, between internal components. If it is too high, it can force the machine to work harder, to overcome the lubricant’s own internal resistance to flow.

What is Viscosity?

Viscosity is the measure of how resistant a material is to motion when you apply force to it.

For example, if you squeeze a tube of toothpaste, how easily does the contents come out of the end of the tube?

There is a formula for measuring viscosity:

Viscosity = sheer stress/shear rate.

You express the result of this formula in centipoise (cP). Centipoise is the equivalent of 1 mPa s (millipascal second).

This applies to absolute viscosity. For kinematic viscosity, the measurement is different, as we will explain later.

  • Sheer stress is the force per unit area that you need to move one layer of fluid in relation to another
  • Shear rate is the measure of the change in speed in the movement of intermediate layers, in respect of each other.

For many liquids, the stress that causes flow is directly proportional to the rate of shear strain. The sheer stress divided by the shear rate is constant for a given fluid, at a specific temperature.

This constant is the dynamic or absolute viscosity. But you can also simply refer to it as the viscosity of a material.

A simple way of seeing viscosity is as the thickness of a fluid, but when you look at fluids with different densities, the clearest way of describing viscosity is as resistance to flow.

Why is Measuring Viscosity Important?

The process conditions of materials is important, both in their production and for their end-use.

The viscosity of a material is also a useful indirect measure of its properties, such as molecular weight and density. These material properties can affect flow behaviour.

Measuring viscosity is a valuable part of quality control and batch consistency.

It is also important to measure viscosity because not all fluids behave in the same way.

Broadly, there are two types:

  • Newtonian, and
  • Non-Newtonian.

Newtonian refers to Newton’s Law of Viscosity, which is the established formula for measuring it.

In Newtonian fluids, the viscosity remains constant, regardless of changes to the shear rate.

Examples of typical Newtonian fluids include water and motor oil.

But, in non-Newtonian fluids, the viscosity fluctuates. These fluids include these types:

  • Plastic
  • Pseudoplastic
  • Rheopectic
  • Thixotropic
  • Dilatant

An everyday example of a non-Newtonian fluid is tomato ketchup, which, when you shake the bottle, becomes runnier. Ketchup has shear thinning properties, because its viscosity decreases as you increase the shear stress.

Kinematic and Absolute Viscosity

There are two ways of expressing viscosity:

  • Kinematic
  • Absolute

You measure kinematic viscosity by observing a fluid’s resistance to flow under the force of gravity.

You measure absolute viscosity by measuring this resistance to flow under an external and controlled force.

The measurement of these two expressions of viscosity are different too.

  • Kinematic viscosity is measured in centistokes (CSt)
  • Absolute viscosity is measured in centipoise (cP).

What Kinds of Viscometer Are There?

There are two basic methods for measuring viscosity:

Either an object passes through a stationary material, or material flows through or past a stationary object.

With either method, you record the time it takes for the operation to happen, which measures the resistance to flow.

Using either of these two methods, there are various kinds of viscometer:

  • Capillary, or glass, viscometer
  • Rotational viscometer
  • Falling ball and falling piston viscometer.

Viscometers measure Newtonian viscosity.

There are also rheometers for measuring non-Newtonian viscosity, and which do this using multiple parameters.

Capillary Viscometer

You will often find a capillary viscometer in a laboratory setting. This is a u-shaped glass tube (which gives it another name, the u-tube viscometer).

The viscosity measuring process involves you submerging the glass tube in a temperature-controlled bath, usually at 40 or 100°C.

There is a precise time reading for this, measured in seconds, which is how long it takes a fixed amount of fluid to flow within the u-tube, between two marked points.

You achieve this flow via suction or by gravity.

Once you have this measurement, you then multiply it by a constant, specific to the type of tube you are using which will calculate either:

  • Kinematic viscosity (if using the suction method), or
  • Absolute viscosity (if using the force of gravity.

Rotational Viscometer

This type of viscometer uses a rotating apparatus, known as a spindle, which you submerge within the fluid you are testing.

The torque on the rotating shaft of the spindle will then measure the fluid’s resistance to flow.

The rotational viscometer measures the absolute viscosity of the fluid.

A common version of this kind of viscometer is the Brookfield viscometer, and a more enhanced version is the Stabinger viscometer.

This uses an electromagnetically controlled spindle to generate rotation within the fluid, eliminating factoring in the bearing friction of a motorised spindle.

Falling Ball and Falling Piston Viscometers

These are not as common methods for measuring viscosity. In falling ball and falling piston viscometer tests, a ball or piston falls into the liquid, and you measure the time between marked points.

However, to do this, you must know the terminal velocity, size and density of the ball or piston you are using.

Rheometers

Rheology is the study of the flow of matter, usually in a liquid state, but it can also apply to certain solids. It examines how materials respond in relation to flow, in response to an applied force.

Technically, viscosity falls under the broader technical category of rheology.

Rheometers are ideal for measuring the viscosity of non-Newtonian fluids. They operate on similar principles to viscometers, but have wider applications.

This is because non-Newtonian fluids have more complex, rheological properties than Newtonian fluids, changing their viscosity when you apply force to them.

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There are four types of rheometer:

  • Capillary
  • Dynamic rotational
  • Torque
  • Oscillatory

Capillary and rotational rheometers are similar to corresponding viscometers.

Torque rheometers measure the torque on mixing screws or motors, which show how relatively hard it is to mix a sample material.

Oscillatory rheometers induce sine wave type shear deformations in sample materials, placing them between two plates and measuring the torque effect.

The main difference between rheometers and viscometers is that rheometers tend to exert some form of shear force on the substances they are testing.

How Do You Prepare Samples for Measurement?

In both rheometric and viscometric measurement, sample preparation can influence on measurement results.

This is especially true when measuring samples at low shear rates.

  • It is important not to shake or stir a sample before measurement, as this can put it under an undefinable shear load, unless the sample has sediment or some other separation. In this case, stirring or shaking, using consistent methods and tools, will be necessary.
  • Your method of application should be consistent too, such as spoon or spatula.
  • Pipette or syringe applications are only suitable for oils, resins or solvents. For other substances, these application methods will increase the shear load, which will reduce measurement values and skew results.
  • Try to ensure there are no bubbles as these can simulate non-Newtonian behaviour and give false measurements.
  • The volume of the sample you use must match the measurement method you are using. Too much or too little of a sample can lead to errors in measurement.
  • Follow guidance for waiting times. Preparation can cause stress to samples, which then need a recovery period. The sample structure may need to regenerate after you have put the measuring system in position, before you can take accurate measurements.
  • You must also ensure you prevent the sample from drying, as this will cause your measured values to be too high.

What Factors Can Affect Viscosity Measurements?

While the process of measuring a fluid’s viscosity might appear simple, there are factors to consider, if these measurements are going to be accurate.

Temperature is a critical factor. The function of the temperature-controlled bath is to maintain a precise temperature throughout the process. You should be able to control the bath temperature to within 0.02°C of your required temperature (usually 40 or 100°C).

There are temperature controlled bath systems that enable you to do this more easily.

In capillary viscometers, the diameter of the u-shaped glass must be precise for accurate measuring. Therefore, these glasses are normally manufactured using low-expansion borosilicate glass. This helps minimise error, along with recalibrating the capillary viscometer annually.

It is also important to rinse and dry thoroughly between measurements, using a residue-free solvent.

Viscometers will vary in size, to measure different types of viscosities. Whatever size the instrument is, he recommended minimum time for the viscometer to take its measurement should be 200 seconds. This allows the fluid to pass between marked points.

Which Industries Benefit from Viscometers and Rheometers?

According to research, the market in viscometers and rheometers is predicted to reach $878.6 million globally by 2023.

A diverse range of industries and sectors uses viscometers and rheometers.

These include:

  • Pharmaceutical, biotech and clinical research
  • Petrochemical and oil
  • Food and drink
  • Chemical
  • Beauty and cosmetics
  • Forensics
  • Environmental testing
  • Manufacturing research and development.

Here are several examples of applying viscosity measurements in various industries.

Adhesives

It is important to measure viscosity in the production of adhesives. Depending on the type of adhesive and its end application, it will need to flow at a certain, optimum rate.

Low viscosity adhesives flow more freely than high viscosity ones. For some, the object will be for the adhesive to stay more firmly in one spot, for others, to spread more widely.

Food

In the food industry, viscosity measurements can help maximise production efficiency, and ensure cost effectiveness.

Where products are piped as part of production, viscosity will affect the speed of this transmission, and how long various foodstuffs take to set or dry, or how long it takes to dispense it into packaging.

Viscosity can have a huge influence on the food production process, and it is a major element in the texture of food.

Any batch inconsistencies can result in products that will not meet consumer standards.

Oil

Viscosity is very important in oil. It will determine the sealing effect of oil and its rate of consumption.

Oil viscosity affects the thermal friction-related temperature of bearings, gear sets and cylinders. It will impact on the efficiency of machinery, and how quickly or slowly it starts and runs under various temperatures.

You measure oil’s kinematic viscosity, which gives its viscosity index (VI) rating. Oil with a higher VI is more efficient, resulting in less consumption and reduced wear and tear in lubrication.

Another factor to measure in oil is its ability to resist shearing during hydrodynamic lubrication.

It is important for an oil’s viscosity to conform to the temperature conditions, speed and load of its lubricated parts.

Concrete

In the concrete industry, the plastic viscosity and yield stress of concrete are vital to its workability and placement.

Concrete’s rheological properties determine its mechanical qualities, durability and overall quality.

To ensure the sustainability of concrete structures, and the usability of the concrete when applying it, it must be of the right viscosity.

Cosmetics

In the beauty and cosmetics industry, measuring viscosity is a critical element in quality control.

For example, a lip balm must be of a high viscosity to ensure it will properly adhere to the skin and protect it.

On the other hand, a body mist needs a low viscosity to flow freely and evenly from its dispenser.

Where cosmetics companies describe products as being luxurious, these are more likely to require a higher viscosity to reinforce this impression.

You can test the viscosity of many oil and water based cosmetic compounds using a capillary viscometer because these are Newtonian fluids. But other compounds will have non-Newtonian characteristics, therefore requiring rheometric testing.

It is important, therefore, to choose the right kind of viscometer, or rheometer, for your range of needs.

How Do You Choose a Viscometer?

As we have seen, there is a broad range of applications for viscometers and rheometers.

What factors should you consider when choosing an instrument for measuring viscosity?

  • What type of viscosity will you need to measure for, absolute, kinematic, or both?
  • Where will you conduct your work, in a laboratory setting, on the move, or elsewhere?
  • How complex or varied will the materials be that you want to measure?

Generally, rheometers are more versatile, and more expensive, instruments than viscometers, but rheometers will measure non-Newtonian fluids.

When it comes to types of viscometers and rheometers, there are various broad instrument categories, covering:

  • Portable, handheld viscometers
  • Glass capillary viscometers
  • Rotational viscometers
  • Ball bearing and air bearing rheometers.

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