What is Viscosity?

Viscosity refers to the amount of resistance shown by material in its motion when an external force is applied to it. Viscosity is generated whenever there is a relative motion between different fluid layers. It is usually measured by the formula:

Viscosity = Shear Stress/Shear Rate

Viscosity is denoted by a centipoise (cP) which is equivalent to 1 mPa s(millipascal second).

Shear stress can be defined as the force per unit area you need to move one layer of the fluid in relation to another. Conversely, the shear rate can be understood as the measure of the change in speed of the liquid due to the movement of intermediate layers with respect to each other.

For several liquids, the stress which causes the flow is directly proportional to the rate of shear strain. At any specific temperature, shear stress divided by the shear rate is always a constant and known as absolute viscosity. 

Rheometers measure properties of liquids

How Does a Viscometer Work?

Viscometers are the instruments used to measure viscosity and other flow properties of liquids. Measuring viscosity helps manufacturers understand the behaviour of liquid in real life. For instance, viscosity is crucial in the lubrication of machines. Having too low or too high viscosity can lower the efficiency of machines. 

What Factors Affect the Viscosity Measurements?

For accurate measurements of the liquid’s viscosity, the following factors need to be taken into consideration:

  • A consistent temperature must be obtained throughout the process for precise readings. Temperature-controlled bath systems are available, which can be used for this process.  
  • The diameter of the u-shaped glass in capillary viscometers must be precise for accurate measurements. 
  • The equipment must be rinsed and dried thoroughly between measurements with a residue-free solvent.
  • For accurate readings, the viscometer must measure for a minimum time of 200 seconds.

What is a Rheometer and its Different Types?

A rheometer is equipment used to measure the viscosity of non-Newtonian fluids. They are similar to viscometers but have a wide range of applications. This is because non-Newtonian fluids have more complicated rheological properties than Newtonian fluids. Non-Newtonian fluids tend to change their viscosity when force is applied to them. 

There are four types of Rheometer:

  1. Capillary A capillary rheometer is used to measure shear viscosity and other rheological properties of liquids. 
  2. Dynamic rotational A dynamic rotational rheometer is used for probing viscoelastic properties and the molecular structure of polymers. 
  3. Torque A torque rheometer measures the torque generated on the mixing of screws or motors. 
  4. Oscillatory An oscillatory rheometer measures the torque effect by inducing sine wave type shear deformations into the sample materials and placing them between two plates. 

Viscometers vs. Rheometers

Viscometers and rheometers are both viscosity measuring equipment. The basic difference between the two of them is that viscometers measure Newtonian fluids’ viscosity, whereas the rheometer measures the viscosity of non-Newtonian fluids. Non-Newtonian fluids, opposite from Newtonian fluids, change their viscosity with the flow conditions. 

Applications of Viscometers and Rheometers

Viscometers and rheometers are usually used to measure the viscosity of the fluids. However, their usage is not limited, and they have a wide range of applications in the field of material science and chemical industries. Some of the industries where their use is prevalent are:

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

All these industries need an accurate measurement of viscosity for the success of their workflows. When certain materials are processed in the industry and need to be piped or pumped, having correct viscosity measurement becomes vital. Having inaccurate reading or not taking the measurements can affect the dipping and coating performance of the manufactured products. 

Apart from the direct measurements, viscosity is beneficial for determining indirect measurements of materials such as their molecular weight and density. It has a direct impact on the flow behaviour of liquids. Hence, viscometers work exceptionally to counter these issues and monitor batch consistency and quality control.

Some common applications of viscometers and rheometers are:

  1. Taking the measurements of the flow of engine oil under distinct temperature conditions.
  2. Determining the flow properties of milk to build efficient, suitable piping systems.
  3. Analysing the viscosity of syrups and jams to ensure batch consistency.
  4. Viscometers can be used to characterise different types of plastics.

So whether it is food, adhesive, oil, concrete, or cosmetics, rheometers and viscometers can be helpful in determining the viscosity and attaining the right proportions for the manufactured products. 

Viscosity Testing Methods

There are multiple methods designed for testing the viscosity. Below-given are some of the regular techniques leveraged in base oil viscosity testing.

1. Capillary Viscometer Test

The apparatus used in a capillary viscometer test is known as the U-tube, whose name comes from its shape ‘U’. The procedure starts by submerging the U tube in a temperature-controlled bath (generally between 40 and 100 degrees C). 

The time taken by the fluids to flow from one marked side of the tube to another via suction or gravity is precisely noted. The absolute (via suction) or the kinematic viscosity (via gravity) is then calculated by multiplying the measured time and the constant associated with the particular tube. 

2. Rotational Viscometer Test

The main apparatus used to measure the viscosity in the rotational test is the rotational spindle. The spindle is submerged into the fluid, and the torque on the rotating shaft is utilised to measure the testing fluid’s resistance to flow.

In this test, there is no involvement of the force of gravity. Hence, it only calculator the absolute viscosity of the fluid.

3. Falling Ball and Piston Viscometer Test

This is a less common way to measure the viscosity of fluids. This method includes testing the viscosity through a falling ball and falling piston. The test initiates by releasing a ball into the fluid and noting down its time to pass from one marked point to another.

To calculate the viscosity, you must know about the size, terminal velocity, and density of the ball or piston used for the test. 

4. Bubble and Vibration Test

Apart from the given tests, viscosity can be tested using bubble or vibration methods on a few uncommon occasions. In the bubble testing, a bubble is released into the apparatus, and the time it takes to rise to a specified mark is noted to calculate the viscosity.

Similarly, vibration in the fluid can be used instead of the bubble to calculate the viscosity using a similar procedure. 

Types of Viscometers

Viscometers are generally categorised into two main types: Capillary viscometers and Rotational viscometers. Other than it, there are a few other types of viscometers such as falling ball, rolling ball, sphere, and piston viscometers. Let us go through each of them in detail. 

Capillary Viscometers

These are commonly known as glass viscometers and are mostly used in laboratory settings. Capillary viscometers measure the viscosity of liquids by calculating the time it takes for the translucent or transparent fluid to flow between two marked points of a capillary tube.

In the case of opaque liquids, reverse-flow viscometers are used to determine the viscosity as they only wet the timing section of the capillary viscometer and ease the process of taking the actual measurements. SV10-100 Sine-Wave Vibro Viscometers are ideal capillary viscometers that provide fast and highly accurate results.

For proper and efficient working of the reverse-flow viscometers, they must be cleaned, dried, and properly refilled before taking a new measurement. Capillary viscometers come in manual and automatic forms. Automatic viscometers leverage thermal sensors for opaque samples and infrared optical sensors for transparent samples. 

Rotational Viscometers

Commonly known as Brookfield type viscometers, Rotational Viscometers use a torsion spring to measure the torque needed to rotate a spindle in the material and measure the viscosity. Different ranges of viscosity can be calculated by changing the rotor speed of the equipment. The process to calculate viscosity using rotational viscometers is simple.

A lightweight magnetic rotor floats and rotates in a liquid-filled tube at a constant speed. The viscous forces of the sample fluid make the momentum for rotation of the rotor. When the speed reaches an equilibrium rotor speed, the viscosity is measured. SV-A Series Vibro Viscometers are perfect for this operation and are known for their ease of operation and fast testing times.

Falling Ball, Rolling Ball, Sphere, & Piston Viscometers

Other than capillary and rotational viscometers, there are a few other forms of viscometers such as rolling and falling ball, sphere, and piston viscometer that can be leveraged to calculate the viscosity of the liquids. 

In a ball-based viscometer, a ball is rolled through a closed and sample-filled capillary inclined at a specified angle to calculate the viscosity. Similarly, in falling piston viscometers, the piston is dropped in the tube, which measures the resistance generated through the material and calculates viscosity. 

Factors Which Affect Viscosity Measurements

Evaluating the viscosity of fluids isn’t as simple as it looks. Various factors need to be kept constant for obtaining accurate measurements, such as:

  • Temperature It is vital to maintain the temperature at a constant unit of typically 0.02 degrees C during the entire duration of the experiment. A temperature-controlled bath system is responsible for maintaining it, and failing which, the calculated viscosity can be misinterpreted. 
  • Viscometer Size A wide range of viscosities can be tested using viscometers. Hence, viscometers come in various size options. To get the precise readings of viscosity, it is advisable to pass the fluid from one marked point to another for an interval of at least 200 seconds. 
  • Viscometer Glass To calculate the viscosity of the fluids, capillary viscometers are also dependent on the inside diameter of the tube. That is why they are made with fully annealed and low-expansion borosilicate glass. Moreover, they come with an associated tube viscometer constant as the correction factor to get the precise readings.  
  • Rinsing In-Between the Measurements The quality of rinsing done within the tube between measurements can also affect the viscosity measurements. Generally, the equipment is rinsed with a residue-free solvent to ensure cleanliness efficiency. However, a visual check should always be done, especially when using oils, to ensure rinsing has been done effectively. 
  • Other Factors Lastly, a few other factors can affect the viscosity measurement, such as proper sample handling, the device’s method, timing, accuracy, and in-depth and continuous quality control.

How Do You Prepare Samples for Measurement?

Sample preparation can significantly impact the measurement results in the case of both rheometric and viscometric measurements. Note these following considerations to prepare samples for the right measurements:

  1. Do not shake or stir the sample before taking measurements, as it can generate an undefinable shear load. 
  2. In case the sample consists of sediments and needs to be stirred, use a consistent method of application such as a spatula or spoon. 
  3. Pipette or syringe should only be used for oils, resins or solvents and not for any other substance as they can increase the shear load and disrupt correct readings. 
  4. Get rid of any bubbles in the sample solution. 
  5. The volume of the sample should be correctly matched with the used measurement method.
  6. The waiting time should be fully utilised in between the measurements and preparatory steps to obtain accurate measurements.
  7. Make sure to prevent the sample from drying, which can lead to incorrect measurements. 

Considerations for Purchasing Viscometers

Buying viscometers can be confusing and challenging if you do not have the needed knowledge. Hence, you must look for the given parameters while choosing a good and reliable viscometer for your operations.

1. Viscosity Range

The range of viscosity for different fluids can vary. Therefore, the viscosity range is the foremost thing to consider while purchasing viscometers. Commonly, Viscosity is stated in centipoise (cP), and 1 cP equals 1 mPa·s. For Thermo Scientific Gilmont Falling-ball Viscometers, the range of viscosity of clear fluids can go up to 200 cP to obtain accurate and economical results.

In the case of Viscotek DSV from Malvern Instruments, the viscosity range lies between <1 mPa·s to 20 mPa·s, and they directly measure the relative viscosity of dilute polymer solutions leveraging pressure sensing technology. SV-A Series Vibro Viscometers offers a range of 0.3-10000cP/mPa.s and are known for their precise measurements.

2. Temperature Control

As mentioned above, maintaining the temperature is extremely crucial while experimenting to get accurate results. The viscosity of most materials gets reduced with the increase in temperature. Besides, materials such as engine oils are subjected to routine temperature variations when in use. 

Therefore, the viscometers must have precise temperature controllers to maintain the temperature. Sine-wave Vibro Viscometer is an efficient device that offers the feature to maintain the temperature and provides effective measurement results.

3. Accuracy

The instrument should be well-engineered and manufactured with the highest quality to achieve the required accuracy. However, the temperature too plays a substantial role in providing accurate results. That is why viscosity measurements are repeatedly taken and average to get the most precise results.

Therefore, when you go out to buy viscometers, check the standardisation of the manufacturer providing the viscometer and always buy from a good and trusted source. A&D Weighing are one of the most trusted brands in Australia known for the genuineness of their products.

4. Sample Volume and Microvolume Capabilities

Having the right volume for your sample is important to get the precise readings from your experiment. Having a low or high volume of samples can affect the effectiveness of the process and lead to incorrect measurements. Hence, look out for products that offer a variable range for the sample volume and have good microvolume capabilities.

5. Automation and Computer Interfacing

Viscometers usually allow only one measurement at a time. However, if you want to take continuous measurements at different measurements, you must leverage a viscometer with an integrated automated system. The RV-10000A Tuning Fork Vibro Rheometer offers this feature and is one of the best rheometers of its segment.

Apart from it, a few automated instruments can also be interfaced with a PC, and you must check this feature while selecting a viscometer for yourself. 

6. Regulatory Requirements and Industry Standards

Make sure to check for ASTM or ISO industry standardisations while choosing a viscometer for yourself. These standardisations ensure that you buy a quality product that has passed the government regulations and will provide you with accurate and precise experiment measurements.

Look out for the A&D for the best, certified, and quality viscometers.

Reference: Measure Fluid Flow With The Most Accurate Viscometers