What Are the types of Rotational Viscometers?

 Vertebral-plate viscometer

The instrument consisted of an upper and lower circular plate. The axis for both the circular plate is on an identical axis. The circular plates and cones can rotate. The polymer melt in the slit formed by the cone and circular plate at an angle of θ sets a barrel viscometer apart. The rotation of the circular plate initiates the cone’s rotation due to the liquid.

Drum Viscometer:

The instrument consists of a flat-bottomed cylinder and a cylinder in the center of the coaxial. The two parallel surfaces between the cylinder and the slit form a slit, and the polymer liquid is filled in the slit. The cylinder is attached to a force-measuring device and connected by springs.

The stepless speed governor is used to rotate the cylinder as the cylinder is caused to turn; the polymer liquid in the slit moves because of the shearing action. The cylinder will continue to rotate because of the fluid until the torque and spring force cease rotation. At this point, the cylinder rotates for a brief period at an angle θ. As soon as equilibrium is attained, the liquid shearing action also achieves a stable state. The torque and rotational speed of the cylinder can be used to determine the sum of each position in the annular seam by the formula.

Falling ball viscometer:

The falling ball viscometer is primarily used for the viscosity measurement of polymers, but its ability to determine melt viscosity is questionable; thereby, it is rarely used. Certain limits, such as difficulty obtaining data, make the instruments less desirable for use. As the ball moves, the value of Y in the liquid varies, making data processing difficult. Hence, a comprehensive analysis is challenging and is only applicable for the highest shear rate near the ball, which may be calculated as 3v/2R. The shear rate during determination is approximately 0.01/second.


The working principle of the Digital Rotary Viscometer:

A synchronous motor is used to rotate the rotor at a constant speed. As the rotor rotates in the liquid, a viscous moment is produced, which acts on the rotor. High viscosity equates to a high viscous moment and vice versa. The intensity of the viscous moment is measured by the sensor, which is processed by the digital computer, and then viscosity is determined.


The digital viscometer uses a microcomputer technology approach, which allows easy range selection, processes the data digitally detected by the sensor, and displays the rotor number and speed. The instrument is composed of 4 different roots and gears of various speeds. A total of 16 combinations can be used to determine the viscosity of the liquid.


What are the factors that affect the measurement accuracy of a rotational viscometer?

Variations in critical factors of viscosity can result in inaccurate measurement results. The factors which can affect the accuracy are:


  1. Air bubbles and the depth of the rotor immersed in the liquid affect the instrument’s accuracy. The rotational viscometer has set strict rules for the depth of the rotor, and manual operation is required as per the requirements. Usually, the rotor is submerged in liquid with foam which disappears soon after the rotor rotates. Often, the rising bubble becomes challenging to tackle. Foam can cause variations in the measurement data. Moving the rotor and tilting it can help to an extent.


  1. The cleaning frequency of a rotational viscometer is essential. The measuring rotor is often covered in dirt and requires frequent cleaning. Special attention must be given to the cleaning method adopted such that no scratches should be present on the rotor as it can cause deviation in the results.


How to choose a rotational viscometer?

Choosing a viscometer can be difficult if the customer is unaware of his requirements. The following tips can help in purchasing a rotational viscometer:


  1. The viscosity value must be consistent with the model, rotor, speed, and temperature conditions. Let’s suppose the sample’s viscosity lies within the 500,000 range. However, choosing a viscometer only by its range is difficult as the rotational speed varies when the sample is in the maximum capacity of the viscometer.
  2. There are two types of fluids: i) Newtonian Fluids and ii) Non-newtonian fluids. Approximately 90% of the fluids belong to the non-newtonian category. As the viscosity value differs for each rotational speed, the viscosity test report must mention the particular test conditions, i.e., machine type, velocity, temperature, etc. The viscosity value fluctuates timely during the testing on non-Newtonian fluid samples.
  3. Understanding the requirements and purpose is essential before buying a rotation viscometer.


What are the critical parameters for choosing a rotational viscometer?

  1. The temperature range and control must be given attention.
  2. The maximum range of viscosity that the instrument can detect.
  3. Whether the viscosity is supposed to be determined at a fixed speed, shear rate, rotor, etc.
  4. The maximum or minimum amount of sample can be used during the process.


How to choose a rotor for a rotational viscometer?

  1. For data comparison, consistency with the test conditions of the comparison data is essential.
  2. If the comparison is not required, observe the conditions under which the viscosity is determined. Choose the corresponding rotational speed as per the requirements.
  3. High speed can contribute to the echo effect, whereas low speed doesn’t equate to the actual application.
  4. A maximum of 5 or more turns are necessary to read the value.
  5. The instrument is accurate to within ±1% of the full scale. The full scale varies for every rotor, and 10–100% of the full scale is also a practical value. To improve the accuracy by 80%, some customers set the measured value to 30–100% of the whole scale.



How does the sample container affect the viscosity measurement of a rotational viscometer?

The sample container can influence the measured value regardless of whether its relative viscosity or absolute viscosity that’s being measured. In the case of relative viscosity, the sample container impacts the final test results. The measured viscosity value is associated with the diameter of the cup and rotor. Large diameters result in a more negligible effect on the viscosity; however, it’s incredibly dependent on the rheological properties of the sample. It’s vital to ensure that the rotor is situated in the middle of the container. The sample containers must have identical specifications with identical upper and lower diameters. Theoretically, a particular diameter and height to meet the immersion depth of the rotor must be met to avoid the effect of the cup wall.


For absolute viscosity measurement, the rotor must be a fixed marching rotor, and a sample cup of DIN or ASTM standard should be used, which cannot be replaced. The rotor and container wall space must be within the specified standard requirements. It ensures ideal laminar flow without eddy, turbulent and other effects. The standard principle recommends the use of a rotor with a small diameter for samples with high viscosity and vice versa.


How to choose a viscosity standard for a portable rotational viscometer?

The instrument must be horizontal when the portable rotating viscometer selects the viscosity standard liquid based on the measured viscosity range.

  1. Bubbles are often produced when the rotor is put in the sample. These bubbles tend to decrease the measured viscosity value. To avoid bubbles, the sample must be positioned inclinedly when the rotor is installed and must not be in contact with the cup wall or bottom.
  2. Keeping the rotor clean and dry during various sample measurements is essential. Residual water can affect the accuracy of the instrument.
  3. The pH limit is 2. Unique rotors are recommended for higher acidity.
  4. The left hand should hold the mandrel and the right hand to rotate the rotor while connecting. This protects the mandrel and hairspring in the fuselage, which helps enhance the instrument’s service life.
  5. It’s crucial to select the value when it is stable, or else there are higher chances of error in the obtained value.
  6. The rotor for the rotational viscometer is determined by the sample’s viscosity to be measured and the range of the rotor close to the number.
  7. Choose a standard viscosity liquid corresponding to the measured viscosity range, and check the instrument before using the viscometer or rheometer. Regular checkups are recommended to ensure accuracy.


What is the difference between Ubbelohde Viscometer and Rotational Viscometer?

Ubbelohde Viscometer, commonly known as Kinematic/Dynamic viscosity mainly focuses on the fluidity determination of liquids. A tangential intern frictional force exists between the layers of flowing liquid. If the liquid can cross the tube, the flow resistance must be overcome by expanding a fraction of the work. A constant temperature water bath and temperature flow time of sample multiplied by the coefficient of the viscometer is kinematic viscosity.


The apparent viscosity of non-Newtonian liquids is measured using rotational viscometers, which measure the dynamic viscosity of distinct Newtonian liquids. For example, to evaluate the rheological properties of non-Newtonian liquids, a custom-made special drum is utilized in conjunction with a conventional drum. The dynamic viscosity is determined.