The viscosity of various liquids is measured by special devices - viscometers. According to the characteristics and design, several types of these devices are distinguished. One of them is a rotational viscometer capable of assessing the permeability of a medium.
Varieties of appliances
Instruments used to measure the viscosity of a liquid are usually divided into three large groups:
Capillary viscometer
Mechanical viscometer
Rotational viscometer
Let's consider each of the species in more detail.
Mechanical devices
The category of mechanical viscometers is a range of different instruments based on the mechanical properties of liquids. These can be resonant, bubble, ball type meters. If the first two types are most often used in the laboratory, then the latter is found in everyday life. Its principle of operation is based on the discovery of Galileo.
Inside the device there is a "booth" where the ball is located. After filling the device with liquid,whose viscosity is to be determined, the ball drops. The exact time required for the ball to fall to the contact area is measured. The conditional viscosity is determined by this time interval.
Capillary type devices
The capillary viscometer in its design has a thin tube with a known diameter. The test fluid flows through this tube. The same liquid is also passed through a tube with a large diameter, inside which no capillary effect is created. Most often, the fluid flows under the force of gravity (i.e. from top to bottom). But in some devices artificial pressure is created. The time taken for the liquid to flow out of both tubes is measured. Next, their difference is calculated. The viscosity value will be proportional to the value of this difference.
Devices of this type are simple but large. Another disadvantage is that the viscosity of the measured liquid should not exceed 12 kPas. This value corresponds to liquids that flow well. Thicker liquids, or those with lumps, cannot be measured in this case.
Rotational viscometer: principle of operation
The design of meters of this type is a cylinder, inside of which a sphere is placed. The inner sphere moves at a certain speed due to the connected electric drive.
There remains a space between the cylinder and the sphere, which is filled with the investigated liquid. In this case, the resistance to the movement of the sphere changes. In these devices, it is precisely the dependence of the resistance that is measuredfluid and rotation speed. These parameters are fixed as a result of the test.
There is not always a sphere inside a cylinder. It can be replaced by a disk, a cone, a plate, or another cylinder. The distance between the outer and inner body is a few millimeters in order to create a friction force. The resistance value is determined by sensors. The more they are set, the more accurate the value will be. Accordingly, the price of the device will increase.
The rotational viscometer is suitable for liquids whose viscosity ranges from a thousand to millions of Pas. The speed of rotation of the inner body plays an important role. It depends on the accuracy of the measurement. The slower the speed, the more accurate the measurement. Instruments with a minimum rotation rate are very accurate, but they are also expensive.
Types of rotational viscometers
The principle of operation of the device described above is typical for the Brookfield viscometer. This is the simplest meter device of this type. But the inner body does not always move. In some cases, the outer cylinder rotates. That is why the rotational viscometer can be of two types: with a fixed cylinder and torsion meters.
The inner body of torsion viscometers is suspended in the center on an elastic thread. When the outer cylinder rotates, the liquid being measured also begins to move. When it rotates, the cylinder also twists. The angle of twist of the inner cylinder is balanced by the friction torque of the rotating fluid.
Measurement error occurs due to the bottom of the inner cylinder. Various scientists have tried to solve this problem in their own way. Most often, the bottom was made concave. When filling the liquid, air remains in the concavity. This reduces the friction on the bottom. Scientists Gatchek, Kuett placed the inner cylinder in protective rings. This reduced the turbulence of its ends. Volorovich used a tall but narrow top hat. In this case, the error due to the bottom became insignificant. A number of scientists used instruments in which the distance between the cylinders was very small. At the same time, the bottom of the device was not filled with liquid.
Rotational viscometer in its design has a lot of options. But it always has the advantages of versatility, small size, small error and low cost. It is thanks to these characteristics that the device has become so popular.
