The viscosity of various liquids is measured by special instruments - 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 the medium.
Types of devices
The devices used to measure the viscosity of a liquid are usually divided into three large groups:
Consider each of the species in more detail.
Mechanical devices
The category of mechanical viscometers represents a number of different devices, the principle of operation of which is based on the mechanical properties of liquids. It can be meters of resonance, bubble, ball types. If the first two types are most often used in the laboratory, then the latter is found in everyday life. His working principle is based on the discovery of Galileo.
Inside the device there is a “cabin” where the ball is located. After filling the device with a liquid, the viscosity of which must be determined, the ball falls. The exact time required to drop the ball to the contact pad is measured. Conventional viscosity is determined by a given time interval.
Capillary 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 allowed through a tube with a large diameter, inside of which a capillary effect is not created . Most often, fluid flows under gravity (i.e. from top to bottom). But in some devices, artificial pressure is created. The time taken for the fluid to flow through 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 they are large. Another drawback is that the viscosity coefficient of the measured liquid should not exceed 12 kPa * s. This value corresponds to fluids that flow well. Thicker liquids, or having lumps, cannot be measured in this case.
Rotational Viscometer: Principle of Operation
The design of meters of this type is a cylinder, into which a sphere is placed. The inner sphere moves at a certain speed due to the connected electric drive.
Between the cylinder and the sphere there remains a space that 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 of the liquid and the speed of rotation that is measured. These parameters are recorded as a result of the test.
There is not always a sphere inside the cylinder. It can be replaced by a disk, cone, plate or another cylinder. The distance between the outer and inner bodies is a few millimeters to create friction. The resistance value is determined by the sensors. The more they are installed, the more accurate the value will be. Accordingly, the price of the device will increase.
The rotational viscometer is suitable for liquids whose viscosity coefficient ranges from thousands to millions Pa * s. The rotation speed of the inner body plays an important role. The measurement accuracy depends on it. The lower the speed, the more accurate the measurement. Devices with a minimum speed of angular rotation are very accurate, but they are also expensive.
Types of Rotational Viscometers
The principle of operation described above is typical for a Brookfield viscometer. This is the simplest device of this type of meter. But the inner body does not always move. In some cases, the outer cylinder rotates. That is why a 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 measured liquid also begins to move. During its rotation, the cylinder is also twisted. The twist angle of the inner cylinder is balanced by the moment of friction of the rotating fluid.
The 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. Thus, friction on the bottom is reduced. Scientists Gatchek, Couette, the inner cylinder was placed in a guard ring. This reduced the turbulence of its ends. Volorovich used a tall but narrow cylinder. At the same time, the error due to the bottom became insignificant. A number of scientists used instruments in which the distance between the cylinders was very small. In this case, the bottom of the device was not filled with liquid.
The rotational viscometer in its design has a lot of options. But it always has such advantages as versatility, small size, small error and low cost. Thanks to these characteristics, the device has become so popular.