Instruments for measuring the viscosity of a liquid. Rotational viscosimeter
The viscosity of various liquids is measuredspecial 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
Instruments used to measure the viscosity of a liquid are usually divided into three large groups:
- Capillary viscometer.
- Mechanical Viscometer.
- Rotational viscometer.
Consider each of the species in more detail.
Mechanical devices
The category of mechanical viscometers isa number of different devices, the principle of operation of which is based on the mechanical properties of liquids. It can be resonance, bubble, ball type meters. If the first two types are most often used in the laboratory, the latter is found in everyday life. His principle of work is based on the discovery of Galileo.
Inside the device there is a "booth", where is locatedball. 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. The conditional viscosity is determined from this time interval.
Devices of capillary type
The capillary viscometer in its design hasA thin tube with a known diameter. The test liquid flows through this tube. The same fluid is also allowed through a tube with a large diameter, inside which a capillary effect is not created. Most often, the fluid flows under the force of gravity (that is, from the top down). But some devices create artificial pressure. Measures the time for which the liquid will flow through both tubes. Then calculate their difference. The viscosity value will be proportional to the value of this difference.
Devices of this type are simple, but have largedimensions. Another disadvantage is that the viscosity coefficient of the liquid being measured should not exceed 12 kPa * s. This value corresponds to liquids that flow well. More dense liquids, or having lumps, in this case can not be measured.
Rotational viscometer: the principle of operation
The construction of measuring devices of this type is a cylinder inside which a sphere is placed. The internal sphere moves at a certain speed due to the connected electric drive.
Between the cylinder and the sphere there is space,which is filled with the liquid being studied. In this case, the resistance to the motion of the sphere changes. In these instruments, it is precisely the dependence of the fluid resistance and rotation speed. These parameters are fixed as a result of the test.
Inside the cylinder there is not always a sphere. It can be replaced by a disk, a cone, a plate or another cylinder. The distance between the outer and inner body is several millimeters to create a friction force. 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,the viscosity coefficient of which is in the range from thousands to millions of Pa * s. The speed of rotation of the inner body plays an important role. The accuracy of the measurement depends on it. The lower the speed, the more accurate the measurement. Instruments with a minimum speed of angular rotation are very accurate, but they are expensive.
Types of rotational viscosimeters
The principle of operation described above is characteristicfor a Brookfield viscometer. This is the simplest 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 gauges.
Internal body of torsion viscometerssuspended in the center on the elastic thread. When the external cylinder rotates, the measured liquid begins to move. When it rotates, the cylinder also turns. The angle of twisting of the inner cylinder is balanced by the torque of the frictional forces of the rotating fluid.
The measurement error is due to the bottominner cylinder. Various scientists have tried to solve this problem in their own way. Most often, the bottom was made concave. When filling the liquid in the concavity, air remains. Thus, friction against the bottom is reduced. Scientists Gatchek, Couette internal cylinder placed in security rings. This reduced the turbulence of its ends. Volorovich used a tall but narrow cylinder. However, 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 viscosimeter by its designhas a lot of options. But it always has such advantages as versatility, small size, small error and low cost. It is thanks to these characteristics that the device became so popular.
