In many industries, liquid and bulk materials are used as technological media. In the modes of in-line production of products, and especially with automatic controls, constant monitoring of the parameters of working materials is required. The most common means of such control is level measurement, during which the degree of filling of one or another capacitive equipment is monitored.
Implementation of technology
In this case, the level is understood as the height of filling the technological installation (tank, reservoir, tank, piston) with the working medium. In itself, knowledge of this value is necessary for the management and control of the production process. In particular, such measurements are a necessary operation in the chemical, oil refining and food industries. Knowing the filling level of the tank for collecting purified oil, for example, the operator can set the optimal parameters for the operation of the pumping pumpstations. And again, many industries operate on automation, so the output data can be processed by controllers, which, even without the participation of an operator, give commands to the executive units, taking into account the information received about the filling level of the controlled apparatus. Depending on the specific technological operation and accounting requirements, different units of level measurement may change - for example, there are methods with a wide measurement range from 0.5 to 20 m, as well as specialized laboratory control schemes that take into account a narrow range from 0 to 500 mm. Direct measurement is carried out by physical, electromagnetic and ultrasonic devices, some of which also record the properties of the medium - chemical composition, pressure, temperature, etc.
Visual controls
The simplest way to solve the problem, in which it is enough to use a standard measuring tool. Roulettes, rulers, sight glasses and other devices are used, which, in principle, can be used in the given conditions of a specific production environment. The most technological means of measuring the level of this type is a remote or bypass indicator. It is installed in the side of the tank by means of threaded, flanged or welded connections. The indication process is provided by a transparent tube that fills up as the liquid level in the target tank rises. More modern bypasses use cylindrical floats with magneticindication system. But even such a design is considered obsolete due to significant limitations in the communication capabilities of interfacing with control electronics and automation equipment.
Float measuring method
Also one of the simplest traditional ways to control the filling level of liquid media. It is based on fixing the position of the float on the very surface of the serviced liquid. Control is carried out according to different principles - mechanical, magnetic and magnetostrictive. In the process of moving, the nature of the connection between the float and the element controlling it, for example, a rigidly fixed lever, changes. The attachment angle changes when the float rises, which is fixed by the measurement system. Typically, this type of level measurement occurs in the process of converting the same angle into an electrical signal. Most often, we are not even talking about taking into account specific indications, but about registering the moment a specific value is reached. In other words, when the float reaches the set height level, the level switch is activated. In the simplest circuits, contacts close, which leads to certain technological actions - for example, the function of a liquid pump stops.
Hydrostatic measurements of fluids
The key measurement factor in this level gauge system is hydrostatic pressure. That is, a pressure gauge with suitable characteristics and a submersible pressure sensor are used. Moreover, an important condition for control isseparation of the sensor from the working medium by a special membrane on the one hand, and on the other hand, atmospheric pressure must be supplied through the capillary supply from the filler. In the process of measuring with this type of level, excess pressure is controlled, the indicator of which affects the characteristics of the generation of a unified signal. Also, an electrical device with a converter is connected to the pressure gauge, which is responsible for notifying about certain changes that have occurred in the controlled environment. As an alternative to this method of measuring hydrostatic pressure, it is possible to control the pressure of a gas that is pumped into an analogue of a capillary tube from the side of the liquid filling the tank. This model of the hydrostatic pressure gauge is called piezometric.
Radar level gauges
In some industries, a universal approach is used to measure the height levels of filling with process media. For working with liquids, gases and bulk materials, radar equipment is optimally suited, the operation of which is based on the analysis of frequency-modulated oscillations. The time of propagation and return of undamped oscillations from special antennas to the serviced environment is measured. Wavebands can vary from one to tens of GHz. The transmit-receive antennas themselves may have a different device and radiation characteristics. To measure the level of liquids in chemical industries, for example, rod antennas are used.with a height measurement range of up to 20 m. For media, the control of which has increased requirements in terms of accuracy, parabolic and planar devices are used. Usually these are areas of technical accounting, where it is important to fix measurements up to 1 mm.
Use of radioisotope techniques
The main specialization of this type of level gauges is the control of bulk materials and liquid media in closed tanks. The principle of operation of the radioisotope apparatus is based on the absorption of gamma rays that pass through the layer of the target medium. Technically, the measurement process is organized using a radiation source and a receiver. The two devices are suspended or mounted on a supporting structure and are controlled by a reversible electric motor that changes their position in height depending on the current filling level. If the system for measuring the level of the working medium is above its surface, then the radiation from the receiving signal will be strong, since there is no obstacle in its path. Therefore, the electric motor from the controller is given a signal to lower the equipment. The position of the measuring fixture will control the signal in the tank by continuously feeding and processing the waveforms.
Ultrasonic control methods
The principle of operation in this case is in many respects similar to radio frequency control, in which a radio signal is emitted and the degree of filling of the production area is fixed by the characteristics of its reflection from the measured mediumcontainers. However, the ultrasonic method uses special acoustic instruments to measure the filling level. That is, sound waves propagate, and the functioning of the equipment is similar to the principles of location. The indicators are fixed according to the time of passage of the distance fluctuations from the emitter to the media separation line and back to the receiving device. The location of the interface is determined from the sides of the air (gas) and target working media. This is how combined high-precision devices work, but in the group of ultrasonic level gauges there are devices that can purposefully control only gas-air (unfilled) or only the working environment.
Microwave Methods
One of the most popular non-contact measurement technologies that combines the techniques and principles of radar electromagnetic control. The most promising technique of this class can be called directional electromagnetic measurement, in which the signal reflection coefficient is determined on the basis of microwave pulses that can penetrate to the bottom of the tank, bypassing various kinds of unwanted impurities and sludge particles. The returned signal, or part of it, is measured for completeness and speed characteristics. Taking into account the time of its passage, the degree of fullness is determined. Microwave methods for measuring the level of working media are widely used in technological tasks of controlling the filling of granular and powder materials. In such industries, probes are usedwith a single suspension on cables, while in relation to liquids, double and rod support structures are used. In general, the optimization of tooling when working with solids justifies itself for reasons of physical and mechanical properties that are associated with technical limitations in the organization of measurement processes.
Conclusion
In recent years, technologies for the development of level gauges for monitoring process media have gone through several fundamentally important stages of development that have changed the principles of such measurements. Among the most important of these are the transition to non-contact measurement methods and the expansion of capabilities when working with aggressive liquids. Today, the same non-contact RF or electromagnetic method can provide accurate control of crude oil, acid, molten sulfur and liquid ammonia.
