Liquid and granular media are used in various areas of production. This raw material can perform different functions, but often the technologies for its use provide for the control of the filling of containers, bunkers and reservoirs with them. For this, special signaling devices are used, which can also be considered as sensors and control relays. In this case, the methods of indication and the principles for determining the filling levels may be different. There are level switches on the market that differ in design, type of sensing element and the presence of additional functions.
Capacitive sensors
These are the most common universal alarms, which, depending on the model, are suitable for either bulk or liquid media. In particular, the levels of powders, granular products, viscous oils, cement slurries, etc. are evaluated using a capacitive signaling device. Some modifications also allow you to detect the presence of foreign bodies, signaling this to the control panel. The principle of operation is based on fluctuations in the electrical capacitance.
As a rule, this type of level switch has a sensitive element thatdirectly contacts with the target material, constantly monitoring the dielectric parameters of the air medium. Fluctuations in the recorded frequencies are converted into a signal fed to the output control relay. The design of the capacitive sensor is varied - manufacturers produce tubular, flexible, cable and other models.
Float switches
Also simple in design and versatile version of the signaling device. The standard device of such a sensor provides for the presence of a float, which is connected to the switching mechanisms of electromagnetic contacts. In the working environment, float detectors can be located both vertically and horizontally, which also expands the range of their applications. And yet there are restrictions on the use of sensors of this type.
For example, they are rarely used in technological processes where high accuracy of readings and additional indication of the properties of the working medium are required. On the other hand, the float level switch is resistant to adverse operating conditions. For this reason, it is used as a recorder of levels of aggressive chemicals, kerosene, sea water, etc. The average maximum temperature for using a float switch is 200°C.
Buoys
In some way, a kind of float signaling devices, but having its own characteristics. The basis of the structure is formed by a buoy, whichsuspended on a cable and, in the same way as float analogs, is connected to contact mechanisms for transmitting readings. The process of immersing the device in a working liquid medium contributes to a change in the load on the cable, which leads to the opening of the contacts.
The model is quite simple in design, but it has significant advantages. For example, a buoy water level indicator is capable of operating at high pressures - about 20 MPa. As for protection from aggressive environments and temperature influences, these qualities are determined by the design and materials of manufacture of a particular model.
Vibrating alarms
A more technological version of the signaling device, which can be used in different environments - loose and liquid. The principle of operation, as the name implies, is based on oscillatory movements. During operation, the sensitive element of the device produces mechanical vibrations in conditions of resonant frequency. The sensor is powered by a piezoelectric generator or other power source.
Immersion of a vibrating level switch provokes a change in the activity of frequency oscillations and electrical parameters in the circuit connected to the sensor. Further, the changed parameters of the electrical circuit are registered in the relay as a discrete output signal. The advantages of vibration alarms include resistance to temperature effects, high pressure and aggressive environments. For many consumers, the high accuracy of such devices is also important - the average error is 1 mm.
Optical sensors
Optical signaling devices use a light beam as a level detector. The source can be an infrared small lamp or a laser. The measurement principle itself is based on the ability of the target material to reflect, refract or transmit light fluxes through itself.
It is important to emphasize that optical devices provide both contact and non-contact analysis. Since this is an electronic level switch, you can also count on the high accuracy of the results provided. As for working materials, these can be both traditional media and specific ones, for example, foam or oil products. In addition, optics make it possible to record the density, viscosity and transparency of materials.
Ultrasonic sensors
Like other ultrasonic measuring equipment, this type of level sensor works based on the evaluation of acoustic vibrations. A special receiver acts as a radiation source. It is installed in a container, after which the process of background propagation of the ultrasonic field is started. At the moment of fixing deviations in the parameters of sound waves, the electronics turns to the output relay, closing or opening the circuit.
Ultrasonic level switches have an average error of 2 mm, but this figure can be improved in the process of finding the most favorable location. As practice shows, ultrasonic models of signaling devices are verydemanding on the conditions of work in the premises. But otherwise, they show consistent performance.
Rotor models of signalers
This is one of the most common versions of electromechanical signaling devices, which is most often used to work with bulk mixtures. Such devices can be used in explosive and food industries to service materials with a density of about 100 g/l and a fraction of up to 50 mm. In particular, the rotary level switch is used to equip tanks for sugar, grain, powder, animal feed, etc.
The sensing element for such models is represented by a rotor blade, which rotates as a result of the transfer of force from a synchronous electric motor. Actually, the moment of level change in the tank is recorded as a result of fixation of the blade by the target medium. Then the electromechanical filling comes into play, which opens the contacts, sending a signal to the controller relay.
Varieties by switch type
Most signaling devices work with indicator contact switches, which are of two types - electromechanical and solid state. In the first case, mechanical contacts or a control relay are used. The strengths of electromechanics include the ability to work with large voltage and current ranges, which improves measurement accuracy.
At the same time, such contacts are not affected by evaporation, so the liquid level switch with electromechanics is allowed to work inrooms with high temperatures. In turn, solid state switches do not have moving mechanical elements subject to wear. That is, in the process of using this device, regular updating of consumables will not be required. In addition, solid-state devices switch faster and respond to the signal acquisition system.
Conclusion
In choosing a suitable device for fixing the filling level of containers with certain materials, it is worth considering many parameters. It is worth starting with the characteristics of the environment and operating conditions of the device. This will give you an idea of the optimal design and measurement scale.
Much of the choice is influenced by the principle of operation of the level indicator, as well as the method of transmitting information to the relay. At a minimum, these parameters will determine the accuracy of the measurement. Do not ignore the additional functionality. The more complex and expensive the signaling device, the more opportunities it will provide for recording the characteristics of the environment. So, in addition to the level of physical filling of the container, the same optical and ultrasonic models can transmit information about the density, degree of viscosity and other qualities of the material.
