Liquid level sensors in the tank allow both the current measurement of the amount of filled liquid and report the achievement of its limit values. Such devices consist of a sensitive sensor that responds to certain physical parameters, and measurement, control and indication circuits. Depending on the application, devices are used that differ in the principle of their operation.
The information presented in the article will help you learn about the principles of operation of different types of sensors and their areas of application. A brief review of their advantages and disadvantages will be carried out, the main manufacturers that have proven themselves in the market will be indicated.
Classification of appliances
Liquid level sensors in the tank can be level gauges or signaling devices. The first of them are designed for continuous measurement of the liquid level at the current moment.time. They use sensors that work on different physical principles. Further processing of the signals coming from them is carried out by analog or digital electronic circuits that are part of the level gauges. The obtained indicators are displayed on the display elements.
Signaling devices warn about reaching a certain value of the liquid level in the tank, pre-set by the setting elements. Their other name is water level sensors in the tank to turn off its further supply. Their output signal is discrete. The warning may be issued in the form of a light or sound alarm. In this case, the operation of the filling or draining systems is automatically blocked.
Methods of level measurement
Depending on the properties of the liquid to be measured in the tank, the following measurement methods are used:
- contact, in which the direct interaction of the liquid level sensor in the tank or its part with the measured medium;
- non-contact, avoiding direct interaction of the sensor with the liquid (due to its aggressive properties or high viscosity).
Contact devices are located in the tank directly on the surface of the measured liquid (floats), in its depth (hydrostatic pressure gauges), or on the tank wall at a certain height (plate capacitors). For non-contact meters (radar, ultrasonic) it is necessary to provide a zone of direct visibility of the surface of the measured liquid and the absence of direct contact withher.
Operating principles
Both level gauges and signaling devices use different operating principles to perform their functions. The following types of devices are most widely used:
- float sensors for liquid level in the tank;
- capacitive;
- hydrostatic liquid level sensors;
- radar type devices;
- ultrasonic sensors.
Float, in turn, can be mechanical, discrete and magnetostrictive. The first three groups of sensors include devices using the contact method of measurement, the other two are non-contact devices.
Mechanical float switches
A light float, constantly on the surface of the liquid in the tank, is connected by a system of mechanical levers to the middle terminal of the potentiometer, which is the arm of the resistance bridge. With a minimum amount of liquid in the tank, the bridge is considered balanced. There is no voltage in its measuring diagonal.
As the tank fills, the float monitors the position of the liquid level by moving the moving contact of the potentiometer through the lever system. Changing the resistance of the potentiometer leads to a violation of the balanced state of the bridge. The voltage that appears in its measuring diagonal is used by the electronic circuit of the display system. Its analog or digital readings correspond to the amount of liquid in the tank at the current time.
Discrete float switches
Discrete signal in the form of a circuitor opening of the contacts of the reed switch is used by the electronic indication and signaling circuit to notify that the liquid level in the tank has reached a certain value. The metal contacts, which are made of a material with a low contact resistance when they are closed, are placed in a hollow insulated glass bulb.
The water level sensor in the tank with a discrete output incorporates a guide in the form of a hollow tube into which liquid from the tank does not enter. The contacts of one or more reed relays are fixed inside the guide. Their location depends on the case in which it is necessary to receive an alarm when the liquid level reaches the set value.
The float of the sensor with a small permanent magnet built into it moves along the guide when the liquid level in the tank changes. The operation of the contact group occurs at the moment it enters the magnetic field of the permanent magnet of the float. The signal through the wires connected to the contacts of the water level sensor in the reed switch tank goes to the alarm circuit.
Magnetostrictive float sensors
Sensors of this type produce a constant signal depending on the level of liquid in the tank. The main element, as in the previous case, is a float with a permanent magnet inside, which takes its position on the surface of the liquid and moves in a vertical plane along the guide.
The inner cavity of the guide, isolated from the liquid, is occupied by a waveguide. It is made of magnetostrictivematerial. At the bottom of the element is a source of current pulses that propagate along it.
When the radiated pulse reaches the location of the float with the magnet, two magnetic fields interact. The result of this interaction is the occurrence of mechanical vibrations that propagate back along the waveguide.
A piezoelectric element is fixed next to the pulse generator, which captures mechanical vibrations. An external electronic circuit analyzes the time delay between the emitted and received pulses and calculates the distance to the float, which is constantly on the surface of the liquid. The indication circuit constantly reports the liquid level in the tank.
Capacitive sensors
The operation of sensors of this type is based on the properties of a capacitor to change its electrical capacitance when the dielectric constant of the material that fills the space between its plates changes. Coaxial capacitors are used, which are a pair of coaxial hollow metal cylinders of different diameters.
The latter are capacitor plates, between which liquid can freely penetrate. The dielectric constants of air and liquid medium have different values. Filling the tank leads to a change in the value of the total dielectric constant of the coaxial capacitor and, accordingly, its electric capacitance.
Frequency of the oscillatory circuit, inthe circuit of which the capacitor is connected changes in proportion to the change in its capacitance. The electronic frequency / voltage converter monitors this change and displays a value proportional to the degree of filling of the tank.
Hydrostatic sensors
Another name for such a device is a detector, or a pressure transducer. They can be stationary, fixed at the bottom of the tank filled with liquid, or portable. In the latter case, pressure transducers are equipped with a cable of considerable length. This allows them to be used for tanks of different geometric sizes.
The sensitive element of a hydrostatic sensor is a membrane that perceives the pressure of a liquid column above it. Its adjustment is made in such a way that atmospheric pressure does not lead to deformation of the membrane. The pressure at the measuring point can be used to determine the height of the liquid column or the degree of filling of the tank.
The amount of membrane deformation is converted into an electrical proportional value, which is then used to display the liquid level in the tank. Corrections are applied that take into account the density of the measured medium and the acceleration of gravity at the point of measurement.
Radar type sensors
The tank liquid level sensor uses a non-contact measurement method based on the properties of this medium of any densityand viscosity to reflect the electrical signal. The frequency of the emitted signal of a radar located above the surface of the measured liquid level changes according to a linear law.
Reflected from the surface, it arrives at the receiving device with a delay determined by the length of the path traveled. Thus, there is a difference between the frequencies of the two signals. By the magnitude of the frequency shift, the analyzing device of the locator determines the path traveled by the signal or the level of the reflecting liquid relative to the location of the radar.
Ultrasonic level sensors
The measurement scheme used for sensors of this type corresponds to that discussed in the previous section of the article. The location measurement method is applied in the ultrasonic wavelength range.
The received data determines the time difference between the emitted transmitter and the signals received by the receiver. Using data on the speed of propagation of ultrasound in the space above the surface of the liquid, the analyzing device determines the distance traveled by the signal, or the level of the liquid in the tank.
Brief overview of manufacturers
Liquid level sensors in the tank "ARIES" allow you to make the necessary measurements at a high level. Advertising of their products can be found on many foreign sites.
Deserves attention to the products of the domestic developer and manufacturer L-CARD, included in the State Register of Measuring Instruments. Alta Group, which has been on the Russian market for more than 10 years, haswell deserved positive feedback.
Conclusion
Liquid level sensors in the tank should be selected based on the conditions of their use, the properties of liquids, the required indicators of measurement accuracy. The most accurate readings can be obtained using radar-type sensors, magnetostrictive meters.
It must be remembered that absolute accuracy requires higher material costs. Float sensors and signaling devices are the simplest devices, but their use is limited by vibration conditions due to the foaming of the liquid, its viscosity, and the aggressiveness of the medium.
The optimal solution, based on the price / quality ratio, is the use of hydrostatic and capacitive sensors, subject to the restrictions imposed on the properties of the measured fluid.
