The liquid level sensors in the tank allow both the current measurement of the amount of liquid charged and the reporting 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 action.
The information presented in the article will help you learn about the principles of operation of sensors of various types and their application areas. A brief overview of their advantages and disadvantages will be carried out, the main manufacturers recommended on the market will be indicated.
Instrument Classification
The liquid level sensors in the tank can be level gauges or alarms. The first of them are designed for continuous measurement of the liquid level at the current time. They use sensors that work on different physical principles. Further processing of the signals arriving from them is performed 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 of the achievement of 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 is discrete. A warning can be issued in the form of a light or sound alarm. At the same time, automatic operation of the systems for filling or draining the liquid occurs.
Level Measurement Methods
Depending on the properties of the liquid whose level in the tank you want to determine, the following measurement methods are used:
- contact, in which the direct interaction of the liquid level sensor in the tank or part thereof with the medium being measured;
- 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 with it.
Principles of action
Both level gauges and signaling devices use different operating principles to perform their functions. The most common devices are the following types:
- liquid level sensors 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 a contact measurement method, the other two relate to non-contact devices.
Mechanical float sensors
A light float, constantly located 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 shoulder of the resistance bridge. With a minimum amount of fluid 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, moving the movable 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 appeared voltage 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 Sensors
A discrete signal in the form of closing or opening of the contacts of the reed relay is used by the electronic indication and signaling circuit to notify about the achievement of the liquid level in the tank of a certain value. Metal contacts made of a material with a low transient resistance when they are closed are placed in a hollow insulated glass flask.
The water level sensor in the reservoir with a discrete output incorporates a guide in the form of a hollow tube into which liquid from the reservoir does not enter. The contacts of one or several reed switches are fixed inside the guide. Their location depends on in which case it is necessary to receive an alarm about the liquid level reaching a predetermined value.
A sensor float with a small permanent magnet embedded in it moves along the guide when the liquid level in the tank changes. The operation of the contact group occurs when 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 capacity, is fed to the alarm circuit.
Magnetostrictive Float Sensors
Sensors of this type give a constant signal, depending on the liquid level in the tank. The main element, as in the previous case, is a float with a permanent magnet inside, occupying its position on the surface of the liquid and moving in a vertical plane along the guide.
The internal cavity of the guide, isolated from the liquid, is occupied by the waveguide. It is made of magnetostrictive material. At the bottom of the element is a source of current pulses that propagate along it.
When the emitted pulse reaches the location of the float with a magnet, two magnetic fields interact. The result of this interaction is the appearance of mechanical vibrations that propagate back through 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 display circuit constantly reports the liquid level in the tank.
Capacitive Sensors
The operation of sensors of this type is based on the properties of the capacitor to change its electric capacitance when the dielectric constant of the material changes, filling the space between its plates. Coaxial type 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 constant of air and liquid medium have different values. Filling the reservoir leads to a change in the value of the total dielectric constant of the coaxial capacitor and, accordingly, its electric capacitance.
The frequency of the oscillatory circuit, in the circuit of which the capacitor is connected, varies in proportion to the change in its capacitance. An 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 pressure transmitter. 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 you to use them for tanks of different geometric sizes.
The sensitive element of the hydrostatic sensor is a membrane that senses the pressure of a liquid column above itself. Its adjustment is made in such a way that atmospheric pressure does not lead to deformation of the membrane. From the pressure at the measuring point, you can determine the height of the liquid column or the degree of filling of the tank.
The amount of deformation of the membrane is converted into a proportional electrical indicator, which is then used to display the liquid level in the tank. Corrections are applied that take into account the density of the medium being measured and the acceleration of gravity at the measurement point.
Radar Sensors
The liquid level sensor of the tank uses a non-contact measurement method based on the properties of this medium of any density and viscosity to reflect an electrical signal. The frequency of the emitted radar signal located above the surface of the measured liquid level varies linearly.
Reflected from the surface, it arrives at the receiving device with a delay determined by the distance 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 considered in the previous section of the article. The location measurement method is used in the ultrasonic wavelength range.
The data obtained determine the time difference between the emitted transmitter and the received receiver signals. 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 liquid level in the tank.
Manufacturers at a Glance
Liquid level sensors in the Aries tank allow you to make the necessary measurements at a high level. Advertising their products can be found on many foreign sites.
Noteworthy are the products of the domestic developer and manufacturer of L-CARD, entered in the State Register of measuring instruments. Alta Group, located on the Russian market for more than 10 years, has well-deserved positive reviews.
Conclusion
The liquid level sensors in the tank should be selected based on the conditions of their use, the properties of the liquids, the required indicators of measurement accuracy. The most accurate readings can be obtained using radar sensors, magnetostrictive meters.
It must be remembered that absolute accuracy requires higher material costs. Float sensors and alarms are the simplest devices, but their use is limited by vibration conditions due to foaming of the liquid, its viscosity, and aggressiveness of the medium.
The optimal solution, based on the price / quality ratio, is the use of hydrostatic and capacitive sensors, provided that the restrictions imposed on the properties of the measured liquid are met.