Introduction
It is well known that sensors transform a physical quantity into an electrical quantity, in this report we will be interested in a sensor that detects the level of a conductive aqueous solution in a tank and transforms it into an electronic signal; this signal will be displayed directly on site or integrated in a process control or control system, knowing that a switching command can start or stop filling devices such as belt conveyors or pumps.
Generally level detection is done by several methods (hydrostatic, acoustic, gamma ray absorption, electrical ... etc.). Ours is one of the electric ones, these are methods using specific sensors, that is to say, directly translating the level into an electrical signal. Their interest lies in the simplicity of the devices and the ease of their implementation.
In the development of this subject we will begin by explaining the conductivity of an aqueous solution; and we will see later a theoretical interpretation of the operating aspect of the sensor, its advantages and disadvantages, its components and its
procedures of realization.
Generally level detection is done by several methods (hydrostatic, acoustic, gamma ray absorption, electrical ... etc.). Ours is one of the electric ones, these are methods using specific sensors, that is to say, directly translating the level into an electrical signal. Their interest lies in the simplicity of the devices and the ease of their implementation.
In the development of this subject we will begin by explaining the conductivity of an aqueous solution; and we will see later a theoretical interpretation of the operating aspect of the sensor, its advantages and disadvantages, its components and its
procedures of realization.
I- The electrical detection of an aqueous solution:
I.1. Detection :
From an electrical point of view it is possible to detect the presence of an aqueous solution thanks to its chemical properties:
- An "aqueous solution" is water containing one or more dissolved substances.
- Pure (distilled) water is practically non-conductive of electric current.
- Aqueous solutions must contain ions (charged particles) to be conductive. Solutions that contain only molecules are not conductive.
- In aqueous solutions, the electric current is due to ion displacement.
- The positive ions move in the direction of the current (towards the - terminal) and the negative ions in the opposite direction (towards the + terminal).
In an ionic aqueous solution, the ions of positive electric charge move towards
the negative electric charge electrode (Cathode), and the negative ions move in the opposite direction (towards the positive terminal: Anode).
the negative electric charge electrode (Cathode), and the negative ions move in the opposite direction (towards the positive terminal: Anode).
It is this two-way movement that constitutes the electric current in a liquid.
I.2. Conductivity concentration
Electrical conductivity is highly dependent on the number of electrons available to
participate in the conduction process.
Conductivity is defined as the ratio of current density (J) to
electric field strength (e) 𝑆 = 𝐽 / 𝑒
participate in the conduction process.
Conductivity is defined as the ratio of current density (J) to
electric field strength (e) 𝑆 = 𝐽 / 𝑒
Sodium Chloride Solution
Most metals are extremely good conductors of electricity because of the large number of free electrons that can be excited in an empty and available energy layer.
In water and ionic and liquid materials, since the electric current is carried by the ions of the solution, the conductivity increases as the ion concentration increases.
Therefore, since the conductivity of the aqueous solutions is generally low, we will adopt a transistor sensitive to this conductivity even if it is weak; our choice fell on transistor BC548 whose characteristics are given in the appendix.
In the following chapter we will discuss the proper fitting for the realization of an
efficient level sensor.
Most metals are extremely good conductors of electricity because of the large number of free electrons that can be excited in an empty and available energy layer.
In water and ionic and liquid materials, since the electric current is carried by the ions of the solution, the conductivity increases as the ion concentration increases.
Therefore, since the conductivity of the aqueous solutions is generally low, we will adopt a transistor sensitive to this conductivity even if it is weak; our choice fell on transistor BC548 whose characteristics are given in the appendix.
In the following chapter we will discuss the proper fitting for the realization of an
efficient level sensor.
II- Functional aspect
II.1. Basic principle
The diagram below represents the basic idea of level detection principle.
If the switch SW1 closed the LED is lit, on the contrary if it is open the transistor is in the off state then the LED will be off due to the absence of current IC.
II.2. Global principle
A probe (electrode, single metal rod of vertical axis isolated from the mass of the tank) has its lower end always immersed in the liquid.
When the level rises, this liquid comes into contact with the end of the second probe located
above the electrical circuit is then closed, a very low voltage current passing between two
electrodes. This variation in the resistance of the medium from the state of insulation to (gas or vapor) in the
conducting state (liquid) produces a voltage variation detected by the electronic circuit (FIG. 2) which
changes the state of a contact or output signal (Remember that we can exploit this output signal for many purposes).
It is easy to place several level detections (very low, low, high, very high ...) by using several probes (electrodes) of different lengths.
Sometimes the longest electrode whose end end must always be immersed in the liquid by the mass of the tank if it is metallic.
An electrode can also be mounted horizontally if there is a lateral tapping on the tank.
Low voltage current is used to avoid the risk of electrocution (passing
in a living organism an electric discharge) and under low intensity so that there is no electrolysis (chemical decomposition of a certain compound bodies in melt or in solution obtained by the passage of an electric current) of the liquid whose level is measured.
When the level rises, this liquid comes into contact with the end of the second probe located
above the electrical circuit is then closed, a very low voltage current passing between two
electrodes. This variation in the resistance of the medium from the state of insulation to (gas or vapor) in the
conducting state (liquid) produces a voltage variation detected by the electronic circuit (FIG. 2) which
changes the state of a contact or output signal (Remember that we can exploit this output signal for many purposes).
It is easy to place several level detections (very low, low, high, very high ...) by using several probes (electrodes) of different lengths.
Sometimes the longest electrode whose end end must always be immersed in the liquid by the mass of the tank if it is metallic.
An electrode can also be mounted horizontally if there is a lateral tapping on the tank.
Low voltage current is used to avoid the risk of electrocution (passing
in a living organism an electric discharge) and under low intensity so that there is no electrolysis (chemical decomposition of a certain compound bodies in melt or in solution obtained by the passage of an electric current) of the liquid whose level is measured.
II.3. diagram
II.4. Advantages
- Simplicity
- Lower cost
- Independence with respect to the physical characteristics of the product (in particular the density)
- Detectors are generally systems that cost less than continuous but reliable measurement devices because the safety of personnel and installations is often based on them.
II.5. disadvantages
- Only suitable for liquids and provided they are inductive (water, acid, solutions).
- Slightly corrosive (otherwise, periodically replace the electrodes, their cost being low).
- Few charged particles likely to settle.
- Pure water, sugar water or oil contain molecules: they are not conductive solutions.
III - Realization
Before proceeding to the realization of circuit in practice one simulates the operation of this circuit by means of a software of simulation to check the validity of assembly and to avoid any risk of deterioration of a component or the whole circuit.
III.1. Components of the circuit
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