To automate many production processes it is necessary to control the water level in the tank; the measurement is carried out using a special sensor that gives a signal when the process medium reaches a certain level. It is impossible to do without level meters in everyday life; a striking example of this is the shut-off valve of a toilet cistern or an automatic system for shutting off a well pump. let's consider different kinds level sensors, their design and operating principle. This information will be useful when choosing a device for specific task or making a sensor yourself.
Design and principle of operation
Design of measuring devices of this type determined by the following parameters:
- Functionality, depending on this device, is usually divided into alarms and level meters. The former monitor a specific tank filling point (minimum or maximum), while the latter continuously monitor the level.
- The operating principle can be based on: hydrostatics, electrical conductivity, magnetism, optics, acoustics, etc. Actually, this is the main parameter that determines the scope of application.
- Measuring method (contact or non-contact).
In addition, the design features are determined by the nature of the technological environment. It's one thing to measure height drinking water in the tank, another is to check the filling of industrial wastewater tanks. In the latter case, appropriate protection is necessary.
Types of level sensors
Depending on the principle of operation, alarms are usually divided into the following types:
- float type;
- using ultrasonic waves;
- devices with a capacitive level detection principle;
- electrode;
- radar type;
- working on the hydrostatic principle.
Since these types are the most common, let's look at each of them separately.
Float
This is the simplest, but nevertheless effective and reliable way measuring liquid in a tank or other container. An example implementation can be found in Figure 2.
Rice. 2. Float sensor for pump control The design consists of a float with a magnet and two reed switches installed at control points. Let us briefly describe the principle of operation:
- The container is emptied to a critical minimum (A in Fig. 2), while the float drops to the level where reed switch 2 is located, it turns on the relay that supplies power to the pump pumping water from the well.
- The water reaches the maximum level, the float rises to the location of reed switch 1, it is triggered and the relay is turned off, accordingly, the pump motor stops working.
It’s quite easy to make such a reed switch yourself, and setting it up comes down to setting on-off levels.
Note that if you choose the right material for the float, the water level sensor will work even if there is a layer of foam in the tank.
Ultrasonic
This type of meter can be used for both liquid and dry media and may have an analogue or discrete output. That is, the sensor can limit the filling upon reaching a certain point or monitor it continuously. The device includes an ultrasonic emitter, receiver and signal processing controller. The operating principle of the alarm is demonstrated in Figure 3.
Rice. 3. Operating principle of ultrasonic level sensor The system works as follows:
- an ultrasonic pulse is emitted;
- the reflected signal is received;
- The duration of signal attenuation is analyzed. If the tank is full, it will be short (A Fig. 3), and as it becomes empty it will begin to increase (B Fig. 3).
The ultrasonic alarm is non-contact and wireless, so it can be used even in aggressive and explosive environments. After initial setup, such a sensor does not require any specialized maintenance, and the absence of moving parts significantly extends its service life.
Electrode
Electrode (conductometric) alarms allow you to monitor one or more levels of an electrically conductive medium (that is, they are not suitable for measuring the filling of a tank with distilled water). An example of using the device is shown in Figure 4.
Figure 4. Liquid level measurement with conductometric sensors In the example given, a three-level alarm is used, in which two electrodes control the filling of the container, and the third is an emergency one to turn on the intensive pumping mode.
Capacitive
Using these alarms, it is possible to determine the maximum filling of the container, and both liquid and bulk solids of mixed composition can act as the process medium (see Fig. 5).
Rice. 5. Capacitive level sensor The operating principle of the alarm is the same as that of a capacitor: the capacitance is measured between the plates of the sensitive element. When it reaches the threshold value, a signal is sent to the controller. In some cases, a “dry contact” design is used, that is, the level gauge operates through the tank wall in isolation from the process medium.
These devices can operate over a wide temperature range and are not affected by electromagnetic fields, and operation is possible at a long distance. Such characteristics significantly expand the scope of application up to severe operating conditions.
Radar
This type of alarm device can truly be called universal, since it can work with any process environment, including aggressive and explosive ones, and pressure and temperature will not affect the readings. An example of how the device works is shown in the figure below.
The device emits radio waves in a narrow range (several gigahertz), the receiver catches the reflected signal and, based on its delay time, determines how full the container is. The measuring sensor is not affected by pressure, temperature or the nature of the process fluid. Dustiness also does not affect the readings, which cannot be said about laser alarms. It is also necessary to note the high accuracy of devices of this type; their error is no more than one millimeter.
Hydrostatic
These alarms can measure both maximum and current filling of tanks. Their operating principle is demonstrated in Figure 7.
Figure 7. Fill measurement with gyrostatic sensor The device is built on the principle of measuring the level of pressure produced by a column of liquid. Acceptable accuracy and low cost made this type quite popular.
Within the scope of the article, we cannot examine all types of alarms, for example, rotary-flag ones, for identifying granular substances (a signal is sent when the fan blade gets stuck in a granular medium, after first tearing out the pit). It also makes no sense to consider the principle of operation of radioisotope meters, much less recommend them for checking the level of drinking water.
How to choose?
The choice of a water level sensor in a tank depends on many factors, the main ones:
- Composition of the liquid. Depending on the content of foreign impurities in the water, the density and electrical conductivity of the solution may change, which is likely to affect the readings.
- The volume of the tank and the material from which it is made.
- The functional purpose of the container is to accumulate liquid.
- The need to control the minimum and maximum level, or monitoring of the current state is required.
- Admissibility of integration into an automated control system.
- Switching capabilities of the device.
This is far from full list for selection measuring instruments of this type. Naturally, for domestic use it is possible to significantly reduce the selection criteria, limiting them to the volume of the tank, the type of operation and the control circuit. A significant reduction in requirements makes it possible to independently manufacture such a device.
Making a water level sensor in a tank with your own hands
Let's say there is a task to automate work submersible pump for water supply to the dacha. As a rule, water flows into a storage tank, therefore, we need to make sure that the pump automatically turns off when it is filled. It is not at all necessary to buy a laser or radar level indicator for this purpose; in fact, you don’t need to purchase any. A simple task requires simple solution, it is shown in Figure 8.
To solve the problem, you will need a magnetic starter with a 220-volt coil and two reed switches: a minimum level for closing, a maximum level for opening. The pump connection diagram is simple and, importantly, safe. The principle of operation was described above, but let’s repeat it:
- As the water collects, the float with the magnet gradually rises until it reaches the maximum level reed switch.
- The magnetic field opens the reed switch, turning off the starter coil, which leads to de-energization of the engine.
- As the water flows, the float drops until it reaches the minimum mark opposite the lower reed switch, its contacts close, and voltage is supplied to the starter coil, which supplies voltage to the pump. Such a water level sensor in a tank can work for decades, unlike an electronic control system.
Country house. Well with pump + storage tank. If you are too lazy to turn on the pump, then you need an automatic switch. The task is as follows: it is necessary to turn on the pump in the well when the water in the storage tank (for example, a barrel) drops below a certain level, and turn off the pump when the water fills up.
Storage containers come in different sizes. Wells are also different. You can inadvertently drain the well if you do not turn off the pump in time. And the pump itself is also not accustomed to working without water.
Therefore, you also need to be able to temporarily turn off the pump in the well if the level has dropped and allow it to turn on if the level is normal.
There are many on the Internet electronic circuits monitoring the upper and lower water levels. From simple ones (a couple of transistors) to microprocessor ones. We will not consider them. The principle of electrical conductivity of water is very often used as a water level sensor. Those. These are, as a rule, electrodes in direct contact with water. The downside is that they tend to oxidize, lose contact with wires, and other benefits of having a “piece of hardware under potential” in water.
This article discusses the implementation of a contactless sensor from scrap materials.
After my pump failed to turn on once again, I decided to make something more original as a sensor than three pieces of iron in the water.
To make one such sensor you will need:
- Polypropylene pipe for water with an internal diameter of 25mm. A pipe made of metal-plastic is not advisable, because... it can be accidentally deformed quite easily, but if you are careful, you can do it.
- A pair of door opening sensors (we remove two reed switches and a magnet from them)
- Cork bottle stopper (wine or other suitable one). The contents of the bottle are not so important, the main thing is that it does not interfere with further work.
- Wire of the required length, heat-shrink tubing, a pair of nylon zip ties, some string and electrical tape.
In general, there was nothing in short supply, everything was found in the barn.
The first thing you need to do is drill (pick) a hole in the end of the plug to insert a magnet inside.
After the magnet is inserted, you need to check that the plug flies freely inside the pipe. Most likely this is not the case. Therefore, by rubbing the cork against a file or sandpaper, we reduce the diameter of the cork.
It should look something like this:
For better sliding inside the pipe, the cork can be coated with varnish (for example, yacht varnish) using the dipping method.
Because The varnish will add some thickness to the cork; you need to adjust the diameter of the cork with a margin. I have the diameter of the plug along with varnish coating less internal diameter pipes by about 3 mm.
Next, we solder the obtained reed switches to the wires, place them in heat shrink and encase it. The distance between the reed switches corresponds to the difference between the lower water level in the well (when the pump needs to be turned off in an emergency) and the upper level when it is allowed to be turned on again.
To prevent water from penetrating inside, the upper edge of the heat shrink should go over the wire and grip it tightly. Therefore, it is better to use a round wire.
The heat shrink should go from the wire suitable to the sensor over both reed switches and end approximately 5 cm after the lower reed switch.
We make the lower edge of the heat shrink about 5 cm larger than necessary and after shrinking we bend the tail upward, fixing it in this position, for example, with another piece of heat shrink.
We collect. Heat-shrinkable reed switches are attached along the tube using any convenient method(nylon ties or electrical tape). When using screeds, it must be taken into account that cold water they can shrink and either burst themselves or damage something. Therefore, you should not tighten them too much.
There is exactly the same limiter on top so that the plug does not float out of the pipe. The limiters must be placed in such a way that, when resting against them, the plug is opposite the reed switch.
Like that:
The assembled structure needs to be tied to a rope and can be used.
It turns out like this:
Before lowering it into the well, we connect everything to the pump control device and check its functionality by turning the pipe over.
The plug should move freely and when it reaches the reed switches, turn the pump off/on.
We lower the pipe into the well to the end (the lower reed switch is just below the pump level). We turn on the pump and try to drain the well. As soon as the pump begins to gasp for air, raise the tube until the plug drops to the lower reed switch and turns off the pump.
Raise it a little higher so that the pump turns off a little earlier than the water runs out and fix it like that. Accordingly, when there is enough water in the well, the plug will turn on the upper reed switch, which allows the pump to work again.
A similar device is installed on the storage tank. It turns on the pump when the water in the tank runs out and turns off when the water is full.
True, this device has some technological features:
- The device is not located inside the container, but outside and works using the method of communicating vessels.
- In this case, you don’t have to install the upper limiter; it is enough for the pipe to be slightly higher than the top edge of the container
- It will not be possible to use two holes and a tie as a lower limiter (water will leak out). Therefore, the lower limit is the bend of the pipe.
Unfortunately, it was not possible to photograph the actual structure installed on the drive. Therefore, I will show you schematically.
When the need arises to control the fluid level, many do this work manually, but this is extremely ineffective, takes a lot of time and effort, and the consequences of oversight can be very expensive: for example, a flooded apartment or a burnt out pump. This can be easily avoided by using float water level sensors. These are devices that are simple in design and operating principle and are affordable.
At home, sensors of this type allow you to automate processes such as:
- monitoring the liquid level in the supply tank;
- pumping groundwater from the cellar;
- turning off the pump when the level in the well falls below the permissible level, and some others.
Operating principle of a float sensor
An object is placed in the liquid and does not sink in it. This could be a piece of wood or foam, hollow sealed plastic sphere or metal and much more. When the liquid level changes, this object will rise or fall with it. If the float is connected to actuator, then it will act as a water level sensor in the tank.
Equipment classification
Float sensors can independently monitor the liquid level or send a signal to the control circuit. According to this principle they can be divided into two large groups: mechanical and electrical.
Mechanical devices
Mechanical valves include a wide variety of float valves for the water level in the tank. The principle of their operation is that the float is connected to a lever; when the liquid level changes, the float moves up or down this lever, and it, in turn, acts on the valve, which shuts off (opens) the water supply. Such valves can be seen in cisterns toilets. They are very convenient to use where you need to constantly add water from the central water supply system.
Mechanical sensors have a number of advantages:
- simplicity of design;
- compactness;
- safety;
- autonomy - do not require any sources of electricity;
- reliability;
- cheapness;
- ease of installation and configuration.
But these sensors have one significant drawback: they can control only one (upper) level, which depends on the installation location, and regulate it, if possible, then within very small limits. Such a valve can be sold called “float valve for containers”.
Electrical sensors
An electric liquid level sensor (float) differs from a mechanical one in that it itself does not shut off the water. The float, moving when the amount of liquid changes, affects the electrical contacts that are included in the control circuit. Based on these signals automatic system control makes a decision on the need for certain actions. In the simplest case, such a sensor has a float. This float acts on the contact through which the pump is turned on.
Reed switches are most often used as contacts. A reed switch is a sealed glass bulb with contacts inside. Switching of these contacts occurs under the influence magnetic field. Reed switches are miniature in size and can be easily placed inside a thin tube made of non-magnetic material (plastic, aluminum). A float with a magnet moves freely along the tube under the influence of liquid, and when it approaches, the contacts are activated. This entire system is installed vertically in the tank. By changing the position of the reed switch inside the tube, you can adjust the moment the automation operates.
If you need to monitor the upper level in the tank, then the sensor is installed at the top. As soon as the level drops below the set level, the contact closes and the pump turns on. The water will begin to increase, and when the water level reaches the upper limit, the float will return to its original state and the pump will turn off. However, in practice such a scheme cannot be used. The fact is that the sensor is triggered by the slightest change in the level, after which the pump turns on, the level rises, and the pump turns off. If the water flow from the tank is less than the supply, a situation arises when the pump is constantly turned on and off, while it quickly overheats and fails.
Therefore, water level sensors to control the pump they work differently. There are at least two contacts in the container. One is responsible for top level, he turns off the pump. The second determines the position of the lower level, upon reaching which the pump turns on. Thus, the number of starts is significantly reduced, which ensures reliable operation the entire system. If the level difference is small, then it is convenient to use a tube with two reed switches inside and one float that connects them. With a difference more than a meter two separate sensors are used, installed at the required heights.
Despite their more complex design and the need for a control circuit, electric float sensors allow fully automated liquid level control.
If you connect light bulbs through such sensors, then they can be used to visually monitor the amount of liquid in the tank.
Homemade float switch
If you have the time and desire, then you can make a simple float water level sensor with your own hands, and the costs for it will be minimal.
Mechanical system
In order to simplify as much as possible design, we will use a ball valve (faucet) as a locking device. The smallest valves (half-inch or smaller) work well. This type of faucet has a handle that closes it. To convert it into a sensor, you need to extend this handle with a strip of metal. The strip is attached to the handle through holes drilled in it with the appropriate screws. The cross-section of this lever should be minimal, but it should not bend under the influence of the float. Its length is about 50 cm. The float is attached to the end of this lever.
As a float you can use two liter plastic bottle from soda. The bottle is half filled with water.
You can check the operation of the system without installing it in the tank. To do this, install the faucet vertically and place the lever with the float in a horizontal position. If everything is done correctly, then under the influence of the mass of water in the bottles, the lever will begin to move down and take a vertical position, and the valve handle will turn with it. Now submerge the device in water. The bottle should float up and turn the valve handle.
Since valves vary in size and the amount of force required to switch them, the system may need to be adjusted. If the float cannot turn the valve, you can increase lever length or take a larger bottle.
We mount the sensor in the container at the required level in a horizontal position, while in the vertical position of the float the valve should be open, and in the horizontal position it should be closed.
Electric type sensor
For self-made sensor this type, except regular tool, you will need:
The manufacturing sequence is as follows:
When the liquid level changes, the float moves along with it, which acts on an electrical contact to control the water level in the tank. A control circuit with such a sensor may look like the one shown in the figure. Points 1, 2, 3 are the connection points for the wire that comes from our sensor. Point 2 is a common point.
Let's consider the principle of operation homemade device. Let's say at the moment of switching on the tank empty, the float is in the low level position (LL), this contact closes and supplies power to the relay (P).
The relay operates and closes contacts P1 and P2. P1 is a self-locking contact. It is needed so that the relay does not turn off (the pump continues to work) when the water begins to rise and the contact of the low pressure unit opens. Contact P2 connects the pump (H) to the power source.
When the level rises to the upper value, the reed switch will operate and open its contact VU. The relay will be de-energized, it will open its contacts P1 and P2, and the pump will turn off.
As the amount of water in the tank decreases, the float will begin to fall, but until it takes the lower position and closes the NU contact, the pump will not turn on. When this happens, the work cycle will repeat again.
This is how a water level control float switch works.
During operation, it is necessary to periodically clean the pipe and float from dirt. Reed switches can withstand a huge number of switchings, so this sensor will last for many years.
To regulate and control the liquid level or solid(sand or gravel) in production and in everyday life they use a special device. It is called a water level sensor (or other substance of interest). There are several varieties similar devices, significantly different from each other in their operating principles. How the sensor works, the advantages and disadvantages of its varieties, what subtleties you should pay attention to when choosing a device, and how to make a simplified model with a relay with your own hands, read in this article.
The water level sensor is used for the following purposes:
Possible methods for determining tank load
There are several methods for measuring liquid level:
- Contactless- often devices of this type are used to control the level of viscous, toxic, liquid or solid, granular substances. These are capacitive (discrete) devices, ultrasonic models;
- Contact- the device is located directly in the tank, on its wall, at a certain level. When the water reaches this indicator, the sensor is triggered. These are float, hydrostatic models.
Based on the principle of operation, the following types of sensors are distinguished:
- Float type;
- Hydrostatic;
- Capacitive;
- Radar;
- Ultrasonic.
Briefly about each type of device
Float models are discrete and magnetostrictive. The first option is cheap, reliable, and the second is expensive, complex design, but guarantees an accurate level reading. However general disadvantage float devices - this requires immersion in liquid.
Float sensor for determining the liquid level in the tank
- Hydrostatic devices - in them all attention is paid to hydrostatic pressure column of liquid in the tank. The sensitive element of the device senses pressure above itself and displays it according to a diagram to determine the height of the water column.
The main advantages of such units are compactness, continuous operation and availability price category. But they cannot be used in aggressive conditions, because they cannot do without contact with liquid.
Hydrostatic liquid level sensor
- Capacitive devices - plates are provided to control the water level in the tank. By changing the capacity indicators, you can judge the amount of liquid. Lack of moving structures and elements, simple circuit devices guarantee durability and reliable operation of the device. But one cannot fail to note the disadvantages - this is the necessity of immersion in liquid, and demanding temperature conditions.
- Radar devices - determine the degree of increase in water by comparing the frequency shift, the delay between the radiation and the achievement of the reflected signal. Thus, the sensor acts as both an emitter and a reflection collector.
Such models are considered the best, accurate, reliable devices. They have a number of advantages:
The only disadvantage of the model is its high cost.
Radar tank liquid level sensor
- Ultrasonic sensors - the principle of operation and the design of the device are similar to radar devices, only ultrasound is used. The generator creates ultrasonic radiation, which, upon reaching the surface of the liquid, is reflected and reaches the sensor receiver after some time. After some mathematical calculations, knowing the time delay and speed of the ultrasound, the distance to the water surface is determined.
The advantages of a radar sensor are also inherent in the ultrasonic version. The only thing is that the indicators are less accurate and the operation scheme is simpler.
Subtleties of choosing such devices
When purchasing a unit, pay attention to the functionality of the device and some of its indicators. Extremely important questions when purchasing a device are:
Options for sensors for determining the level of water or solids
DIY liquid level sensor
You can make a basic sensor to determine and control the water level in a well or tank with your own hands. To perform the simplified version you need:
A self-made device can be used to regulate water in a tank, well or pump.
In industry and everyday life, there is a constant need to monitor the levels of liquids in containers. Measuring devices are classified as contact and non-contact. For both options, the water level sensor is located at a certain height of the tank, and it is triggered, signaling or giving a command to change the mode of its supply.
Contact devices operate on the basis of floats that switch circuits when the liquid reaches specified levels.
Non-contact methods are divided into magnetic, capacitive, ultrasonic, optical and others. The devices have no moving parts. They are immersed in controlled liquid or granular media or fixed to the walls of tanks.
Float sensors
Reliable and cheap devices for monitoring liquid levels using floats are the most common. Structurally, they may differ. Let's look at their types.
Vertical arrangement
A float water level sensor with a vertical rod is often used. There is a round magnet placed inside it. The rod is a hollow plastic tube with reed switches located inside.
A float with an attached magnet is always located on the surface of the liquid. Approaching the reed switch, the magnetic field triggers its contacts, which is a signal that the container is filled to a certain volume. By connecting contact pairs in series through resistors, you can constantly monitor the water level based on the total resistance of the circuit. The standard signal varies from 4 to 20 mA. The water level sensor is most often placed at the top of the tank in an area up to 3 m long.
Electrical circuits with reed switches may differ even if the mechanical part is similar in appearance. Sensors are located at one, two or more levels, giving a signal about how full the tank is. They can also be linear, transmitting a signal continuously.
Horizontal arrangement
If it is not possible to install the sensor from above, it is attached horizontally to the wall of the tank. A magnet with a float is installed on a lever with a hinge, and a reed switch is placed in the housing. When the liquid rises to the upper position, the magnet approaches the contacts and the sensor is triggered, signaling that the limit position has been reached.
In case of increased contamination or freezing of the liquid, a more reliable float water level sensor is used on flexible cable. It consists of a small sealed container located at depth with a metal ball with a reed contact or toggle switch inside. When the water level coincides with the position of the sensor, the container turns over and the contact is activated.
One of the most accurate and reliable float sensors are magnetostrictive. They contain a float with a magnet that slides along metal rod. The principle of operation is to change the duration of passage of an ultrasonic pulse through the rod. The absence of electrical contacts significantly increases the clarity of operation when the interface reaches a given position.
Capacitive sensors
The non-contact device responds to the difference between the dielectric constant of different materials. The water level sensor in the tank is installed outside the side wall of the tank. In this place there should be an insert made of glass or fluoroplastic so that the interface between the media can be distinguished through it. The distance at which the sensitive element detects changes in the controlled environment is 25 mm.
Hermetically sealed capacitive sensor makes it possible to place it in a controlled environment, such as a pipeline or a tank lid. However, it may be under pressure. In this way, the presence of liquid in the closed reactor is maintained during the technological process.
Electrode sensors
A water level sensor with electrodes placed in a liquid responds to changes in electrical conductivity between them. To do this, they are secured with clamps and placed at the extreme upper and lower levels. Another conductor is installed in pair with the longer one, but usually a metal tank body is used instead.
The water level sensor circuit is connected to the pump motor control system. When the tank is full, all electrodes are immersed in liquid and a control current flows between them, which is a signal to turn off the water pump motor. Water also does not flow unless it touches the exposed upper conductor. The signal to turn on the pump is a decrease in the level below the long electrode.
The problem with all sensors is the oxidation of contacts in water. To reduce its influence, use stainless steel or graphite rods.
DIY water level sensor
The simplicity of the device makes it possible to make it yourself. This requires a float, a lever and a valve. The entire structure is located at the top of the tank. A float with a lever is connected to a rod that moves the piston.
When the water reaches the upper limit level, the float moves a lever that acts on the piston and closes the flow through the lower pipe.
As the water flows, the float lowers, after which the piston again opens the hole through which the tank can be refilled.
At making the right choice and manufacturing a water level sensor, assembled with your own hands, works reliably in the household.
Conclusion
The water level sensor is indispensable in the private sector. With it, no time is wasted when monitoring the filling of the tank in the garden, the level in the well, borehole or septic tank. A simple device will start or turn off the water pump in time without the help of the owner. Just don’t forget about its prevention.