In industry and everyday life, there is always a need to identify different levels in containers. For these tasks level sensors of various designs are used. Depending on the filling medium of the tank, one or another sensor is used, sometimes, for the sake of simplicity and saving time and money, use sensors combined, that is, made with their own hands. These are uncomplicated designs that use sensors of completely different types. Basically, such sensors are used where there is no simple access to the measurement environment or the measurement site is very aggressive to human health.
- 1 Types of level sensors
- 2 DIY water level sensor
3 Selection of level sensors
- 3.1 Examples of sensors, their parameters and scope
Types of level sensors
- float sensors the simplest in construction, with the widest distribution, combined with an electrical relay, represent a different type of float attached through a wire, thread, or rigidly to the contact lobe, which closes when changing position the float.
- capacitive sensors Levels are plates located on both sides of the tank with the material. The principle of operation lies in the change in capacity depending on the distribution of the amount or composition of the material in the tank. Basically, such sensors are used to determine the level of bulk materials.
- radar sensors the levels operate on the principle of "emitter - receiver of frequency waves", the surface of the material reflects the radiation of the sensor, the receiver picks up this radiation and compares the radiated signal with the reflected one and, by frequency shift, determines the level in tank. The determination of the level occurs by comparing the frequency time shift.
- ultrasonic sensors The levels operate on the principle of radar, only radiation occurs by ultrasound. Similarly to the radar ones, the difference of the signal emitted from the reflected one is compared and the distance to the surface of the material (liquid) is calculated.
- hydrostatic sensors levels work by measuring an indirect value - the pressure of a liquid column. The pressure is proportional to the level of the liquid in the tank.
Most modern level sensors have in their design an electronic relay with a converter. The electronic circuit is designed to convert the measured value into a standard signal. The signal can be analog and discrete. Analog can be current 0..20mA and a signal called current loop 4..20mA or voltage 0... 5V, 0..10V.
Level sensors are used to protect the pump motor from the dry run, they regulate the motors of the pumps of the wells that fill any tanks with water and not only in the system of cold and hot water supply.
DIY water level sensor
Let's see, on the example of pumping water from a pit, how to make control in an automatic cycle to maintain the water level not above the set.
We have a pit with a very non-clean type of liquid, consisting of water and coolant impurities for the cutters of the metal-cutting machine.
All types of sensors were considered, however, for the price and simplicity of performance, a combined design came up, consisting three meter long wire (depth of the pit), attached to the float (large plastic container with air), on the surface of the wire is attached to a spring with a petal.
The signal is taken as a conventional 24V discrete signal from a conventional inductive sensor. He works on the petal. When the water level in the pit grows, the float rises, weakening the spring. At the end of the spring is attached petal, it rises due to the spring extension force. On the petal, in turn, fulfills an inductive sensor, applying a pump motor relay to the coil, forcing it to pump out the water from the pit. In order to avoid frequent switching off of the engine, in the sensor-coil circuit, there is a switch-off delay relay with a setting for 10 minutes.
Thus, the next time the sensor is triggered, the relay will work again and the cycle will repeat.
Of course, to protect the engine from running dry is advisable. put the leak sensor in the pipethrough which the emulsion is pumped. But in our case, simplicity of the design was important. Instead of an inductive sensor, you can use two plates in contact with each other, which will be even more economical.
If water or other liquid has a homogeneous composition, then you can use the end of a metric single-electrode level sensor.
For example DU-1N manufacturer "Relsib", designed to measure the level in various types of liquids. The sensor can operate in a wide temperature range. The body is not subject to corrosion, consists of high-quality stainless steel. Ceramics and fluoroplastic are used as insulation, it provides excellent insulation protection. Resistant to many mechanical loads. Measurements are independent of fluid density. And does not require additional care during work.
Selection of level sensors
When choosing level gauges following objectives:
- Type of measured fluid. Its characteristics, we find the passport density of the measured material. To what substances is a substance, is it dangerous to humans or not.
- The material of the container in which the measurement will be made. It depends on him principle of operation of the level gauge.
- Do you need an analog standard signal from the sensor or is it preferable to work as a relay? Some models have built-in circuits for processing an analog signal and convert it to a bit logic signal.
- It is necessary to know the limits of the measured value, in very long vessels, with a rapidly changing volume, the float constructive will not work stably. In this case, the preferred radar gauge.
- Modern level meters Equip LCD screen with the display of parameters in real time and the ability to save the maximum and minimum values. In the sensor parameters, several trigger levels are set, each level has its own discrete output. Sets the density of the medium in the settings.
- Take into account sanitary standards for the application. For example, in the food industry for the measurement of water, beverages are high requirements. Required stainless steel.
- The need for certificates. For example, some Rostechnadzor meters must check and approve for use at hazardous facilities. Some require a permit from the sanitary-epidemiological service mainly for water and food.
- Sensor availability in an explosive environment. Such are used in petrochemical plants. The manufacturer guarantees that, due to such systems, the combustible environment in which it is located will not ignite during the entire service life.
- It is necessary to take into account the possibility of mechanical effects on the system, vibration, electromagnetic waves, aggressive media.
- The presence of temperature drops of the system, the maximum possible state.
- How important is the accuracy of level measurements, one of the most important parameters.
Examples of sensors, their parameters and scope
- Sensor capacitor DE-1. Scope: in explosive environments of the metallurgical, chemical industry, etc. Tracking the magnitude of the level, as well as the temperature values of liquid media and bulk materials. Power is provided 8.. 30V DC. used in emergency protection systems.
- ESU-1K level alarm. The body is made of PTFE and steel. Placed in explosive environments. The power source is located outside the hazardous environment. power supply 127... 220V. Measurement of liquid materials such as water, alcohol, oil. The sensing element is placed in the measured material, the principle of the capacitive. The material of the power unit is aluminum alloy.
- RU-305 level switch. Monitoring the status of the level of liquid substances. Intrinsically safe. Temperature of use is -50. + 50C. Not used in chemically aggressive environments. Strictly work in one position, the slope is unacceptable. The measurement works by moving the float with a magnet. Fulfills by triggering reed switches. Accuracy of measurements up to 5mm. 220V supply, current 1A.
- Level indicator SU-100. Measurement of liquid and solid levels. 24V power supply. The composition includes an electromagnetic relay, a sensitive element is placed directly in the test liquid. Measure the ultimate position of solids: sand, gravel, grain.
Rosemount 5600 Radar Transmitter. Non-contact measurement of any kind of substances. The correct installation is important, the measurement accuracy depends on it. Power supply 24-240V. The device does not tolerate electromagnetic radiation. Explosion proof housing. Has a display unit. For the gage developed its own application that simplifies the setting. It is used to measure the temperature in the tank. For proper full use of the capabilities of the gauge, a qualified instrument setup is required. It is necessary to use the parameters when setting up:
- the distance between the reference point and the level;
- internal diameter of the pipe;
- connection length of the gage.
There is a huge number of types of sensors on sale. The choice must be stopped on the most economically advantageous variant for a specific project.