1 Overview
The material level meter refers to a sensor that continuously detects the change in the height of the solid material in the container in real time. This kind of sensor usually outputs 4-20ma or 1-5V standard signal and is connected with display instrument or computer system, and can also be connected with computer system through RS-485 or field bus.
2 categories
1. Mechanical;
2. Capacitance level gauge;
3. Radio frequency admittance level meter;
4. Tuning fork level gauge;
5. Vibrating bar level gauge;
6. Heavy hammer level gauge.
3 Applications and Features
1. Contact measuring instrument
1. The material level detection process of the heavy hammer type material level meter is controlled by the signal sent by the controller. When the sensor receives the detection command, the motor rotates forward, and after the worm gear and worm are decelerated, the gear shaft and the spool are driven to rotate, so that the wire rope Lower it, drive the heavy hammer to descend from the top of the warehouse, when the heavy hammer falls to the material surface and the measured surface is lifted up and loses weight, the wire rope is slack, and the sensitive lever action makes the micro switch contact, the controller gets the signal and sends the motor reverse command, heavy The hammer rises and returns until it hits the top switch and the motor stops.
Turn, the heavy hammer returns to the original position of the warehouse top to complete a detection process.
Advantages: The measurement is not affected by the density of the medium and the size of the particles.
Disadvantages: The dust inside the machine is easy to fall, which affects the measurement effect; the mechanical wear is serious, requiring frequent maintenance, and the cost is large; the heavy hammer is prone to be buried by the material, and the hammer head is dropped and the belt is broken.
2. Capacitive material level meter The principle is to form a capacitor between the electrode inserted into the silo and the silo wall. When the material level in the silo changes, the capacitance changes, and the corresponding control signal is obtained through the conversion circuit.
Advantages: no mechanical wear, easy installation and maintenance; depending on the range size and control methods, the electrodes are designed as rod (rod) type or steel cable (heavy steel cable) type, which can be used in various silos; the price is low.
Disadvantages: If there is material on the electrode (probe) or the warehouse wall, it will often cause the controller to malfunction, thus affecting the measurement effect. The probe and the level switch should be checked regularly and verified.
3. The basic principle of the resistance-rotating material level gauge is that after the synchronous micro-motor decelerates, it drives the detection blade to rotate at a speed of 2.5~5r/min. When the material level of the material to be measured rises and the rotation of the blade is blocked, the detection mechanism will rotate around the main shaft. displacement. The displacement first makes a micro switch act to send out a material level signal. Then another micro switch acts to cut off the power of the motor to stop it.
Advantages: simple switch structure, easy maintenance; low price.
Disadvantage: Not suitable for working in high temperature.
2. Non-contact measuring instruments
1. The working principle of the gamma ray level gauge is to set an isotope source on one side of the silo and a detector on the other side. The isotope source emits gamma rays to the detector. If the material level in the silo is lower than it, the detector detects the material empty. Signal; if the material level is higher than it, the material blocks and absorbs gamma rays, and the material full signal is obtained.
Advantages: The daily operation and maintenance workload is small, and the operation is simple; according to the shape of the silo and the process requirements, the gamma ray level gauge can be installed in different positions.
Disadvantages: The radioactive source pollutes the environment; the attenuation of the radioactive source makes the material level control unreliable.
2. The ultrasonic level gauge mainly uses the principle of echo ranging, and calculates the distance from the transducer to the surface of the material by measuring the time when the transducer transmits and receives the sound wave. The level meter is suitable for bulk and granular solid level measurement.
Advantages: easy installation, reliable operation, low maintenance; competitive price.
Disadvantages: Ultrasound must be propagated by means of a medium. The material level measurement of cement plants usually uses air as the propagation medium, and changes in air temperature, humidity, etc. will affect the propagation speed of ultrasonic waves. Therefore, in some occasions with temperature, pressure, steam, etc., this material The level meter cannot work normally; the dust in the air of the silo also attenuates the ultrasonic signal, which affects the measurement effect; because the surface of the powder silo material level is very loose when the material is unloaded, the ultrasonic signal is strongly attenuated, so the measurement effect of the powder silo material level poor.
3. The radar level gauge uses the echo ranging principle. Its horn or rod antenna emits microwaves to the surface of the material to be measured. When the microwaves propagate to the surface of materials with different relative permittivity, they are reflected and received by the antenna. The time difference between the transmitted wave and the received wave is proportional to the distance between the material surface and the antenna, and the distance can be known by measuring the propagation time.
Advantages: Since microwave is an electromagnetic wave, it propagates at the speed of light and is not affected by the characteristics of the medium, so in some occasions with temperature, pressure, steam, etc., the ultrasonic level gauge cannot work normally, but the radar level gauge can be used; this level gauge is mostly It is a two-wire integrated product, saving a lot of cables; software debugging is convenient.
Disadvantages: The two-wire radar level gauge requires a high quality of 24VDC power supply, and the AC harmonics generally cannot exceed ±30VAC; the internal power module of the radar level gauge is easily damaged by other large current interference, so the level gauge should be removed from the power outage; measurement For solid materials, it is easy to generate interference echoes and reduce the measurement effect, so it is not suitable for the measurement of solid materials.
4. Guided wave radar level gauge This is a variant of radar level gauge, which usually works by pulse wave. Unlike radar level gauges, microwave pulses do not propagate through space, but propagate through a wave guide extending from the top of the warehouse (or two) to the bottom of the warehouse. The wave guide can be a metal hard rod or a flexible metal cable. The microwave pulse propagates downward along the outside of the rod or cable, and is reflected on the surface of the tested material. The echo is received by the antenna, which can be calculated from the time difference between the launch and the echo pulse. out the transmission distance.
Advantages: Compared with ultrasonic and radar level gauges, the work is stable and reliable; like radar level gauges, the intensity of the reflected signal depends on the dielectric or conductivity of the material to be measured, but the guided wave type can measure lower dielectrics rate material.
Disadvantages: Double rods (cables) and coaxial tubular waveguides are easily affected by external structures due to the concentration of electric field energy; materials are easily accumulated between the rods (or inside the tubes), resulting in false echoes and affecting the measurement effect; buried in The cable in the material will generate a large pull-down force due to the gravity of the material, causing the cable to be pulled off and other faults.
Post time: 06-04-22