The level sensor is a branch of the level sensor. The level measurement is a branch in the level measurement. The level refers to the interface position between liquid, powdery solid and gas in the storage container or industrial production equipment. According to the detailed use, it is divided into liquid level sensor, level sensor and boundary sensor. All three sensors have their own unique application environment, and the level sensor is widely used in thermal power plants.
The level sensors currently used in thermal power plants mainly include heavy hammer, nuclear radiation, ultrasonic and capacitive.
1. Heavy hammer level sensor
The heavy hammer type level sensor is composed of a servo motor, a wire rope suspended with a heavy hammer, a material level sending device and a display instrument with a microcomputer. After starting, the microcomputer sends a falling hammer signal, and the servo motor rotates to lower the weight. When the weight hits the material surface, the transmitter sends a signal to the microcomputer, causing the weight to stop falling and sending a hammer signal. The servo motor reverses the weight. The hammer rises and sends a level signal value to the display meter. After the weight is raised to the top of the warehouse, the motor stops, and the above action is repeated after a delay. There are also devices on the meter that have upper and lower limit alarms and so on. The heavy hammer level sensor can solve the measurement problem within a certain range. The typical measuring range is up to 60m, and this measurement has high measurement accuracy regardless of steam and dust.
Heavy hammer level sensor
2. Nuclear radiation level sensor
The g-rays emitted by the radioactive sources Co-60 (half-life 5.26 years) and Cs-137 (half-life 32.2 years) are able to penetrate the walls of the container and the contents of the container. A g-ray receiver is installed on the lower side of the silo. As the height of the material level changes, the intensity of the g-ray passes through the material layer. The receiver detects the intensity of the injected g-ray and displays the material level through the display instrument. height.
This is a non-contact measurement method that does not require perforation on the container to damage the container, so it is suitable for measurement in hazardous materials and in harsh environments such as high temperature and high pressure. G-rays have a detrimental effect on the human body, but for limited doses, there is no danger under proper protection.
3. Ultrasonic level sensor
An ultrasonic generator and receiver are placed on the top of the silo opposite the loading surface. The ultrasonic waves emitted by the generator are reflected by the air layer to the surface, and a part of the reflection is received by the receiver. The time elapsed from the time when the ultrasonic wave is transmitted to the reception multiplied by the speed of sound can calculate the height of the material level. Since the temperature of the air affects the speed of sound wave propagation, it is also necessary to measure the air temperature to correct the speed of sound. Ultrasonic level sensors are suitable for measuring bulk material levels with large particle sizes.
Ultrasonic level sensor
4. Capacitive level sensor
The capacitive level sensor is a capacitor between the probe of the measuring container and the inner wall of the container, between the two probes or between the probe and the concentric measuring tube, and works by the principle that the inter-electrode capacitance is proportional to the material level when the dielectric constant of the material is constant. .
Capacitive level sensor is characterized by no moving parts, regardless of material density, but requires a large difference between the dielectric constant of the material and the dielectric constant of the air. The varying dielectric constant is compensated for continuous measurement and is required. High frequency circuit.