Source: Thermal Control Circle
Part I
A level switch, as the name suggests, is a switch used to control the liquid level. In terms of form, it is mainly divided into contact type and non-contact type. Non-contact type such as capacitive type liquid level switch, contact type such as: float type liquid level switch, electrode type liquid level switch, electronic liquid level switch. Capacitive level switches can also be implemented using the contact method.
Part II
1. Float level switch
The float level switch structure is mainly designed and produced based on the principle of buoyancy and static magnetic field. The position of the float with a magnet (referred to as the float) in the measured medium is affected by buoyancy: a change in the liquid level leads to a change in the position of the magnetic float. The magnets in the float act with sensors (reed switches) to generate a switching signal.
2. Tuning fork level switch
The working principle of the tuning fork level switch is to make the tuning fork vibrate at a certain resonant frequency by a pair of piezoelectric crystals mounted on the base. When the tuning fork of the tuning fork level switch is in contact with the measured medium, the frequency and amplitude of the tuning fork will change, and these changes of the tuning fork level switch are detected, processed and converted into a switching signal by the intelligent circuit to achieve the purpose of liquid level alarm or control. In order to allow the tuning fork to reach the tank, the tuning fork switch is usually mounted to the side or top of the tank using a flange or threaded process fitting.
3. Capacitive level switch
The measuring principle of the capacitive level switch is that the level of the solid material changes and the size of the area covered by the probe changes, resulting in a change in the capacitance value. The probe and the tank wall (made of conductive material) form a capacitance. When the probe is in air, it measures a small initial capacitance value. When there is material injected into the tank, the capacitance value will increase accordingly with the increase of the area covered by the probe by the material, and the switching state will change.
4. External liquid level switch
The external liquid level switch is a non-contact liquid level switch realized by "frequency conversion ultrasonic technology", which is widely used in liquid detection of various liquids. The measuring probe is mounted on the outer wall of the vessel and is a completely non-contact detection instrument for detecting the liquid level from the outside of the tank. The instrument measuring probe emits ultrasonic waves, and detects its residual vibration signal in the container wall, when the liquid overflows the probe, the amplitude of this residual vibration signal will become smaller, and this change is detected by the instrument and outputs a switch signal to achieve the purpose of liquid level alarm.
5. RF admittance level switch
RF admittance level control technology is a kind of capacitive level control technology developed, anti-hanging, more reliable, more accurate, more applicable to the level control technology, "RF admittance" in the meaning of "admittance" is the reciprocal of the impedance in electricity, it is composed of resistive components, capacitive components, inductive components, and "RF" is high frequency, so radio frequency admittance technology can be understood as the use of high frequency measurement admittance. The high-frequency sinusoidal oscillator outputs a stable measurement signal source, using the bridge principle to precisely measure the admittance on the sensor mounted in the vessel to be measured, and in direct action mode, the output of the meter increases as the level rises. The difference between RF admittance technology and traditional capacitive technology is the diversity of measurement parameters, the drive three-terminal shielding technology, and the addition of two important circuits. Due to the introduction of measurement parameters other than capacitance, especially the resistance parameter, the signal-to-noise ratio of the measurement signal of the instrument increases, which greatly improves the resolution, accuracy and reliability of the instrument, and the diversity of measurement parameters also effectively expands the reliable application field of the instrument.
6. Resistive rotary level switch
The material resists the rotation of the rotating blades, so that the overload detector of the switch acts, and the relay sends out on-off and off-off switching signals, so that the external control circuit sends a signal alarm and controls the feeder at the same time. For example, when the switch is controlled as a high position: when the material touches the blades, the switch sends an alarm signal and stops the feeder at the same time. When the switch is controlled as a low position, the switch sends an alarm signal when the material leaves the blade and the feeder is started at the same time.
7. Electromagnetic liquid level switch
Electromagnetic proximity switch, also known as inductive proximity switch, when energized, the oscillation loop (coil, etc.) emits electromagnetic waves to the front with the assistance of the core CORE, and then returns to the proximity switch, when there is metal at the front end of the proximity switch, because the metal absorbs the electromagnetic, the proximity switch is converted into a switch signal through electromagnetic attenuation, and the output is controlled after the signal processing is completed.
8. Electronic level switch
The working voltage of the electronic liquid level switch is DC5V-24V, the water level is detected by the built-in electronic probe, and then the detected signal is processed by the chip, when the liquid level of the measured liquid reaches the action point, the output DC5V-24V can be directly used with the PLC or with the control board, so as to realize the control of the liquid level.
9. Photoelectric liquid level switch
The photoelectric level switch uses infrared detection, using the principle of refraction and reflection of light, and the light will be reflected or refracted at the interface of two different media. When the measured liquid is in a high position, the measured liquid and the photoelectric switch form a kind of interface, and when the measured liquid is in a low position, the air and the photoelectric switch form another kind of interface, and these two interfaces make the reflected light intensity received by the light receiving crystal inside the photoelectric switch different, that is, corresponding to two different switching states.
10. Ultrasonic level switch
The fork probe of the piezoelectric crystal inside the ultrasonic level switch is separated by air, and a crystal vibration frequency of 1.5MHz transmits the sound signal to the middle of the air gap, when the probe is immersed in the liquid, the crystal and the sound wave are coupled, and the ultrasonic level switch changes the state.