This article describes the characteristics of the liquid level sensors.
The working principle of liquid level sensor
Use the principle of static pressure measurement: When the liquid level transmitter is put into a certain depth in the measured liquid, the pressure formula of the sensor on the liquid surface is: Ρ=ρ.g.H+Po where:
P: pressure on the face of the transmitter
Ρ: the density of the measured liquid
G: local acceleration of gravity
Po: Atmospheric pressure on the liquid surface
H: the depth of the transmitter into the liquid
At the same time, the pressure of the liquid is introduced into the positive pressure chamber of the sensor through the gas-conducting stainless steel, and then the atmospheric pressure Po on the liquid surface is connected to the negative pressure chamber of the sensor to offset the Po on the back of the sensor.
Make the pressure measured by the sensor: ρ.g.H. Obviously, by measuring the pressure P, the liquid level depth can be obtained.
liquid level sensor characteristics
1. High precision, good stability, and strong anti-disturbance function.
2, the liquid level sensor adopts a three-level lightning protection plan, which can resist the consequences of lightning strikes.
3, the temporary stability of full scale and zero position can reach 0.1%FS/year.
4. Solid structure, no moving parts, high reliability, long application life, reverse maintenance and current limiting maintenance circuits, the positive and negative poles will not damage the transmitter when the device is connected
5. Convenient installation, simple structure, economical and durable.
6. The liquid level sensor is suitable for multi-point pressure parameter network collection, and the bus supports 255 devices, which can simultaneously have: 4-20mA output and RS485 communication functions, and the output current discrimination rate is better than 3uA.
Common liquid level measurement methods
Due to the wide application range of liquid level sensors, different detection methods have their advantages and disadvantages. Let's take a look at the common liquid level measurement methods.
1. Floating ball detection method is the simplest and oldest detection method, and the price is relatively cheap. The main method is to detect the change of the liquid level through the up and down movement of the float. It is a mechanical detection. The detection accuracy is easily affected by buoyancy, and the repeatability is poor. Different liquids need to be recalibrated. It is not suitable for viscous or impurity liquids, which can easily cause blockage of floating balls. At the same time, it does not meet the application requirements of the food hygiene industry.
2. Capacitance measurement Capacitance measurement mainly measures the height of the
material level by detecting the change in capacitance value caused by the change of the liquid level or the height of the bulk material. There are many types, including capacitive liquid level gauges that can output analog quantities, liquid level capacitive proximity switches, and capacitive proximity switches can be installed on the side of the container for non-contact detection. When choosing, it must be noted that capacitive sensors are easily affected by different container materials and solution properties.
3. static pressure measurement
The measurement method uses a pressure sensor installed at the bottom, which detects the liquid pressure at the bottom and converts and calculates the height of the liquid level. The reference value of the liquid pressure at the bottom is the atmospheric pressure connected to the top or the known pressure. This detection method requires the use of a high-precision, flush-type pressure sensor, and the conversion process requires constant calibration.
4. photoelectric refraction measurement
This detection method emits a light source inside the sensor, which is totally reflected by the transparent resin to the sensor receiver, but when it encounters the liquid surface, part of the light will be refracted to the liquid, so that the sensor detects the decrease in the amount of light returned by the total reflection to monitor the liquid level. This detection method is cheap, simple to install and debug, but it can only be applied to transparent liquids, and only outputs switching signals.
5. Tuning fork vibration measurement
The tuning fork measurement is only a switch output, and cannot be used to continuously monitor the liquid height. The principle is: when the two vibrating forks are filled with liquid or bulk material, when the resonance frequency changes, the switching signal is sent out depending on the change of the detection frequency. It can be used for height monitoring of high-viscosity liquids or solid bulk materials, mainly for anti-overflow alarms, low-liquid level alarms, etc. It does not provide analog output. In addition, in most cases it needs to be installed on the side of the container.
6. Since the principle of ultrasonic measurement is to calculate the liquid level by detecting the time difference between ultrasonic transmission and reflection, it is easily affected by the energy loss of ultrasonic propagation. It also has the characteristics of easy installation and high flexibility, and can usually be installed in high places for non-contact measurement. However, when used in environments containing steam, powder, etc., the detection distance will be significantly shortened. It is not recommended to use in a wave-absorbing environment such as foam.
7. Microwave principle measurement
Its name has many different names in the industry. It has the advantages of laser measurement, such as: easy installation, calibration, flexibility, etc., and it is better than laser detection, such as no need to repeat calibration and multi-function output, etc. , It is suitable for all kinds of liquid level detection with foam, and is not affected by the color of the liquid, and can even be applied to high-viscosity liquids. The interference from the external environment is relatively small, but its measuring height is generally less than 6 meters.
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