In such as food and beverage, cosmetic, pharmaceutical, semiconductor and other industries automation production process, in order to ensure the consistency of product quality, production process directly by human monitoring and intervention time has passed, level measurement and monitoring plays an increasingly important role, can say, liquid level measuring and monitoring directly affects the quality of the product, even relates to whether the production process can proceed smoothly.
At present, the simple mechanical, rough type of measuring and monitoring, are increasingly being high reliability, high stability and simple installation and debugging, compact size, application of various measurement methods.
In addition, because of the complexity of the liquid level testing environment and variability, and also presents a different challenge for the application of the sensor.
For example: high viscosity liquid level detection, waste water containing impurities level monitoring, liquid level height measurement and high corrosive liquid with bubble level alarm, and so on.
According to various demand on the market, researchers have developed a variety of measurement technology and level measurement methods.
The following will introduce several common ways of level measurement.
1, the ball float measurements: the way to the most simple, the oldest way of testing, the price is relatively cheap.
Mainly by floating ball of lifting up and down to detect the liquid surface, its for mechanical testing, test precision is easily affected by buoyancy, repeat accuracy is poor, the different liquid needs to be calibrated.
Do not apply to the viscosity or impurities in the liquid, easy to block, the floating ball at the same time, do not meet the requirements of the application of food hygiene industry.
2, a tuning fork vibration measurement: tuning fork type measurement is only switch output, cannot be used for continuous monitoring of liquid height, more representative is a tuning fork level switch
Its principle is: when the liquid or powder filling two vibrating fork, resonance frequency change, rely on the detection and signal switch frequency change.
It can be used to the height of the high viscosity liquid or solid powder monitor, mainly for spill-proof alarm, low liquid level alarm, etc. , does not provide analog output, in addition, in most cases need opening installed on the container side.
3, ultrasonic measurement: because of its principle by detecting ultrasonic sending and the reflection of the time difference to calculate liquid level height, easy to be affected by the energy loss in the ultrasonic propagation.
Easy to install, high flexibility, etc, it has usually can be installed in high non-contact measurement.
But when used for steam, the environment such as powder layer, detection distance will be shortened obviously, therefore, is not recommended in such as bubble of absorbing environment use.
The time domain reflection)
/ principle of guided wave radar/microwave measurements: its name in the industry with a variety of different term, it has the advantages of laser measurement, such as: easy to install, calibration, good flexibility, etc. , but more is better than that of the laser detection, such as do not need to repeat the calibration and multifunctional output, suitable for all kinds of bubble in liquid level detection, is not affected by the liquid color, can be applied to high viscous liquid, even relatively small influence of the external environment, but its measurement generally less than 6 m.
5, laser measurement: laser sensors based on optical detection principle, through the surface reflects light to the receiver, the light small and concentrated, easy to install, calibration, good flexibility, can be applied to the powder or liquid level of the continuous or limit alarm, etc.
But it doesn't fit in a transparent liquid (
Transparent liquid refraction of light, easily lead to the light cannot be reflected to the receiver)
, including bubble or steam environment (
Cannot penetrate bubble or vulnerable to steam interference)
, volatile liquid (
Easy to cause false action)
, such as vibration environment use.
6, photoelectric refractive measurement: the test way in light source, through the sensor inside the light through a transparent resin total reflection to sensor receiver, but when you meet the surface part of the light will be reflected to the liquid, and sensors to detect total reflection back less of the quantity of light to monitor liquid level.
The test way cheap, simple installation, debugging, but can only be used in transparent liquid, only switch output signal at the same time.
7, capacitance measurements: mainly through capacitance measurement due to liquid level or bulk material height change of capacitance change to measure the material level height.
Its type is more, can output analog capacitive liquid level meter, capacitive proximity switch, capacitive proximity switch can be installed on the container side for non-contact detection.
When the choice must pay attention to, capacitance sensor easily affected by different container material and solution properties, such as plastic containers and hanging is easy to affect the analog output of capacitance sensor.
8, static pressure measurement, the measurement method used to install at the bottom of the pressure sensor, by detecting the bottom of the liquid pressure, transformation to calculate the height of liquid level, with the top in the liquid pressure at the bottom of the reference for atmospheric pressure or pressure is known.
The measurement requirements, flush type pressure sensor with high precision, calibration requires constant conversion process at the same time, the advantage for detecting liquid level height without limit, but the higher the altitude, the sensor accuracy is higher, the long time use or replacing liquid need to repeat the calibration.
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