There is no good or bad for liquid level sensor in principle and structure, but the testing and using of environmental is complexity and variability, which would be different requirements for liquid level sensor. So, how do we choose the appropriate level sensor, has become very important, will directly affect the product usage, capital cost, stability etc. For example: high viscosity liquid level detection, waste water containing impurity liquid level, liquid with foam height measuring, high corrosive liquid height and level alarm, etc., have had to choose suitable liquid level sensor. All kinds of liquid level sensor have own advantages and disadvantages, decided to whether we choose the suitable or not.
Common liquid level measurement methods
In order to select the best liquid level sensor, we need to know state and properties of measured liquid, and the advantages and limitations of different means of detection, comprehensive analysis, to select the most suitable sensor. The most common detection technology has a float liquid level sensor, laser liquid level sensor, TDR (time domain)/guided wave radar/microwave original liquid level sensor, acoustic liquid level sensor, tuning fork vibratory level sensor, photoelectric refraction type liquid level sensor, static pressure type liquid level sensor, capacitive liquid level sensor, etc. Below according to the principle of all kinds of sensors are briefly introduced.
1. Float detection: this method is the simplest and oldest detection method, and the price is relatively cheap. It is mainly used to detect the change of liquid surface through the fluctuation of float. It is a mechanical test, and the detection accuracy can be affected by buoyancy, the repetition accuracy is poor, and different liquids need to be recalibrated. It is not suitable for viscous or impurities liquid, which can cause float clogging, and it is not in line with the application requirements of food hygiene industry.
2, Laser measurement: laser sensors based on optical detection principle, through the surface reflect 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 is not suitable for application in 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 (cause misoperation), vibration environment, etc.
3, TDR (time domain)/principle of guided wave radar/microwave measurements: its name in the industry with a variety of different term, it has the merits of the laser measurement, such as: easy to install, calibration, good flexibility, etc., and the more superior to laser detection, such as do not need to repeat the calibration and multifunctional output, it is applicable to various foam containing liquid level detection, is not affected by the liquid color, even can be applied to high viscous liquid, relatively small influence of the external environment, but its measurement height generally is less than 6 m.
4. Ultrasonic measurement: because of its principle, the liquid level height is calculated by detecting the time difference between the ultrasonic sending and reflection, so it is susceptible to the energy loss of ultrasonic propagation. It is also easy installation and high flexibility. It is usually installed on high ground for non-contact measurement. However, when using the environment such as steam and powder layer, the detection distance will be significantly shortened, and it is not recommended to use in the absorbing environment, such as foam etc.
5. Vibration measurement of tuning fork: the tuning fork type measurement is only for the on –off output, and cannot be used for continuous monitoring of liquid height. The principle is: when the liquid or powder is filled with two vibrating forks, the resonant frequency is changed, and the switching signal is transmitted by the change of detection frequency. 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, and in most cases need opening installed on the container side.
6, Photoelectric refractive measurement: light source is inside the sensor, the light through a transparent resin total reflect to sensor receiver, but when meet the level surface, part of the light will be reflected to the liquid, and sensors to detect the less of total reflection back of the light quantity to monitor liquid level. The detection method is cheap, easy to install and calibrate, but can only be applied to transparent liquid, and only output on-off signal.
7, Static pressure measurement: the method used to the pressure sensor installed at the bottom, by detecting the bottom of the liquid pressure, transformation to calculate the height of liquid level, the liquid pressure reference at the bottom is top atmospheric pressure or pressure is known. This detection way, need flush type pressure sensor with high precision, calibration requires constantly in conversion process at the same time, its advantage is detectable level without height limit, but the higher height the higher precision demand, for a long time use or replacing liquid need to recalibrate.
8, Capacitance measurement: mainly through capacitance value change measured level height due to liquid level or bulk material height change. It has many types, was to be output analog capacitive liquid level gauge, liquid level 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 is easily affected by the different container material and solution properties, such as plastic containers and hang material is easy to affect the analog output of capacitance sensor.
How to choose the best measure
1. Compare with the advantages and disadvantages of different detection methods.
2. What are the functions to be achieved?
3. What is the output signal? The output signal is usually used on-off, analog signal, NPN/PNP, etc.
Generally, the son-off/digital output is used for alarm or protection function, such as overflow alarm and low level protection of pump, etc. The analog output is mainly used for process control, including filling capacity, liquid level display, feeding speed control, etc.
4. Understand the properties of liquid, including state, color, causticity, viscosity and impurities. Do need to conform to food hygiene certification? According to our needs, find the right sensor. For example: cosmetic cream filling process, the need to monitor the storage tank high, low level, first of all, we learned that cream for fluid state, has a larger viscosity, translucent milky white color and non-corrosive, without food hygiene certification, in accordance with our request, we can choose the types are: tuning fork, TDR measurement and ultrasonic measurement.
5. Final evaluation, including installation and calibration of products, application temperature, pressure range, price, etc., can even borrow sample test from suppliers. Such as float liquid level sensor, liquid level switch performance stable, and low price, is the first choice of liquid level measurement; TDR and ultrasonic need to be mounted on top of tank; Tuning fork can be installed on top or side; the calibration perspective.