Home > Knowledge > Content
A brief introduction to some level meters
Dec 14, 2018

A brief introduction to some level meters

I.Foreword

In all kinds of sensors army, liquid level meter plays an important role, it is our production and life security. There are more than ten kinds of liquid level meters in the market, and the commonly used ones in enterprises are buoy liquid level meter, float ball liquid level meter, differential pressure type liquid level meter, guided wave radar liquid level meter, etc.

A brief introduction to some level meters

Ii. Float level meter

1. Working principle the buoy level meter is composed of four basic parts: buoy, spring, magnetic steel chamber and indicator.The buoy immersed in liquid is subjected to the combined action of downward gravity, upward buoyancy and spring force.When these three forces are in equilibrium, the buoy is at rest in a certain position.When the liquid level changes, the buoyancy force of the buoy changes correspondingly, and the equilibrium state is broken, thus causing the elastic force change, that is, the stretching of the spring, to achieve a new balance.The spring's expansion causes it to shift from the magnetic steel it is rigidly connected to.Thus, the liquid level is indicated by the magnetic induction element and the transmission device inside the indicator.

2. Features and applicable occasions

2.1 field instruction and remote transmission compatibility;

2.2 large measurement range, up to 3000mm;

2.3 reliable operation, good accuracy and sensitivity;

2.4 high temperature and high pressure resistance, strong corrosion resistance;

2.5 on-site debugging is convenient and easy to check and maintain.Because it is intuitive, stable, high reliability, and therefore for continuous production of oil refining, chemical industry in the important containers, equipment, such as tower type, storage tank intermediate container and other liquid level measurement is very applicable, but not suitable for high viscosity medium liquid level measurement.

3. Fault phenomena and handling

3.1 high output: check whether the process variables are out of range;Check wiring terminals, pins or sockets;Check the power supply voltage;Electronic circuit component malfunction.

3.2 unstable output: check the line voltage;Whether there is intermittent short circuit, open circuit or multi-point grounding;Circuit board malfunction.

3.3 no output or low output: check the line voltage;Whether there is short-circuit or multi-point grounding;Check polarity of signal line;Check circuit resistance;Inspection range;Circuit board fault;The booty accumulated inside the buoy.

A brief introduction to some level meters

III.Float level meter

1. Working principle

The structure of the float level meter is mainly designed and manufactured based on the principle of buoyancy and static magnetic field.The position of floating ball with magnet in the medium under test is affected by buoyancy, and the change of liquid level leads to the change of magnetic float position.Magnets and sensors (reed switches) in the float act to change the number of elements (such as constant resistance) that are connected to the circuit, thereby changing the electrical quantity of the instrument circuit system.That is, changes in the position of the magnetic float cause changes in electrical quantities.Changes in electrical quantities are measured to reflect the liquid level in the container.

2. Features and applicable occasions

2.1 simple structure and convenient use

2.2 stable performance, long service life, easy installation and maintenance can be applied to almost all kinds of industrial automation process control of liquid level measurement and control, can be widely used in oil processing, food processing, chemical industry, water treatment, pharmaceutical, electric power, papermaking, metallurgy, shipbuilding and boiler in the field of liquid level measurement, control and detection.

3. Fault phenomena and handling

3.1 field change, the display does not change with the liquid level: check whether the shaft and the transmitter are in good contact;Check the power supply voltage;Check zero point and range;Sensor fault;Circuit board malfunction.

3.2 actual liquid level changes and no on-site changes: the external balance bar is separated from the rotating shaft;The heavy hammer is not adjusted properly;Loose and detached internal connectors;Club deformation;The float ball falls off;The float ball breaks;Medium evaporation

Iv. Differential pressure type liquid level meter

1. Working principle

Differential pressure liquid level juice works by utilizing the principle that static pressure generated by liquid column changes correspondingly when liquid level changes in the container, as shown in figure 1.Differential pressure transmitter at one end of the liquid phase, the other end of the gas phase according to the principle of hydrostatics, we know that the transmitter positive pressure chamber pressure is: Pl=P gas ten rho gH.Where H: liquid level height;Rho: medium density;G: gravity acceleration;P gas: vapor pressure.Figure 1 differential pressure transducer to measure the level diagram differential pressure transmitter negative pressure gas chamber pressure P2 = P, is the differential of pressure chamber are as follows: Δ P = P1 and P2.In general, the density of the medium under test is known.So if you measure the differential pressure, you know the level.

 

A brief introduction to some level meters

2. Features and applicable occasions

2.1 high sealing and leakage prevention can be achieved

2.2 measure liquid level safely and reliably under high temperature, high pressure, high viscosity and strong corrosion

2.3 there is no blind area in the whole process of measurement, the display is eye-catching, the reading is intuitive, and the measurement range is large with liquid level alarm and control switch, which can realize the upper and lower limit alarm and control of liquid level or boundary level.Easy installation, easy to achieve remote transmission and automatic adjustment, industrial applications more.For the more mature liquid level measuring instrument, the measurement accuracy is higher and the maintenance is less.Single flange (single lead line) level meter is generally used in open or atmospheric pressure containers, sealed with pressure equipment should choose double flange (double lead line) level meter.

3. Fault phenomena and handling

3.1 large change in liquid level: large fluctuation of medium or serious vaporization;The upper lead line or the lower lead line is not clear;The medium has crystallization;Loss of pressure transfer medium in capillary tube;Damage of diaphragm box;The heat tracing temperature is too high.

3.2 no change in display: the cut-off valve is not opened;Lead line blockage;The range and zero point are not adjusted properly;There is debris accumulation at the membrane box;The capillaries are compressed;Circuit board malfunction.

V.Guided wave radar level meter

1. working principle

Guided wave radar level gauge is based on the time-domain reflection principle of electromagnetic wave, microwave pulse is not travel through space, but spread by metal wave guide rod, when in contact with the liquid surface, the conductors in the gas and liquid conductivity of different wavelengths, and that the impedance of the waveguide body, there was a sudden change to produce a liquid level primitive impulse, at the same time at the top with a predefined waveguide impedance and the impedance to produce a reliable basic pulse, liquid level pulse radar liquidometer detected after comparing with basic pulse, and liquid level height is calculated.

2. Features and applicable occasions

2.1 measurement is not affected by tank shape

2.2 it is not affected by dielectric constant, temperature, pressure and density

2.3 it is not affected by surface fluctuation of object level, dust, steam and foam

2.4 measurement length can be flexibly changed without calibration

2.5 the measurement results are of high accuracy, repeatability and resolution

2.6 applicable pressure range up to 40bar guided wave radar level meter is applied to water liquid storage tank, acid and alkali storage tank, slurry storage tank, solid particles and small storage tank.All kinds of conductive, non-conductive media, corrosive media.Such as coal bunker, ash bunker, oil tank, acid tank and so on.

3. Comparison with ordinary radar level meter

3.1 general radar is non-contact measurement, guided wave radar is contact measurement, which means guided wave radar needs to consider the corrosive and adhesion of medium, and too long guided wave radar installation and maintenance is more difficult.Ordinary radar can be used interchangeably, while guided wave radar cannot be used interchangeably because the length of guided wave rod (cable) is fixed according to the original working condition. Therefore, the selection of guided wave radar is more troublesome than that of ordinary radar.When measuring solid materials, waveguide radar should also consider the force of waveguide rod (cable), which is also because of the force of the general waveguide radar measurement distance is not very long, and the general radar in 30, 40m tank application is common, even can be measured to 60m.In addition, the general guided wave radar has the function of bottom detection, which can be modified according to the measured value of bottom echo signal to make the signal more stable and accurate.

3.2 however, guided wave radar has obvious advantages in some special working conditions, such as mixing in the tank and large medium fluctuation. The measured value of guided wave radar fixed at the bottom is more stable than flexible radar in such working conditions.There are also small tank inside the object level measurement, due to the installation of small measurement space (or more interference in the tank), the general general radar is not applicable, then the advantages of guided wave radar is shown;In the case of low dielectric constant, the measurement principle of both radar and guided wave radar is based on the difference of dielectric constant. Because the waves emitted by ordinary radar are divergent, when the dielectric constant is too low, the signal is too weak and the measurement is unstable, while guided wave radar wave propagates the signal along the guide wave rod relatively stable.

4. Fault phenomena and handling

4.1 fluctuation of liquid level, output percentage and loop value: reconfiguration probe length and deviation;Rely on other equipment to confirm accurate liquid level;Adjust damping coefficient;Reconfigure loop values.

4.2 Regardless of the liquid level, the output value is the same: confirm the probe length;Adjust the bias value to the exact value.

4.3 No liquid level signal: check the dielectric constant of the medium;The liquid level is in the top transition zone, which is not set during configuration;Circuit board or 16-pin connector is not working properly;Check probe length configuration;There may be media bridging the probe;The dielectric constant is not selected correctly.4.4.4 output or maximum, or minimum, inaccurate: the medium is not pure, such as oil with water;Bridging medium or sundries on the probe;Guide rod blockage;Having a foam or goop; viscous.There are sundries in the top seal of the probe

A brief introduction to some level meters

Vi. Commonly used liquid level meter

1. Installation, use and matters needing attention

1.1 upper and lower flanges shall not be subject to force;

1.2 the table body should be vertical;

1.3 reliable connection of accessories;

1.4 considering the convenience of operation, observation and maintenance in the future;

1.5 generally, the upper cut-off valve is opened before the lower cut-off valve is opened for operation;

1.6 avoid large parts of vibration as far as possible.

2. Type selection principle of liquid level meter

2.1 consider working conditions, such as the nature of media, working temperature, working pressure, whether the container is closed or open, etc.

2.2 consider work requirements, reliability, measurement accuracy, measurement range, etc.

2.3 economic requirements. Considering the above requirements, a suitable liquid level meter is selected.

Conclusion this paper introduces the working principle, characteristics and application situation of several commonly used liquid level meters, application failure and troubleshooting, matters needing attention in installation and use, and selection principles. Give the reader reference when applying