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More than 20 kinds of liquid level measurement methods

Level including the level and level two. Level also includes thesignal level and continuous level measurement device two. Levelsignal device is a fixed position of the level of severalmeasurements, for the level of the upper and lower limit alarm.Continuous level measurement is a continuous liquid levelmeasurement, is widely used in petroleum, chemical, food processingand many other areas, has very important significance. In thispaper, more than 20 continuous level measurement methods werecompared.
A glass tube method, glass method, two-color water, artificial footlaw review
  tube method: This method uses the principle ofwork connected device, as shown in Figure 1-1 [1]. Figure 1 - testvessel; 2 - glass tube; 3 - indicates the scale feet; 4,5 - valve;6,7 - connecting pipe. Scale level instructions directly from thescale reading.

   Glass method: glass can be installed throughthe connectivity can also be installed in the vessel wall openings,and passages in series to increase the range of the glass. Levelvalue read directly from the glass scales.
   Color indicator method: The method uses optical principles, sothat the water is green, leaving the water vapor in red, whichindicates the water level [2].
  Artificial foot inspection method: The method used to measure thetank level. Measurement, the surveyor to put the oil dipstick, andin the foot mound brought in contact with the bottom of the tankdipstick. According to oil on the dipstick mark, read the oillevel; according to dipstick end water cream color change todetermine the height of the water cushion, high oil and water todetermine the high [3].
  Above 4 methods are manual methods of measurement, withmeasurement of simple, reliable, intuitive, low costadvantages.
2 blowing method, differential pressure method, HTGLaw
  blowing method: This method works as shown inFigure 2-1 [4]. Figure 1 - filter; 2 - valve; 3 - throttlingelement; 4 - rotameter; 5 - transmitter. Inflatable tube becausethe pressure is approximately equal to the static fluid columnpressure, so P = ρgH

  Where, ρ-fluid density; H-level. It can bemeasured by the level of static pressure P H. Blowing method formeasuring corrosion resistance, a hanging cloud of liquid property,mainly used in less demanding precisionapplications.
    Differential pressure method: This method worksas shown in Figure 2-2 [4] . In the figure, 1,2 - valves; 3 -differential pressure transmitter. The open container or pressurevessel, valves 1 and gas pressure pipes can save cited. Therelationship between pressure difference and the level for the ΔP =P2-P1 = ρgH

Where: ΔP-transmitter is, negative pressure chamber pressuredifference; P2, P1-cited pressure pipe pressure; H-level.Differential pressure transmitter pressure difference will betransformed into 4 ~ 20 mA DC signal. If the pressure in the lowerlimit of measurement range corresponding to the output signal isgreater than or less than 4 mA, then the need to zero by adjustingthe spring migration migration technology, so that is equal to 4mA.

  HTG Method: The method used in tank differential pressure levelmeasurement, shown in Figure 2-3. The figure: P1, P2, P3-precisionpressure sensor; RTD-temperature detection devices; HIU-interfaceunit. P1 is located near the bottom of the tank shell at, P2 8 feethigher than the P1, P3 in the tank shell near the top of Office.For the pressure tank, pressure sensor P3 omitted. Based pressuresensor P1, P2, P3 were measured pressure p1, p2, p3,then

   Where: G-oil weight; Sav-tank average cross-sectional area;ρav-range pressure sensor P1, P2 average density between the oil; gis the acceleration due to gravity; H is the pressure sensor P1, P2distance between; h is the oil height; h0 is the height of thepressure sensor P1. RTD used to measure the oil temperature to thetemperature compensation of measured values. HTG measurementsystems lower prices, but low level measurement accuracy, theinstallation shall be in the tank wall opening.
  Above 3 methods are used to measure the liquid level of thepressure difference.
3 float method, float method, the float method, the servo method,Shen cone
float method: This method uses the float as the liquid levelmeasurement devices, and disk drive coding or encoding with otherdisplay devices, or connecting to remote electronic transmittertransmission measurement signal.

    Float method: This method uses the middle holeof the magnetic liquid level sensor tube as shown in Figure 3-1.Stainless steel casing from the hole through the middle buoys,fixed in between the tank top and tank bottom. Level changes drivenhollow magnetic cylinder (containing a permanent magnet) moves upand down along the casing, and a magnet to attract the casing wallmoved up and down along the casing, the second instrument based onthe amount of movement calculated magnet level.
  Float method: The method uses leverage the work shown in Figure3-2 [4]. The figure: 1 - Float; 2 - connecting rod; 3 - shaft; 4 -balance weight; 5 - leverage. Float around the shaft to follow thelevel change rotation, rotation pointer on the drive shaft and theother end and leveraged balance weight balance, while the dialindicates the level value. Float within a float type and the law isoutside the float type two, shown in Figure 3-2. Float method ismainly used to measure temperature, high viscosity of the liquid,but the smaller scale.

   Servo Method: The method uses wave integralcircuit to eliminate jitter, extend life and improve the levelmeasurement accuracy. Servo level gauge for measuring modern highprecision, has reached the 40 m range accuracy of less than 1 mm,and generally have a measured density distribution and the averagedensity function.

   Shen tube method: Shen tube position changeswith the liquid level, but the variation does not change with thelevel equal amount. In Figure 3-3a in [4] , the relationshipbetween position level with the float asfollows:

   The formula: ΔH-level variation; C-spring constant; A-Shen tubecross-sectional area; ρ liquid density; ΔX-Shen Bin location of thechange. Typically, the location of the change ΔX float level changeis far less than the amount of ΔH. Figure 3-3b is the torque tubeShen cone principle [4], the figure: 1 - Shen Bin; 2 - Lever; 3 -torque tube; 4 - shaft; 5 - shell. Shen tube position changes withthe level, the role of the lever, the torque tube shaft torsionalangle changes, the second instrument based on the change in twistangle calculated level.
  5 kinds of methods are used more than the buoyancy principle towork.
4, capacitance, resistance method, inductivemethod
  capacitance method: non-conductive fluid usedto measure the capacitance of Principle in Figure 4-1 [4]. Figure4-1, the capacitance of two concentric cylindrical electrodes,whose capacity CH for the

   The formula: ε1-measured relative permittivity of liquid; ε2-gasmedium relative permittivity; H-capacitive sensor immersed inliquid depth (m); l-capacitance sensor vertical height (m); R-inthe underside of the plate cylinder radius (m); r-outer radius ofthe plate cylinder bottom (m). Since R, r, l are all fixed, as longas the use of ε1, ε2, CH will be able to calculate the liquid levelH. Figure 4-2 is used to measure the capacitance of conductiveliquid Principle [4], the formula slightly. Capacitive level meterlower price, easy to install and can be applied to high temperatureand pressure of the occasion. But the capacitance level measurementrepeatability lower, requiring regular maintenance andre-calibration is not very long working life.

  Resistance Method: The method [ 5], especiallyfor measurement of conductive liquids, sensitive device has aresistance characteristic, the resistance value changes with theliquid level, so the change in resistance value will be sent to thesecondary circuit which are liquid. Probe type using trackingmethod to measure the level to level up the case as an example toillustrate the principles of level measurement, when the liquidlevel rises, lift the probe completely out of the liquid, thenslowly lower the probe to find surface, then When the probe and thelocation of the liquid that is new to correspond with the level.Probe type is characterized by a high precision measurement,control circuit complexity.
  Inductive method: This method [5] for conductive liquid levelmeasurement, in particular liquid metal. The principle of inductivemethod, the liquid level change makes the inductor inductance,mutual inductance, or permeability changes, thus changing theamount of the secondary circuit can be sent to the appropriatelevel value. Inductive method is the most widely usedhigh-frequency level meter. The measurement principle of the levelmeter, the frequency modulated signal through the RF cable coupledto the transmission line sensor resonant circuit, resonant circuitafter the output voltage detection circuit and RF cables sent tothe low-pass filter, and then low-pass filter output voltagecontrol tuning circuit, new oscillation frequency until theresonant sensor resonant circuit is in full state to date, theoscillation frequency at this time that the sensor's inductancecorresponds to correspond with the level.
   Use of the above 3 methods are generated level sensor changesthe electrical parameters of the method to measure liquidlevel.
5 magnetostrictive, ultrasonic, modulation type optical,microwave
  magnetostrictive method: The method used tomeasure the tank level of the principle shown in Figure 5-1 [6].Figure 5-1, two floats were used to detect oil and gas interfaceand the oil-water interface. Have possession of the float within aset of permanent magnets, used to generate a fixed magnetic field.Measurement, given the low current level gauge head "ask" pulsemagnetic field generated by the current downward conduction alongthe waveguide. Float when the current magnetic field and magneticfield meet, have a "return" pulse (also called "Bird distorted"pulse). Ask pulse and return pulse that corresponds to the timedifference between the oil-water interface and the high oil and gasinterface. Magnetostrictive level gauge easy to install, highaccuracy, but the liquid density and temperature changes will bringthe measurement error [7], float along the outside of theprotection tube waveguide move up and down easilystuck.

   Ultrasonic: transducer to convert electricpower ultrasonic pulses, fired surface, the surface reflectivityand then again from the ultrasonic transducer converts electricalsignals. Ultrasound is a mechanical wave, propagation attenuation,the interface reflection strong signal, and transmitting andreceiving circuit is simple, and thus is widely used; but theultrasonic propagation velocity by the media, density,concentration, temperature, pressure and other factors, themeasurement accuracy is low.
   Microwave Method: Microwave the antenna (mostly for the apertureantenna, there are planar antenna) radiated by the antenna afterreflection surface, and then transmitting the signal from thesecondary circuit computing the time difference with the receivedsignal level obtained. Principle of continuous wave radar levelgauge as shown in Figure 5-2, the liquid level meter using thetriangular wave form of frequency modulation, and transmittingsignals by mixing the received signal and the difference betweenthe signals obtained after analysis, microwave transmission time tocalculate the liquid level. By microwave transmission medium speed,temperature, pressure, dielectric constant of the liquid is small,but the volatility of the liquid interface, the liquid surface ofthe foam, the microwave dielectric constant of the liquid mediumhas great influence on the strength of the reflected signal. Whenthe pressure exceeds the set value, the pressure on the liquidlevel measurement accuracy will have a significant impact. Lessthan the specified value for the dielectric constant of the liquid,most of the radar level instruments are required by the waveguide,the waveguide corrosion, bending and tilt can affect measurementaccuracy. For example: When the air is high h 20 m, and verticalwaveguide tilt angle α long as more than 0.573 °, Δh is the errorlevel caused by more than 1 mm, which proved that the angle α (indegrees) small, Δh meet:

  Radar level gauge is particularly suitable forhigh pollution or high viscosity products such as asphalt. Radarlevel measurement repeatability higher, without regular maintenanceand re-calibration, measurement accuracy is higher, but the priceis higher, difficult to measure the oil-waterinterface.
   Modulated optical method and microwave method is similar, butwith phase or frequency modulated optical signals instead ofmicrowave signals. Figure 5-3 is a schematic diagram of laser radarlevel gauge [8]. But the optical signal by water vapor, oil vaporgreater impact, and the level fluctuation is very sensitive, andmust be susceptible to contamination by the opticallens.

  Above 3 methods are spread by detecting thesignal of time to determine the liquid level. Let the receivedsignal and transmitting signal time difference is t, the emptyhigh-h = vt / 2, v for the wave propagationspeed.
6 magnetic flap method, vibration method, nuclear radiation,optical fiber sensor Law
  Principle magnetic flap as shown in Figure6-1a shows [1], 1 - flap indicates components; 2 - float; 3 -connecting pipe components ; 4 - adjustment screws; 5 - put drainplug. Float with a group of permanent magnets, with the level go upand down movement, driven by magnetic coupling component magneticflap flip. When the liquid level rises, the magnetic flap red sideout; level falls, the white side out. Therefore, according to thecolor magnetic flap level can be determined. In the float magnetand magnetic flap magnetic structure shown in Figure 6-1b [5], thedistance between each piece flap was 10 mm. Magnetic flap with afew series to increase the range of devices.

  The principle of vibration method in Figure6-2 [9]. By the rail vibration level gauge, test stands, hammershock, vibration sensor, servo and other components. Servo controlof crawling up and down and hammer excitation, excitation by thefree vibration after the vibration sensor, the detection signalobtained by the FET transformation frequency at maximum power, andfinally when the natural frequency of the empty / fluid levelrelations bit. The liquid level measurement methods need to shockhammer, servo and other mechanical moving parts, and its workinglife is not very long, regular maintenance and re-calibration,installation is also more complex.
  Radiation: radioactive radiation in the decay process will ray,common rays α, β, γ-rays. Where, γ-ray penetrating power, longrange, so the liquid level measurement of nuclear radiation iswidely used. Experimental results show that γ-ray intensity throughthe material before and after the change occurs, and meet thefollowing relationship [5]:
   The formula: J0-through material strength before; J-throughmaterial strength after; μ-material on the γ-ray attenuationcharacteristics; d-material thickness. Liquid level meter from theradiation sources, detectors and processing circuits. Most usedsources of cobalt or cesium -137 -60. Ionization chamber detectors,counting tube, scintillation counter, such as several, whose roleis to detect the intensity of radiation penetrating substance.Radiation using non-contact level gauge installed, as shown inFigure 6-3. Figure 6-3a using point sources, detectors, measuring asmaller range; Figure 6-3b using point sources, linear detectors,the larger measuring range; Figure 6-3c using linear radioactivesources, detectors , the largest measuring range. In addition toγ-ray, the neutron radiation can also be used to measure liquidlevel. Neutron penetrating power is very strong, stronger than 10times more γ-rays can penetrate up to 9-inch wall thickness steelcontainer [10]. Ray level gauge, easy installation, precisionmeasurements can meet the needs of a large tank, there are certainapplications.

  Fiber Optic Sensing Method: Literature [ 11]proposed a fiber liquid level sensor, when the liquid levelchanges, the elastic diaphragm pressure sensor have the sensitivityof displacement, led reflective film movement, the feelings of theprobe light intensity changes, in order to estimate the liquidlevel. [12] proposed another kind of fiber optic liquid levelsensor, according to the probe in the gas and liquid media feel thedifference in light intensity to determine the location of theprobe and control probe track level changes, resulting in levelvalue.
7 Conclusion
  In this paper , more than 20kinds of liquid level measurement methods were compared. Inpractice, the price should be based on the measurement accuracy,the characteristics of the measured medium and other factors, areasonable choice of the type of level gauge.

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