1 Buidangthanh-3i@mail.hut.edu.vn Ha Noi, March 2012 1 MEASUREMENT TECHNOLOGY LEVEL MEASUREMENT BUI Dang Thanh, NGUYEN Thi Lan Huong School of Electrical Engineering, Hanoi University of Science and Technology 1 Dai Co Viet road, Hà Nôi, Viêt Nam Buidangthanh-3i@mail.hut.edu.vn Ha Noi, March 2012 2 Presentation outline 1. Introduction  Discrete-Level Detectors  Continuous-Level Detectors 2. Measurements Using the Effects of Density 3. Time-of-Flight Measurements 4. Level Measurements by Detecting Physical Properties Introduction  What is level ?  is defined as the filling height of a liquid or bulk material, for example, in a tank or reservoir.  They have two classifications: discrete and continuous.  Discrete-level detectors can only detect whether the material is at a certain level.  The continuous-level detector provides an analog signal that is proportional to the material level. 3 Buidangthanh-3i@mail.hut.edu.vn Ha Noi, March 2012 Introduction  Representation of a tank with a liquid or solid material (hatched area), the product to be measured.  The level sensor can be mounted (a) contacting product at the bottom, (b) as a contactless instrument on top, (c) as an intrusive sensor, or (d) at the sides as a level switch. 4 Buidangthanh-3i@mail.hut.edu.vn Ha Noi, March 2012 2 Discrete-Level Detectors  Discrete-level detectors determine when a liquid has reached a certain level.  An application of this type would be determining when to stop the fill cycle of a washing machine. Buidangthanh-3i@mail.hut.edu.vn Ha Noi, March 2012 5 Continuous-Level Detectors  Continuous-level detectors provide a signal that is proportional to the material level. Buidangthanh-3i@mail.hut.edu.vn Ha Noi, March 2012 6 Buidangthanh-3i@mail.hut.edu.vn Ha Noi, March 2012 7 Measurements Using the Effects of Density 1. Displacer 2. Float 3. Pressure Gages 4. Balance Method * Displacer Buidangthanh-3i@mail.hut.edu.vn Ha Noi, March 2012 8 3 Displacer  Displacers measure the buoyancy of a solid body that is partially submerged in the liquid. The change in weight is measured.  Quantities of a solid body immersed into a liquid. The forces F can be calculated from Equations 2, 3, and 4. r = density; b = length of the body; Ld = dipped length. Buidangthanh-3i@mail.hut.edu.vn Ha Noi, March 2012 9 Displacer  The cross section A of the body is assumed to be constant over its length b . The weight of force F G due to gravity g and mass m is:  The buoyant force F B accounts for the partial length L d that is submerged with the remainder of the body in the atmosphere: Buidangthanh-3i@mail.hut.edu.vn Ha Noi, March 2012 10 Displacer Buidangthanh-3i@mail.hut.edu.vn Ha Noi, March 2012 11  Combining 2 Equations in previous slide gives the resulting force to be measured by an appropriate method:  The result for level Ld, related to the lower edge of the displacer is: Displacer Buidangthanh-3i@mail.hut.edu.vn Ha Noi, March 2012 12  Level, interface and density sensor using the effects of buoyancy.  And the surrounding density pL can be calculated: 4 Float Buidangthanh-3i@mail.hut.edu.vn Ha Noi, March 2012 13 Float  Floats are similar to displacers, but are swimming on the liquid’s surface due to the buoyancy. Hence, the density of the float must be lower than the density of the liquid. Buidangthanh-3i@mail.hut.edu.vn Ha Noi, March 2012 14 Float  If the float is very flat, it is called a “sensing plate”. This plate is mechanically guided, e.g., by a servo control, on the surface until uplift is detected. For solids, specially shaped perpendicular floats are helpful. Buidangthanh-3i@mail.hut.edu.vn Ha Noi, March 2012 15 Pressure Gages Buidangthanh-3i@mail.hut.edu.vn Ha Noi, March 2012 16 5 Pressure Gages  A hydrostatic pressure p , caused by the weight of the product, is present at the bottom of a tank, in addition to the atmospheric pressure p0 :  Pressure gages at the bottom of the tank measure this pressure. In process tanks with varying atmospheric pressure, a differential pressure measurement is achieved by measuring the difference between the pressure at the bottom and that at the top of the tank, above the liquid. Buidangthanh-3i@mail.hut.edu.vn Ha Noi, March 2012 17 Pressure Gages  Level gaging by hydrostatic pressure measurement. The bottom pressure p is proportional to level. Buidangthanh-3i@mail.hut.edu.vn Ha Noi, March 2012 18 Pressure Gages • Figure (b) in previous slide shows a vertical arrangement with three sensors; the measurements of p1 and p2 are used to compensate for the influence of density pL, and to calculate the level: Buidangthanh-3i@mail.hut.edu.vn Ha Noi, March 2012 19 Time-of-Flight Measurements Buidangthanh-3i@mail.hut.edu.vn Ha Noi, March 2012 20 1. Ultrasonic 2. Microwaves 3. Laser/Light * 6 Basic Principle  Although different types of physical waves (acoustic or electromagnetic) are applied, the principle of all these methods is the same: a modulated signal is emitted as a wave toward the product, reflected at its surface and received by a sensor, which in many cases is the same, (e.g., the ultrasonic piezoelectric transducer or the radar antenna). Figure in next slide demonstrates the principle of operation. The measuring system evaluates the time-of-flight t of the signal: Where v is the propagation velocity of the waves. Buidangthanh-3i@mail.hut.edu.vn Ha Noi, March 2012 21 Basic Principle Buidangthanh-3i@mail.hut.edu.vn Ha Noi, March 2012 22 • (a) Representation of time-of- flight measurements. The emitter couples a wave (ultrasonic or electromagnetic) into the atmosphere that propagates the wave toward the liquid. Its surface reflects the wave and a sensor receives it. • (b) Due to the propagation velocity v, a time delay is measured between emission and receipt of the signal. This example is characterized by a modulated burst. The time scale is arbitrary.  One can generate an unmodulated pulse, a modulated burst as in Figure 11.6(b), or special forms. Table 11.1 lists the main properties of the three preferred types of waves, used for time-of-flight level gaging.  The very short time spans of only a few nanoseconds for radar and laser measurement techniques require the use of time expansion by sampling or special evaluation methods Basic Principle Buidangthanh-3i@mail.hut.edu.vn Ha Noi, March 2012 23 Buidangthanh-3i@mail.hut.edu.vn Ha Noi, March 2012 24 Ultrasonic 7 Ultrasonic  Ultrasonic waves are longitudinal acoustic waves with frequencies above 20 kHz. Ultrasonic waves need a propagation medium, which for level measurements is the atmosphere above the product being measured.  Sound propagates with a velocity of about 340 m s–1 in air; but this value is highly dependent on temperature and composition of the gas, and also on its pressure. In vacuum, ultrasonic waves cannot propagate.*  Piezoelectric transducers are utilized as emitter and detector for ultrasonic waves, a membrane coupling it to the atmosphere.  The sensor is installed as in Figure slide22(b), the signal form is as in Figure slide22(b). Level gaging is, in principle, also possible with audible sound 16 Hz to 20 kHz or infrasonic waves less than 16 Hz. Buidangthanh-3i@mail.hut.edu.vn Ha Noi, March 2012 25 Ultrasonic  Another procedure is to propagate the waves within the liquid by a sensor mounted at the bottom of the tank.  The velocity of sound in the liquid must be known, considering the dependence on temperature and type of liquid.  This method is similar to an echo sounder on ships for measuring the water depth. For more information about time-of-flight ultrasound evaluation techniques, refer to * Buidangthanh-3i@mail.hut.edu.vn Ha Noi, March 2012 26 Buidangthanh-3i@mail.hut.edu.vn Ha Noi, March 2012 27 Microwaves Microwaves  Microwaves are generally understood to be electromagnetic waves with frequencies above 2 GHz and wavelengths of less than 15 cm. For technical purposes, microwave frequencies are used up to max. 120 GHz; in practice, the range around 10 GHz (X-band) is preferred.  The usually applied time-of-flight measurements with microwaves are RADAR-based [8, 9]. The term “RADAR” is generally understood to mean a method by means of which short electromagnetic waves are used to detect distant objects and determine their location and movement. It is an acronym from RAdio Detection And Ranging Buidangthanh-3i@mail.hut.edu.vn Ha Noi, March 2012 28 8 Microwaves Buidangthanh-3i@mail.hut.edu.vn Ha Noi, March 2012 29 Microwaves Buidangthanh-3i@mail.hut.edu.vn Ha Noi, March 2012 30 FIGURE 11.8 Design of a compact industrial level radar system. The converter above the flange includes the complete microwave circuitry, signal processing stages, microprocessor control, display, power supply, and output signal [6]. Microwaves  For level measuring systems, a small radiation angle is desirable in order to avoid interfering reflections from the tank wall or tank internals as much as possible. The larger the aperture area, the smaller the radiation angle and the higher the antenna gain. The power balance is given by the general radar equation: Buidangthanh-3i@mail.hut.edu.vn Ha Noi, March 2012 31 Microwaves  The reflection factor R of the product’s surface is dependent on the dielectric permittivity ε r of the liquid or bulk material:  In level measurement situations, the reflecting area is so large that it intersects the beam cross section completely; therefore, D2 is approximately proportional with distance d2. Thus also, the received power decreases proportionately with d2, as derived in [8]: Buidangthanh-3i@mail.hut.edu.vn Ha Noi, March 2012 32 9 Microwaves  This is not the case if the waves propagate within an electromagnetic waveguide, like a vertical tube dipping into the liquid, called a stilling well. Here, the propagation is nearly without losses.  An alternative method using electromagnetic waves is to propagate them in a cable. Next Figure illustrates the operation with a cable dipped into the liquid or bulk material. Where the dielectric permittivity of the surrounding medium changes, part of the wave is reflected. This method can be applied to interface measurements too. Buidangthanh-3i@mail.hut.edu.vn Ha Noi, March 2012 33 Microwaves Buidangthanh-3i@mail.hut.edu.vn Ha Noi, March 2012 34  Principle of operation of a wire- conducting high- frequency level measurement system. Level Measurements by Detecting Physical Properties Buidangthanh-3i@mail.hut.edu.vn Ha Noi, March 2012 35 1. Ultrasonic 2. Microwaves 3. Laser/Light * Basic method  Electrical Properties • The sensor must be in direct or indirect contact with the product to detect its electrical properties. For continuous measurement, only part of the intrusive sensor must be in contact with the product to detect the difference in dielectric permittivity or conductivity.  Capacitive • In most applications, a rod electrode is arranged vertically in the tank. The electrode can be (1) noninsulated if the liquid is nonconductive, or (2) insulated. The metallic vessel acts as a reference electrode. Buidangthanh-3i@mail.hut.edu.vn Ha Noi, March 2012 36 10 Basic method Buidangthanh-3i@mail.hut.edu.vn Ha Noi, March 2012 37 References 1. http://www.yokogawa.com 2. http://www.abb.com 3. http://www.wikipedia.org 4. http://www.automation.siemens.com 5. Moderm control techonlogy – components & system Buidangthanh-3i@mail.hut.edu.vn 38 Ha Noi, March 2012 Buidangthanh-3i@mail.hut.edu.vn Ha Noi, March 2012 39 Thank you for your attention! . high- frequency level measurement system. Level Measurements by Detecting Physical Properties Buidangthanh-3i@mail.hut.edu.vn Ha Noi, March 2012 35 1. Ultrasonic 2. Microwaves 3. Laser/Light. can be applied to interface measurements too. Buidangthanh-3i@mail.hut.edu.vn Ha Noi, March 2012 33 Microwaves Buidangthanh-3i@mail.hut.edu.vn Ha Noi, March 2012 34  Principle of operation of. sides as a level switch. 4 Buidangthanh-3i@mail.hut.edu.vn Ha Noi, March 2012 2 Discrete -Level Detectors  Discrete -level detectors determine when a liquid has reached a certain level.  An