Lecture Sensors and analytical devices: Level measurement - Nguyễn Công Phương

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This chapter include all of the following content: Dipsticks, float systems, pressure-measuring devices (hydrostatic system), capacitive devices, ultrasonic level gauge, radar (microwave) sensors, nucleonic (or radiometric) sensors, other techniques, choice between different level sensors.

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Nội dung Text: Lecture Sensors and analytical devices: Level measurement - Nguyễn Công Phương

Nguyễn Công Phương Sensors and Analytical Devices Some Basic Measurement Methods, Level Measurement
Contents A. Introduction B. Sensors Characteristics C. Some Basic Measurement Methods D. Measurement Systems sites.google.com/site/ncpdhbkhn 2
Some Basic Measurement Methods I. Sensor Technologies II. Temperature Measurement III. Pressure Measurement IV.Flow Measurement V. Level Measurement VI.Mass, Force, and Torque Measurement VII.Translational Motion, Vibration, and Shock Measurement VIII.Rotational Motion Transducers sites.google.com/site/ncpdhbkhn 3
Level Measurement 1. Introduction 2. Dipsticks 3. Float Systems 4. Pressure-Measuring Devices (Hydrostatic System) 5. Capacitive Devices 6. Ultrasonic Level Gauge 7. Radar (Microwave) Sensors 8. Nucleonic (or Radiometric) Sensors 9. Other Techniques 10. Choice between Different Level Sensors sites.google.com/site/ncpdhbkhn 4
Introduction • Required in a wide range of applications. • Can involve the measurement of solids in the form of powders or small particles as well as liquids. • Some applications require a high degree of accuracy. • Others only need an approximate indication of level. • Simple devices such as dipsticks or float systems are relatively inexpensive. • Devices offer good measurement accuracy: pressure- measuring, capacitive, ultrasonic, radar, radiation. sites.google.com/site/ncpdhbkhn 5
Ordinary Dipstick • A simple means of measuring the level of liquids approximately. • The least expensive device available. • Consist of a metal bar on which a scale is etched. http://www.expandingknowledge.com/Jerome/Motorcycle/ • The bar is fixed at a known position in Model/2009HondaCBR1000RRABS/Maintenance/Engine_Oil/ 2010_06_11_Level_Inspection/Main.htm the liquid-containing vessel. • A level measurement is made by removing the instrument from the vessel & reading off how far up the scale the liquid has wetted. • A human operator is needed to remove & read the dipstick  used in relatively small & shallow vessels. http://www.gobluedevil.com/blog/how- sites.google.com/site/ncpdhbkhn do-i-fix-a-rear-main-seal-leak-2/ 6
Optical Dipstick • Allows a reading to be obtained without removing the dipstick from the vessel. • Applicable to larger, deeper tanks. • Light from a source is reflected from a mirror, passes round the chamfered end of the dipstick, & enters a light detector after reflection by a second mirror. • When the chamfered end comes into contact with liquid, its internal reflection properties are altered & light no longer enters the detector. • By using a suitable mechanical drive system to move the instrument up & down & measure its position, the liquid level can be http://moodle.sbc.ac.uk/sharedContent/repository/to monitored. _be_packaged/Engineering,Technology%20and%20Ma nufacturing/Manufacturing/The%20Main%20Methods %20of%20Measuring%20Fluid%20Level/SC2_RESOURC E_SCREEN_1/SC2_RESOURCE_SCREEN_1.html sites.google.com/site/ncpdhbkhn 7
Level Measurement 1. Introduction 2. Dipsticks 3. Float Systems 4. Pressure-Measuring Devices (Hydrostatic System) 5. Capacitive Devices 6. Ultrasonic Level Gauge 7. Radar (Microwave) Sensors 8. Nucleonic (or Radiometric) Sensors 9. Other Techniques 10. Choice between Different Level Sensors sites.google.com/site/ncpdhbkhn 8
Float Systems (1) • Simple & inexpensive. • Provide an alternative way of measuring the level of liquids approximately. • Consist of a float on the surface of the liquid whose position is measured by means of a suitable transducer. • Inaccuracy: ±1%. http://www.moellermarine.com/sites/moellermarine/ files/gauge_sender_wiring_diagram.jpg • Widely used in monitoring the level in motor vehicle fuel tanks. • Maintenance requirements are high. http://avstop.com/ac/apgeneral /fuelsystemindicators.html sites.google.com/site/ncpdhbkhn 9
Float Systems (2) • An alternative system: the float-and-tape gauge. • Used in greater number. • It has a tape attached to the float & passes round a pulley situated vertically above the float. • The other end of the tape is attached to either a counterweight or a negative- rate counterspring. • Popular in many applications, http://instrupedia.blogspot.com/2012/02 /level-measurement-direct-methods.html but maintenance requirements are high. sites.google.com/site/ncpdhbkhn 10
Level Measurement 1. Introduction 2. Dipsticks 3. Float Systems 4. Pressure-Measuring Devices (Hydrostatic System) 5. Capacitive Devices 6. Ultrasonic Level Gauge 7. Radar (Microwave) Sensors 8. Nucleonic (or Radiometric) Sensors 9. Other Techniques 10. Choice between Different Level Sensors sites.google.com/site/ncpdhbkhn 11
Pressure-Measuring Devices (1) P • Measure the liquid level basing h on the principle that the g hydrostatic pressure due to a liquid is proportional to its depth & hence to the level of its Pressure transducer surface. • In the case of open-topped vessel, the level can be measured http://www.pressuresensor- by inserting a pressure sensor at knowhow.com/blog/2012/12/10/hydrostatic -level-measurement-2/ the bottom of the vessel. • One source of error: imprecise • P: the measured knowledge of the liquid density. pressure • If the liquid density varies with • ρ: the liquid density temperature, temperature • g: the acceleration of measurement may be needed. gravity sites.google.com/site/ncpdhbkhn 12
Pressure-Measuring Devices (2) • If liquid-containing vessels are totally sealed, the liquid level can be calculated by measuring the differential pressure between the top & the bottom of the tank. P h • The differential pressure g transducer used is normally a standard diaphragm type. • One source of error: imprecise knowledge of the liquid density. • If the liquid density varies with www.designfax.net temperature, temperature measurement may be needed. • δP: the differential measured pressure • ρ: the liquid density • g: the acceleration of gravity sites.google.com/site/ncpdhbkhn 13
Pressure-Measuring Devices (3) • The bubbler unit: uses a dip pipe that reaches to the bottom of the tank & is purged free of liquid by a steady flow of gas through it. • The rate of flow is adjusted until gas bubbles are just seen to emerge from the end of the tube. • The pressure in the tube, measured by a pressure transducer, is then equal to the liquid pressure at the bottom of the tank. • Advantage: avoidance of the large maintenance problem associated with leaks at the bottom of the http://automationwiki.com/index.php?titl tanks at the site of pressure e=Bubbler_Level_Measurement tappings required by alternative methods. sites.google.com/site/ncpdhbkhn 14
Level Measurement 1. Introduction 2. Dipsticks 3. Float Systems 4. Pressure-Measuring Devices (Hydrostatic System) 5. Capacitive Devices 6. Ultrasonic Level Gauge 7. Radar (Microwave) Sensors 8. Nucleonic (or Radiometric) Sensors 9. Other Techniques 10. Choice between Different Level Sensors sites.google.com/site/ncpdhbkhn 15
Capacitive Devices • Widely used for measuring the level of both liquids & solids in powdered or granular form. • Perform well in many applications, but become inaccurate if the measured substance is prone to contamination by agents that change the dielectric constant. http://www.spiraxsarco.com/resources/steam-engineering- tutorials/the-boiler-house/methods-of-detecting-water- level-in-steam-boilers.asp • Suitable for use in extreme conditions measuring liquid metals A (high temperatures), liquid gases C (low temperatures), corrosive d liquids (acids, etc.), & high- • ε: the dielectric constant pressure processes. • A: the area • Inaccuracy: ±1 – ±2%. • d: the distance sites.google.com/site/ncpdhbkhn 16
Level Measurement 1. Introduction 2. Dipsticks 3. Float Systems 4. Pressure-Measuring Devices (Hydrostatic System) 5. Capacitive Devices 6. Ultrasonic Level Gauge 7. Radar (Microwave) Sensors 8. Nucleonic (or Radiometric) Sensors 9. Other Techniques 10. Choice between Different Level Sensors sites.google.com/site/ncpdhbkhn 17
Ultrasonic Level Gauge • One of a number of noncontact techniques available. • Used primarily to measure the level of materials that are either in a highly viscous liquid form or in solid (powder or granular) form. • Operating principle: http://zone.ni.com/devzone/cda/ph/p/id/66 – Energy from an ultrasonic transmitter above the material is reflected back form the material surface into an ultrasonic receiver. – Measurement of the time of flight allows the level of the material surface to be inferred. • Alternative version: transmitter & receiver are at the bottom (for liquids only). • Has a small sensitivity to temperature, humidity, air pressure. sites.google.com/site/ncpdhbkhn 18
Level Measurement 1. Introduction 2. Dipsticks 3. Float Systems 4. Pressure-Measuring Devices (Hydrostatic System) 5. Capacitive Devices 6. Ultrasonic Level Gauge 7. Radar (Microwave) Sensors 8. Nucleonic (or Radiometric) Sensors 9. Other Techniques 10. Choice between Different Level Sensors sites.google.com/site/ncpdhbkhn 19
Radar (Microwave) Sensors • Use microwave radar as an alternative technique for noncontact measurement. • Very expensive. • Provide successful level measurement in applications that are otherwise very difficult: – In closed tanks, where the liquid is turbulent, – In the presence of obstructions & steam condensate. • Also used for detecting the surface of solids in powder or particulate form. • Operating principle: – Direct a constant amplitude, frequency-modulated microwave signal at the liquid surface. – A receiver measures the phase difference between the reflected signal & the original signal transmitted directly through air to it. – The phase difference is linearly proportional to the liquid level sites.google.com/site/ncpdhbkhn 20