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Viscocity, Flow and Humidity Sensors [Compatibility Mode].pdf
- 1. EMT 4103 SENSORS AND TRANSDUCERS ON 3RD JUNE 2020 LESSON 3 1. Viscosity sensors and transducers 2. Humidity sensors and transducers 3. Flow sensors and transducers By GRACE WAIRIMU
- 2. Lesson expectations • At the end of the lesson, you should understand the types, designs, working principles and applications of the following sensors and transducers: 1. Viscosity 2. Humidity/moisture 3. Flow
- 3. Viscosity Sensors and Transducers In food industry, viscosity of raw materials such as dough, butter and ice cream has a direct effect on the quality of the product. In cement industry. Assembly e.g flow of glue through tubes Types: 1. Capillary and tubes viscometers/viscosimeters 2. Falling body viscometers 3. Rotational body viscometers e.g. cone and plate, parallel plates, coaxial cylinders viscosimeters.
- 4. Viscometers or viscosimeters 4. Falling piston viscometers 5. Oscillating piston viscometers 6. Vibrational e.g. quartz, quartz crystal viscosimeters 7. Electromagnetically spinning sphere viscometers 8. Bubble viscometers 9. Krebs viscosimeters 10. Rectangular slit viscosimeters.
- 5. Capillary and tube viscometers where R is the radius (m) of the tube, L is its length (m), P is the pressure difference (N/m2) across the ends and V is the volume of liquid flowing in time T (m3/s). For non-Newtonian fluids, corrections must be made for shear rate variations. For any given viscometer, R, L and V are constants
- 6. Falling body viscometer • where R is the radius (m) of the sphere, g is the acceleration due to gravity (m/S2) and ρs and ρl are the specific gravities (g/m3) of the sphere and liquid respectively and V is the velocity (m/s) of the sphere. For non-Newtonian fluids, correction for the variation in shear rate is very difficult.
- 7. Rotational viscometers where G is the couple (Nm) formed by the force exerted by the torsion wire and its deflection, R1 and R2 are the radii (m) of the inner and outer cylinders, h is the length of the cylinder (m) and ω is the angular velocity (rad/s) of the rotating cylinder. Torque required to turn an object in a fluid function of its viscosity. One driven and other suspended by tension wire.
- 8. Moisture and Humidity Sensors and Transducers 1. Mechanical methods 2. Gravimetric methods 3. Water separation 4. Neutron moderation 5. Phase change 6. Equilibrium relative humidity measurements 7. Ultrasonic methods 8. Optical methods.
- 9. Moisture and Humidity Sensors and Transducers 9. Low resolution magnetic resonance (NMR) 10. Elec methods: a) Capacitive moisture meter b) Microwave energy c) Freq. change in a quartz crystal d) Elec. Conductivity 11. Hygrometers
- 10. Importance of humidity measurement • Physical properties and storage stability of most solids depend on moisture E.g. medicine, foodstuffs, plastics, textiles, paper • Moisture content in milk etc
- 11. Humidity sensors and transducers • Humidity Capacitive sensors
- 12. Hygrometers • Materials, such as hair, skin, membranes, and thin strips of wood, change their length as they absorb water. • This change in length is directly related to humidity. • Operating temp about 70°C. • Accuracy approximately ±5% • Other devices use hydroscopic materials that change their electrical properties with humidity
- 13. Types of hygrometers Indirectly measure humidity by sensing changes in physical or electrical properties in materials due to their moisture content • Laminate hygrometer • Hair hygrometer • Resistive hygrometer • Capacitive hygrometer • Piezoelectric hygrometer
- 14. Two types of hygrometers, using (a) metal/wood laminate, and (b) hair.
- 15. Hair hygrometer • Simplest and oldest hygrometer, made using hair. • Human hair lengthens by 3% when the humidity changes from 0% to 100%. • The change in length controls a pointer for visual readings, or to control a transducer, such as an LVDT, for an electrical output. • The accuracy of approximately 5% for the humidity range 20% to 90%, over the temperature range -15° to 70°C.
- 16. Piezoelectric or sorption hygrometers • Use two piezoelectric crystal oscillators. • One is used as a reference and is enclosed in a dry atmosphere, and the other is exposed to the humidity to be measured. • Moisture increases the mass of the crystal, which decreases its resonant frequency. • By comparing the frequencies of the two oscillators, the humidity can be calculated. Moisture content of gases from 1 to 25,000 p/m can be measured.
- 17. Hydrometers: (a) resistive, and (b) capacitive.
- 18. Ultrasonic and mechanical methods • Noncontact • The presence of water changes the speed of propagation of ultrasonic waves through liquids. Mechanical Density changes in many liquids and slurries can be measured and related to moisture content, resolution up to 0.2% moisture. • By measuring the moisture level-dependent viscosity of liquids, pastes and slurries.
- 19. The psychrometer (wet and dry bulb hygrometer) • Has two temperature sensors, • one exposed to the atmosphere and one enclosed in a wet wick. • Air is blown across • the sensors, which causes evaporation and a reduction in temperature in the wet sensor. • The temperature difference between the sensors is related to the humidity level.
- 20. Dew point meter The dew point is the temperature at which the sample becomes saturated with water.
- 21. Assignment, to be submitted in 2 weeks time Describe the following laboratory techniques for measuring moisture content. • 1.Water separation • 2.Gravimetric methods • 3.Phase-change methods • 4.Equilibrium relative humidity measurement
- 22. Level Sensors and Transducers • Resistance tape devices. • Radar microwave / antenna methods • Pressure-measuring devices (hydrostatic systems) • Ultrasonic level gauge • Radiation • Capacitive • Hot-wire elements / carbon resistor elements • Fibre-optic level sensors • Vibrating level • Rotating paddle switches
- 24. Pressure measuring devices(hydrostatic systems) hydrostatic systems: (a) open-topped vessel; (b) sealed vessel; (c) bubbler unit
- 25. Ultrasonic level gauge Measuring interface positions: (a) liquid/liquid interface; (b) liquid/precipitate interface
- 26. Radiation methods Caesium-137 is a commonly used gamma- ray source. The radiation level measured by the detector I is related to the length of liquid in the path x according to: where I0 is the intensity of radiation that would be received by the detector in the absence of any liquid, µ is the mass absorption coefficient for the liquid and ρ is the mass density of the liquid.
- 27. Capacitive devices: level sensors
- 28. Hot-wire elements/carbon resistor elements
- 31. Flow Sensors and Transducers 1. Differential pressure (obstructive-type) meters • Orifice plate, • venturi tube, • flow nozzle 2. Turbine meters 3. Variable area flowmeters (rotameters) 4. Electromagnetic (EM) flowmeters 5. Ultrasonic flowmeters • Doppler shift ultrasonic flowmeters 6. Transit time flowmeters 7. Positive displacement flowmeters
- 32. Differential pressure (obstructive-type) meters Figure 20: Obstruction devices: (a) orifice plate; (b) venturi; (c) flow nozzle; (d) Dall flow tube
- 33. Orifice plate
- 34. Venturi tube
- 35. Flow nozzle
- 36. Turbine meters multi-bladed wheel mounted in a pipe along an axis parallel
- 37. Electromagnetic flowmeter EM flowmeters are limited to measuring the volume flow rate of electrically conductive fluids.
- 38. Under Faraday's law of induction, moving conductive liquids inside of a magnetic field generates an electromotive force (voltage) in which the pipe inner diameter, magnetic field strength, and average flow velocity are all proportional. In other words, the flow velocity of liquid moving in a magnetic field is converted into electricity. (E is proportional to V × B × D)
- 40. Doppler shift ultrasonic flowmeter Scattering elements (solid particles, gas bubbles or eddies) within the flowing fluid, deflect the ultrasonic energy output from the transmitter such that it enters the receiver. The scattering elements cause a frequency shift between the transmitted and reflected ultrasonic energy, and measurement of this shift enables the fluid velocity to be inferred.