Flow Sensors. - mass flow rate - volume flow rate - velocity. - stream line parabolic velocity profile - turbulent vortices. Methods of measurement

Transcription

1 Flow Sensors Flow - mass flow rate - volume flow rate - velocity Types of flow - stream line parabolic velocity profile - turbulent vortices Methods of measurement - direct: positive displacement (batch sensors, metering pumps) - indirect: measurement of velocity or kinetic energy

2 Flow Sensors Classification: a) volume flow rate Q V t Q v =, m = V t ρ b) mass flow rate Q m = m t c) velocity Qv = vs, Qm = vsρ

3 Flow Sensors Volume flow rate sensors: - rotameters (float) - batch/oval gear - velocity: - turbine, paddle wheel - vortices - electromagnetic - ultrasonic - with moving marks - obstruction devices Mass flow rate sensors: - thermal - Coriolis force

4 Flowmeters with float Rotameter Gravity Float (buoy) Float acts as force balance indicator Glass tube Flow viscosity insensitive =>sharp edges on float

5 Velocity based flowmeters Turbine flowmeter linearity 0 1% threshold 2 3% range

6 Velocity based flowmeters Turbine flowmeters frequency of pulses f proportional to velocity: f = Lower limit of accuracy: 3 5% linearity: 0,1% KQv K constant of flowmeter QV volume flow Criterion of nonlinearity: Qv ω D r 3 = f ω r D η 2 D diameter of turbine pipe η viskoziy of fluid lin.dependence of ang, velocity of rotor ω r on velocity of flow v drop of pulse amplitude for low v (not in Hall sensor)

7 Paddle-wheel sensor + cheaper - less precise Velocity based flowmeters

8 Velocity based flowmeters

9 Velocity based flowmeters Vortex Shedding flowmeter Karman vortices f = Sr a v Bluff body f frequency of vortices A characteristic dimension of obstacle Sr Strouhal number (char. for certain shape of obstacles) Detection of vortices: thermoanemometers ultrasonic detectors pressure detectors Accuracy ~ 1%

10 Marking flowmeters Mark - conductive (injection of electrolyte to liquid) - optical (injection of colouring agent) -thermal - ionisation (admixture of radioisotope) Principle: measurement of time interval of mark transit between two points in direction of liquid flow Correlation based velocity measurement

11 Ultrasonic flowmeters Pulse ultrasonic flowmeter is based on addition of vectors velocity of liquid flow and velocity of ultrasound propagation. The measured value is time of propagation of pulse from transmmiter to receiver. v = L t2 t 2cosα t t t 1 time interval of propagation from (V 2,P 2 ) to (V 1,P 1 ) t 2 time interval of propagation from (V 1,P 1 )tok (V 2,P 2 )

12 Ultrasonic flowmeters Doppler type of ultrasonic flowmeter (non wetted type) Works in continuos wave mode CW (not in pulse mode) Similarly to radar trafic speed measurement measure Doppler shift of frequency Principle: reflection of ultrasonic wave from bubbles or dispersed particles

13 Induction (electro-magnetic) flowmeters analogy to Hall effect Usual accuracy: through flow type 0,2%, immersion type 2%

14 Induction (magnetic) flowmeters Construction of flowmeter with saddlebacked coils

15 Induction (magnetic) flowmeters Immersion type

16 Sensors of flow with obstruction devices Pressure drop (pressure difference) on obstructions devices (orifice,flow nozzle, Venturi tube) Q v 2 πd p1 p2 = αε 2 4 ρ 1 p d = ρ v 2 2 Q v volume, α expansion coefficient ε diameter Accuracy 2% (0,5%)

17 Sensors with conversion of flow to deformation (drag-force flowmeters) On the target immersed in the flowing medium acts drag - force F d F d = C d S ρ v 2 2 C d constant of the target S crossection area ρ density of liquid v velocity Accuracy several % good dynamic response - resonant frequency up to 200 Hz

18 Oval gear flowmeters, metering pumps Badgermeter co. Used for balance (audit) flowmetering 1 dm 3 /h 10 3 dm 3 /h

19 Thermal mass flowrate sensors (flowmeters) Exchange of heat between source and surroundings (fluid) - measurement of cooling of heat source (thermoanemometer) - measurement of warming up of fluid Thermoanemometer 2 ways of operation: -constant temperature of wire (feedback- excellent dynamic response) -constant current

20 Thermal sensor of mass flowrate Mode of operation of thermoanemometers -const. current (change of flow velocity => change of temperature => => change of resistance => bridge not balanced) -const.temperature of wire (bridge balanced for maximum flow, drop of v => less cooling effect => => drop of heating current) Output current 2 i = a + b Q m a <= transfer of heat to surroundings b <= phys. properties of fluid

21 Thermal sensor of mass flowrate Differential thermoanemometer Response to the step change of velocity 1 constant current 2 constant temperature

22 Thermal sensor of mass flowrate Differential thermoanemometer At v = 0...R 1 =R 2 at v > 0 cooling R 1 and warming up R 2 (<= heat from R H ) Suitable for MEMS increased sensitivity, elimination of temperature influence, suitable for small flowrates (10-4 mm 3.s -1 )

23 Differential sensor Thermal sensor of mass flowrate Thomas principle Bridge evaluates the temperature difference θ 1 -θ 2 measured by S 1 and S 2 ( ϑ2 ϑ Qm = A 1) P A constant C p specific heat capacity of fluid P Q thermal flux from heating windings T sensor of small mass flowrate ( shunt ) q

24 Coriolis flowmeter Coriolis force F c is perpendicular to the axis of rotation and direction of movement F C depends on - angular velocity f (ot/s) - mass of the body m (kg) - velocity of body w (m/s) F C = 2 m (w ω) = 4π m w f

25 Coriolis flowmeter F C l = 2vω m = 2 ω m = 2Q t ω l Tube filled by liquid flowing with velocity w: when rotating around axis z Coriolis force F c acts on liquid F c is perpendicular to the axis of rotation and direction of flow and has a tendency to bow the tube m

26 Coriolis flowmeter One of rarely occuring principles of direct mass flowrate measurement Coriolis flowmeter = type with U tube Magnetic force F m causes vibrations of tube around the axis ω. F c produces twisting of tube.!sediments in U tube -> linear tube M t = 2dF = 4dbωQ Q m C m

27 Level sensors Analogue output Level sensors Binary output (level switch) Liquids only Slurry Powders, granules

28 Level sensors Many principles, but only a few really massively used 90% of applications just 4 types: Pressure / differential pressure Float Ultrasound Radar

29 Pressure Open vessel Closed vessel => pressurized vapours above level

30 Pressure bubbler Pressure sensor not in contact with liquid

31 Float + reed switch (~analog)

32 Float + reed switch (binary)

33 Float + flag

34 Float + weight (Archimedes)

35 Float + Magnetostrictive Balluff

36 Ultrasonic (time of flight) Beam width can be critical in narrow vessels

37 Ultrasonic level switch Transmission from source to detector changes when water fills the space

38 Radar Measurement: TDR (time domain radar) FMCW (frequency-modulated, continuous wave) Arrangement: Antenna (dish or conical) Waveguide (+ better signal)

39 Radar TDR

40 Radar FMCW

41 Radar antennae - parabolic Tanker (crude oil) radar sensors - SAAB

42 Radar antennae - cone Tanker (crude oil) radar sensors - SAAB

43 Radar guided wave

44 Special sensors less frequently used Tuning fork Yo-yo Paddle wheel Optical Conductive Dip stick

45 Tuning fork

46 Yo-yo

47 Motor must survive static (jammed) operation Paddle wheel

48 Optical switch Braking the condition of total reflection

49 Conductive

50 Dip stick Normal dip stick in every car (oil level checking) Here: a high-tech dip stick