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Pressure reference instruments
• McLeod gauge• Trapped gas in capillary (isothermal
compression)• Capillary force, Boyle’s law and gh of
Hg
• Barometer• Inverted tube in a bath of fluid used to
measure atmosphere• Rise of fluid (usually Hg low vapor
pressure) gives the pressure• Needs to be corrected for temperature
effects
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Pressure reference instruments
• Manometer• Height difference gh gives difference in pressure, P
• Inclined manometer• Low pressure application
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Known!
Less relative error
These techniques suited to static (or slowly varying) measurements and not necessarily easily integrated into a system
Pressure transducers
• Actually a sensor & transducer
• Elastic element deforms or deflects under pressure (sensor)
• Secondary element converts elastic deflection to an observable signal (transducer) voltage, mechanical rotation of a pointer etc
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Bourdon Tube
• Mechanical dial gauge
• Deflection of tube due to different internal and external pressures
• Measurements from 104 to 109 Pa with error generally 0.5 – 2%
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Diaphragm
• Sensing element: Thin, elastic, circular plate supported along its circumference pressure difference across membrane causes a deflection
• Membranes can be metal, non-metal, plastic, neoprene etcmaterial selection determined by pressure range and fluid it is contact with
• Have good linearity and resolution• Can be designed to have low mass & stiffness gives a
wide frequency response, high natural frequency, small damping ratio and short rise/settling times
• Well suited for both static and transient measurements!
• Need to add a 2nd element to translate the deflection into a usable signal (transducer)
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Diaphragm
• Transducer options:• Strain gauge resistance
depends on applied strain
• Capacitance fixed metal plate forms a capacitor. Capacitance varies with deflection and can be measured through a change in voltage
• Piezoelectric deformation of a piezoelectric crystal induces a surface charge which can be measured
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Pressure calibration
• Static calibration comparison with reference pressure measurement techniques (gh, dead weight tester etc)
• Dynamic calibration rise time determined from step change in P (shock tube)
• Frequency response found from period pressure input (eg. reciprocating piston cylinder)
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Pressure measurement in moving Fluids
• 2 contributions static pressure and dynamic pressure (ie. additional pressure due to fluid flow)
• We can measure the static pressure with pressure taps to flow
• We can measure total pressure by inserting a tube and stopping the flow (Pitot tube)
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Pitot tube
• If we connect the static pressure and total pressure tubes together, we form the basis of a Pitot static tube (Prandtl tube) way to measure velocity!
• From Bernoulli, we know that dynamic pressure is:
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2
1
2
1
1
2
and 2
2
D
T D S S
T S
UP
UP P P P
P PU
So a Pitot-static tube provides a pointwise means to measure velocity
Velocity Measurement Techniques
• Two broad types:• Local (pointwise) velocity profiles
• Global flow rate vs P physics can be inferred
• Pointwise velocity measurements:• Pitot-static tube
• Thermal anemometry
• Doppler anemometry
• Global velocity measurements• PIV
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Pointwise measurements can be determined at multiple locations to create a velocity field but this is rather tedious particularly if you want well a well resolved velocity field
Velocity Measurement Techniques
Thermal anemometry
• Basic principle measure heat transfer out of a fin
• Two types hot wire and hot film
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Velocity Measurement Techniques
Doppler anemometry
• Basic principle Doppler effect think radar gun for speed checking
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Velocity Measurement Techniques
Particle image velocimetry
• Expensive, technically advanced
• But full-field, essentially instantaneous
• Only suitable for systems that have “optical access” and particles to scatter the light!
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Things to consider
• Spatial resolution
• Velocity range
• Sensitivity (K)
• Dynamic velocity or static?
• Probe access (blockage, optical access etc)
• Operating environment (hostile, flammable?)
• Calibration
• Cost
• Ease of use
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