me 322: instrumentation lecture 16
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ME 322: Instrumentation Lecture 16. February 26, 2014 Professor Miles Greiner. Announcements/Reminders. HW 6 due Friday No lab this week Career Fair, Tomorrow , February 27, 2014 Internships Prepare for permanent employment next year www.unr.edu/engineering/careerfair - PowerPoint PPT PresentationTRANSCRIPT
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ME 322: InstrumentationLecture 16
February 25, 2015Professor Miles Greiner
Lab 6 calculations (Excel demo)
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Announcements/Reminders
• HW 6 due Friday• Career Fair, Tomorrow, February 26, 2015
– Internships– Prepare for permanent employment next year– www.unr.edu/engineering/careerfair– Differential Fees pay for this
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Lab 6 Air Volume Flow Rate and Centerline Speed in a Wind Tunnel
• Plexiglas Tube and Schedule-40 Pipe have different diameters• Control flow rate using a variable speed blower
– Cover blower exit for very low speeds• For a range of flow rates, measure
– Volume flow Q rate using a Presso Venturi Tube (in pipe)– Centerline speed VC using a Pitot-Static Tube (in Plexiglas tube)
• For both measure pressures difference using calibrated transmitters/digital multimeters
• Both VC and Q increase with blower flow rate– Is VS < VC < VP?
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Instrument Schematic
• Measure atmospheric conditions PATM and TATM – Using hand-held digital-barometer – = 0.5 kPa, = 1°C (95%?)
• Need PStat to find pressure– Is PStat < = or > than PATM?– Use 40-in-WC transmitter to find Gage Pressure PG = PATM – PS – PS = PATM - PG
• To measure Pitot-Static tube pressure difference PP – Use 3-in-WC transmitter
• To measure Venturi tube pressure difference PV – Use 40-in-WC transmitter
DTubeDPipe
PP PG
PV Static
Total+ +--IV
IP IG
+-
Variable SpeedBlower Pipe
PlexiglasTube
Pitot-Static Probe VC
Venturi TubeQ
Atm40 in WC
3 in WC 40 in WC
BarometerPATM TATM
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Summary• Before Experiment• Measure tube diameter• Calculate transmitter uncertainties• Use hand held barometer to measure
– PATM
– TATM • °C
Tatm Patm Dpipe Dtube Apipe Atube K W40-inch W3-inch
[°C] [mbar] [inch] [inch] [m2] [m2] [-] [Pa] [Pa]22 873 2.07 2.25 0.002165 0.002565 0.381 25 1.9
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During Experiment• For each blower setting measure currents and find the values &
uncertainties – Pressure Measurements
• P = rgh = rg(FS)(I – 4mA)/16 mA• WP = 0.0025rg(FS)
– Static Pressure, PStat = WOB (Work on Board)• WOB• WOB
– Air density WOB• WOB
– Volume flow rate WOB• WOB
– Centerline speed WOB• WOB
– Check Pipe Reynolds numbers, = • (300 K)• Venturi calibration (KPresso) is within 2% for 54,000 < < 137,000
Blower Condition
IV
[mA]IP
[mA]IG
[mA]Blower off 4.01 4.02 4
1 8.9 14.9 52 8.59 14.34 4.993 8.33 13.17 4.884 7.74 12.05 4.785 7.15 11.4 4.726 6.74 10.87 4.687 6.39 10.4 4.628 6.09 8.38 4.439 5.59 7.96 4.410 5.09 6.18 4.21
Blower off 4.02 4.02 4.01
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Consistency Check
• For a given volume flow rate Q– VS = Q/A– VP = 2VS
• What area should we use?– APipe or ATube
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Measured Results
• Determine speed and flow rate uncertainty for a range of blower speeds
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Demonstrate Excel Calculations
• Lab 6 Sample Data– http://
wolfweb.unr.edu/homepage/greiner/teaching/MECH322Instrumentation/Labs/Lab%2006%20Fluid%20Flow/Lab%20Index.htm
• Values and uncertainties• Pressure Units• Error Bars
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Pressure Transmitter Uncertainty• Pressure
• = 998.7 kg/m3, g = 9.81 m/s2
• FS = (3 or 40 inch)
• Manufacturer stated uncertainty: 0.25% Full Scale– (95%?)– For FS = 3 inch WC
• PFS = rWghFS = (998.7 kg/m3)(9.81 m/s2) (3 inch) = 746.6 Pa• wP = 0.0025 PFS = 1.9 Pa
– For FS = 40 inch WC• PFS = rWghFS = (998.7 kg/m3)(9.81 m/s2) (40 inch) = 9954 Pa• wP = 0.0025 PFS = 25 Pa
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Static Pressure• PStat = PATM – PG
– Use for , RAir = 0.2870 kPa-m3/kg-K– So want PStat in [kPa]
• Inputs– PATM
• Measure using barometer• = 500 Pa = 0.5 kPa (95%)
– PGAGE • Measure using 40 inch WC gage• = 25 Pa = 0.025 kPa (95%)
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Static Pressure Uncertainty
• PStat = PATM – PG (Linear Sum?)
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Transmitter Pressures
3” or 40” WC
FS Wρ
Pitot 3” WC 1.9 PaVenturi
and Gage
40” WC 25 Pa
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Gas Pressure and Density