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ME 322: InstrumentationLecture 16
February 25, 2015
Professor Miles Greiner
Lab 6 calculations (Excel demo)
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
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?
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
IPIG
+-
Variable SpeedBlower Pipe
PlexiglasTube
Pitot-Static Probe VC
Venturi TubeQ
Atm40 in WC
3 in WC 40 in WC
BarometerPATM TATM
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
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 41 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
Consistency Check
• For a given volume flow rate Q– VS = Q/A
– VP = 2VS
• What area should we use?– APipe or ATube
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
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
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%)