ME 322: InstrumentationLecture 16

February 25, 2015Professor 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

IP IG

+-

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 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

Consistency Check

• For a given volume flow rate Q– VS = Q/A– VP = 2VS

• What area should we use?– APipe or ATube

Measured Results

• Determine speed and flow rate uncertainty for a range of blower speeds

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%)

Static Pressure Uncertainty

• PStat = PATM – PG (Linear Sum?)

Transmitter Pressures

3” or 40” WC

FS Wρ

Pitot 3” WC 1.9 PaVenturi

and Gage

40” WC 25 Pa

Gas Pressure and Density