ME 322: Instrumentation
Lecture 16
February 24, 2016
Professor Miles Greiner
Lab 6 calculations (Excel demo)
Regional Science Olympiad
• Will be held 8 am to 4 pm Saturday, March 5th 2016
– On campus: SEM, PE and DMS
• ME 322 students who participate in observing and
judging the events for at least two hours (as
reported) will earn 1% extra credit.
• To sign up, contact Rebecca Fisher,
rnfisher@unr.edu, (775) 682-7741
– Today is the last day to sign up
• Details
– You cannot get extra-credit in two courses for the same
work.
– If you sign-up but don’t show-up you will loose 1%!
Announcements/Reminders
• HW 6 due Friday
– NEW: Upload L6PP to WebCampus
– Joe Young will hold office hours today after class (PE 2)
– Marissa will hold a Lab 6 tutorial on Friday
• Time and place by email?
• General Advising Session for Upper Division
Students
– Friday, March 11 from 1:30pm to 2pm in WRB 2030
– Conducted by Dr. Padilla
– For juniors and seniors, or students who are taking 300and 400-level classes in mechanical engineering
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 (and outlet cover)
• 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
• Need air density to calculate both
• Both VC and Q increase with blower flow rate
– Is VS < VC < VP?
Instrument Schematic
Variable Speed
Blower
Pipe
Venturi Tube
Q
Plexiglas
Tube
DTube
DPipe
40 in WC
Pitot-Static Probe
VC
- PV +
IV
Static
Total
+
3 in WC
•
Measure atmospheric conditions PATM and TATM
PP -
•
𝑃
Need 𝜌𝐴𝑖𝑟 = 𝑅 𝑆𝑡𝑎𝑡𝑇, so need 𝑃𝑆𝑡𝑎𝑡
𝐴𝑖𝑟
– Use 40-in-WC transmitter to find Gage Pressure 𝑃𝐺 = 𝑃𝐴𝑇𝑀 − 𝑃𝑆𝑡𝑎𝑡
– 𝑃𝑆𝑡𝑎𝑡 = 𝑃𝐴𝑇𝑀 − 𝑃𝐺
•
𝐼𝐺
To measure Pitot-Static tube pressure difference PP
𝐼𝑃
To measure Venturi tube pressure difference PV
𝐼𝑉
– Use 3-in-WC transmitter
– Use 40-in-WC transmitter
- PG + Atm
IG
IP
– Using hand-held digital-barometer
– 𝑤𝑃𝐴𝑇𝑀 = 0.5 kPa, 𝑤𝑇 = 1°C (95%?)
•
Barometer
PATM
TATM
40 in WC
Summary
•
•
•
•
Before Experiment
Measure tube diameter
Calculate transmitter uncertainties
Use hand held barometer to measure
– 𝑃𝐴𝑇𝑀 , 𝑊𝑃𝐴𝑇𝑀 = 0.5 𝑘𝑃𝑎
– 𝑇𝐴𝑇𝑀 , 𝑊𝑇𝐴𝑇𝑀 = 1°C
Tatm
Patm
Dpipe Dtube
Apipe
[°C] [mbar] [inch] [inch]
[m2]
22
873 2.07 2.25 0.002165
Atube
K
[m2]
0.002565
[-]
0.381
W 40-inch W 3-inch
[Pa]
25
[Pa]
1.9
During Experiment
• For each blower speed measure transmitter currents, and find values & uncertainties
– Transmitter Pressure:
•
•
P = 𝜌𝑊 gh = rg(FS)(I – 4mA)/16 mA, 𝜌𝑊 = 998.7 kg/m3
𝑤𝑃𝑝 = 1.9 𝑃𝑎; 𝑤𝑃𝑣 = 𝑤𝑃𝐺 = 25 𝑃𝑎 𝑎𝑠𝑠𝑢𝑚𝑒 95%
Blower
Condition
Blower off
1
2
3
4
5
6
7
8
9
10
Blower off
– Static Pressure, 𝑃𝑆𝑡𝑎𝑡 = 𝑃𝐴𝑇𝑀 − 𝑃𝐺 (Linear Sum)
•
•
2
𝑊𝑃𝑆𝑡𝑎𝑡
= Work on Board
𝑊𝑃𝑆𝑡𝑎𝑡 = ____ (units!)
– Air density 𝜌𝐴𝑖𝑟 =
•
𝑊𝜌𝐴𝑖𝑟 2
𝜌𝐴𝑖𝑟
𝑃𝑆𝑡𝑎𝑡
;
𝑅𝐴𝑖𝑟 𝑇
RAir = 0.2870 kPa-m3/kg-K
= WOB
– Volume flow rate 𝑄 = 𝐴𝑝𝑖𝑝𝑒 𝐾𝑝𝑟𝑒𝑠𝑠𝑜
•
𝑊𝑄 2
𝑄
= WOB
– Centerline speed 𝑉𝑐 = 𝐶
•
𝑊 𝑉𝑐 2
𝑉𝑐
2𝑃𝑣
𝜌𝐴𝑖𝑟
2𝑃𝑝
𝜌Air
= WOB
– Check Pipe Reynolds numbers, 𝑅𝑒𝑃𝑖𝑝𝑒 =
𝑉𝑃𝑖𝑝𝑒 𝐷𝑃𝑖𝑝𝑒 𝜌𝐴𝑖𝑟
𝜇𝐴𝑖𝑟
=
4𝑄𝜌𝐴𝑖𝑟
𝜋𝐷𝑃𝑖𝑝𝑒 𝜇𝐴𝑖𝑟
𝑁𝑠
•
𝜇𝐴𝑖𝑟 = 1.846𝑥10−5 𝑚2 (300 K)
•
Venturi calibration, KPresso = 0.3810 is within 2% for 54,000 < 𝑅𝑒 < 137,000
IV
[mA]
4.01
8.9
8.59
8.33
7.74
7.15
6.74
6.39
6.09
5.59
5.09
4.02
IP
IG
[mA] [mA]
4.02
4
14.9
5
14.34 4.99
13.17 4.88
12.05 4.78
11.4 4.72
10.87 4.68
10.4 4.62
8.38 4.43
7.96
4.4
6.18 4.21
4.02 4.01
Consistency Check
• For eac volume flow rate 𝑄 (show calculations next time)
– 𝑉𝑆𝑙𝑢𝑔 = 𝑄/𝐴 (APipe or ATube)
– 𝑉𝑃 = 2𝑉𝑆𝑙𝑢𝑔
• 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%20Fl
uid%20Flow/Lab%20Index.htm
• Values and uncertainties
• Pressure Units
• Error Bars
Pressure Transmitter Uncertainty
• Pressure
– 𝑃 = 𝜌𝑊 𝑔ℎ = 𝜌𝑊 𝑔(𝐹𝑆)
𝐼−4 𝑚𝐴
16 𝑚𝐴
• 𝜌𝑊 = 998.7 kg/m3, g = 9.81 m/s2
• FS = (3 or 40 inch)
2.54 𝑐𝑚 1 𝑚
1 𝑖𝑛𝑐ℎ 100 𝑐𝑚
= 0.0762 𝑜𝑟 1.016 𝑚
• Manufacturer stated uncertainty: 0.25% Full Scale
– (95%?)
– For FS = 3 inch WC
• PFS = rWghFS =
2.54 𝑐𝑚
(998.7 kg/m3)(9.81 m/s2) (3 inch)
1 𝑖𝑛𝑐ℎ
• wP = 0.0025 PFS = 1.9 Pa
1𝑚
100 𝑐𝑚
= 746.6 Pa
– For FS = 40 inch WC
• PFS = rWghFS =
kg/m3)(9.81
m/s2)
(998.7
(40
• wP = 0.0025 PFS = 25 Pa
2.54 𝑐𝑚 1 𝑚
inch)
1 𝑖𝑛𝑐ℎ 100 𝑐𝑚
= 9954 Pa
Static Pressure
• PStat = PATM – PG
– Use for 𝜌𝐴𝑖𝑟 =
𝑃𝑆𝑡𝑎𝑡
𝑅𝐴𝑖𝑟 𝑇
, RAir = 0.2870 kPa-m3/kg-K
– So want PStat in [kPa]
• Inputs
– PATM
• Measure using barometer
• 𝑤𝑃𝐴𝑇𝑀 = 500 Pa = 0.5 kPa (95%)
– PGAGE
• Measure using 40 inch WC gage
• 𝑤𝑃𝐺𝐴𝐺𝐸 = 25 Pa = 0.025 kPa (95%)
Static Pressure Uncertainty
• PStat = PATM – PG (Linear Sum?)
– 𝑤𝑃𝑆𝑡𝑎𝑡
2
=
=
2
𝛿𝑃𝑆𝑡𝑎𝑡
2
𝑤𝑖
𝑖=1
𝛿𝑥𝑖
2
2
1𝑤𝑃𝐴𝑇𝑀 + −1𝑤𝑃𝐺
2
= 1 0.5kPa
• 𝑊𝑃𝑆𝑡𝑎𝑡 = 0.5006 𝑘𝑃𝑎
+ −1 0.025kPa
2
Gas Pressure and Density