ME 322: Instrumentation Lecture 11 February 11, 2015 Professor Miles Greiner Pitot probe operation, non-linear transfer function, fluid density, uncertainty, example
Announcement/Reminders Friday: HW 4 due – Monday: President’s Day Holiday – Wednesday: HW 5 due, Midterm Review – Next Friday: Midterm I Thanks for helping at the Science Olympiad. Ms. Davis was very appreciative, and you will receive extra credit. Evening with Industry (UNR Society of Women Engineers) – Wednesday, February 25, 2015 (two weeks) – Networking 5:30pm-6:30pm (business attire) – Dinner and Keynote Speaker 6:30-8pm – $25 (or free tickets from ME Office, first come first served) – If you pickup a free ticket, please show-up How is Lab 4 going?
Lab 5 Sample Report
Fluid Speed
Pressure Method for Measurement When an obstruction is placed in a flow it causes the fluid to decelerate and stop, and increases the pressure – P S = Static Pressure Measured by an observer traveling with the fluid, or on a flat surface parallel to the flow – P T = Total (or Stagnation) Pressure Measured at stagnation point, where V = 0 Pitot probes are designed to transmit P T Pitot-Static probes transmit both P T (inner tube) and P S (outer tube) V PSPS PSPS P T > P S
Stagnation Point Along the blue line, the fluid decelerates and stops (at the Stagnation Point) Viscosity does not play an important role in this process Problem: If the flow changes so the stagnation point is not at the opening, then the probe will not transmit P T. V, Speed P S, Static Pressure , density V = 0, P = P T
Related Devices Boundary Layer probes transmit P T near walls Keil probes transmit P T even when flow direction changes Aircraft probes measure air speed Pitot-Static probes are used in Wind Tunnel Labs (6 and 11)
Bernoulli Equation ReadingMeasurand V, P S, V = 0, P T
Ideal (inviscid) Transfer Function wPwP wVwV
To use Pitot Probe
Uncertainty in V
How to Find Density
Water Properties (Appendix B of Text)
British Units
Air (at 1 ATM, not other pressures)
Example A Pitot-static probe is used to measure air speed in a wind tunnel. If the air temperature and (static) pressure are T = 27±1 ° C (95%) and P = 86±2 kPa (95%), and the difference between the total and static pressures is P = 55±3 Pa (95%), what is the confidence interval for the speed? – Solution (first identify, then do) ID: – Fluid, – Do all uncertainties have the same certainty-level? – Likely or Maximum Uncertainty Do: on white board
Fluid Flow Rates A dA V,
Many Flow Rate Measurement Devices Each relies on different phenomena When choosing, consider – cost, stability of calibration, imprecision, dynamic response, flow resistance Rotameters (variable area) Turbine Vortex (Lab 11) Laminar Flow Coriolis Variable Area
Pitot-Static Probe Concentric tubes – Port for inner tube at stagnation point, measures total (or stagnation) pressure, P T (pressure observed after the flow is stopped) – Port for outer tube at side, measures static pressure, P S (observed when moving with the flow) Use a pressure transmitter to read – P = P T – P S Measurand: U V, Speed P S, Static Pressure , density V = 0 P = P Total = P T P = P S P = P T