Lecture 11

Pressure Measurements

Techniques to measure pressure.

Pressure Measurement

• PressurePressure = Force/Area

Units of force per unit area

• Pressure measurements.– All pressure measurements are relative

measurements.– The measurement of a pressure at a point is relative

to, or referred to, a reference pressure.

Pressure References

• Absolute Pressure:– Measured relative to a zero pressure or a

perfect vacuum.

• Gauge Pressure:– Measured relative to the local ambient

pressure.

• Differential Pressure:– Measured pressure between two point.

Barometer

• Aneroid from Greek aneros – “not wet”

Mercury Barometer

• Measurement of Atmospheric Pressure

Manometers

• Manometers are one of the oldest pressure measuring devices.

How could you make a U-tube manometer more sensitive?

• Change Manometer fluid.

• Incline the manometer.

• Combine inclined and straight sections.

Inclined Manometer

Pressure Transducers

• What is a pressure transducer?– A pressure transducer is a device that

converts a pressure signal to an electric signal.

• What type of devices can be used to create a pressure transducer?

• Strain gage devices• Capacitance devices• Piezoelectric devices

Pressure TransducerStrain gage device

Strain gage is attached to a flexible membrane. When the membrane is deflected the resistance in the strain gage changes.

Pressure Transducer Capacitance device

The capacitance between two plates changes with the distance between the plates.

Pressure Transducer Piezoelectric device

Piezoelectric device take advantage of the electrical properties of crystals such as quartz. These crystals generate an electrical charge when they are strained.

Pressure Sensitive Paint

Pressure Probes to Measure Velocity

• Pitot Static Probe

Measuring Velocity Components

• Five hole Pressure Probe

How would you calibrate this probe?

Pressure Models

• Airfoils, wings and complete airplane models equipped with pressure ports can be used to determine model loads and load distribution.

• Pressure models provide more detail on how the loads are generated than a force balance.

• Disadvantage is pressure measurement of model forces is more time consuming.

How to you create a pressure model?

• Suppose you have designed a new airfoil section and you want to determine its aerodynamic performance and its pressure distribution, how would make such a model?