20-06-2013, 03:24 PM
Pressure Measurements
Pressure Measurements.ppt (Size: 1.76 MB / Downloads: 56)
Pressure definition
Pressure is the action of one force against another over, a surface. The pressure P of a force F distributed over an area A is defined as:
P = F/A
Static, dynamic, and impact pressures
Static pressure is the pressure of fluids or gases that are stationary or not in motion.
Dynamic pressure is the pressure exerted by a fluid or gas when it impacts on a surface or an object due to its motion or flow. In Fig., the dynamic pressure is (B − A).
Impact pressure (total pressure) is the sum of the static and dynamic pressures on a surface or object. Point B in Fig. depicts the impact pressure.
Definition Of Pressure
Absolute pressure
The pressure is referenced to zero absolute pressure and has units of
psia. Absolute pressure can only have a positive value.
Gauge pressure
The pressure is referenced to atmospheric pressure and by convention
is measured in the positive direction, i.e. 7 psig.
Vacuum pressure
The pressure is referenced to atmospheric pressure and by convention
is measured in the negative direction, i.e. -50 mm Hg.
Manometer
With both legs of a U-tube
manometer open to the
atmosphere or subjected to the
same pressure, the liquid
maintains the same level in
each leg, establishing a zero
reference.
Bellows Resistance Transducer
Bellows or a bourdon tube with a variable resistor.
Bellow expand or contract causes the attached slider to move along the slidewire.
This increase or decrees the resistance.
Thus indicating an increase or decrease in pressure.
Piezoelectric
When pressure, force or acceleration is applied to a quartz crystal, a charge is developed across the crystal that is proportional to the force applied.
Piezoelectric devices can further be classified according to whether the crystal’s electrostatic charge, its resistivity, or its resonant frequency electrostatic charge is measured.
Depending on which phenomenon is used, the crystal sensor can be called electrostatic, piezoresistive, or resonant.
Piezoelectricity
displacement of electrical charge due to the deflection of the lattice in a naturally piezoelectric quartz crystal
The larger circles represent silicon atoms, while the smaller ones represent oxygen.
Quartz crystals is one of the most stable piezoelectric material
Pressure Sensor
A typical Quartz crystal sensor with inbuilt micro-electric circuitry and a diaphragm.
These sensors measure dynamic pressures, and are not generally used for static pressure sensing.
Proper and accurate alignment of the sensor is very important for higher sensitivity.
Sensors used in high temperature conditions(e.g. combustion chamber of an engine) use either recess mounting, baffled diaphragm or thermal protection coatings to reduce negative signal effects.