Pressure measurement Notes | EduRev

: Pressure measurement Notes | EduRev

 Page 1


Objectives_template
file:///G|/optical_measurement/lecture7/7_1.htm[5/7/2012 11:54:06 AM]
 Module 2: Review of Probes and Transducers
 Lecture 7: Pressure measurement
 
The Lecture Contains:
Review of Probes and Transducers
Piezoelectric Pressure Transducers
Static Tube
Pitot Tube
Manometers
Velocity Measurement Using Pitot Tube
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
Page 2


Objectives_template
file:///G|/optical_measurement/lecture7/7_1.htm[5/7/2012 11:54:06 AM]
 Module 2: Review of Probes and Transducers
 Lecture 7: Pressure measurement
 
The Lecture Contains:
Review of Probes and Transducers
Piezoelectric Pressure Transducers
Static Tube
Pitot Tube
Manometers
Velocity Measurement Using Pitot Tube
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
Objectives_template
file:///G|/optical_measurement/lecture7/7_2.htm[5/7/2012 11:54:06 AM]
 Module 2: Review of Probes and Transducers
 Lecture 7: Pressure measurement
 
Piezoelectric Pressure Transducers
Instantaneous fluid pressure acting on a surface can be measured using pressure transducers. These
consist of piezoelectric crystals that get charged when their faces are deformed. The electrical signal
is suitably amplified before the local pressure is measured as voltage. Pressure transducers are
primarily suitable for measurement of pressure fluctuations. Their operating characteristics deteriorate
as the forcing frequency approaches zero. Pressure acting on a surface can also be estimated by
installing strain gauges on it. This is accomplished by determining the change in resistance of the wire
due to longitudinal strain. Fluid pressure can also be measured relative to atmospheric pressure using
a capacitance pick-up. Here the differential pressure changes the capacitor spacing and hence the
capacitance. When this element is placed in an electrical circuit the change in capacitance is
observed as a change in output voltage. The pressure difference across the faces of the capacitor is
developed commonly by a pitot or a static tube. Capacitor pick-ups vary significantly in construction
depending on the method used to fabricate a capacitor element. This could be a thin metal diaphragm
fixed on its boundary or a thin deformable layer on a semi-conductor substrate that forms a part of an
amplifier circuit. Piezoelectric transducers are also used in `lift-drag-moment' balance that directly
measures forces and moments on objects placed in wind tunnels. The design of the balance is
usually complicated by the fact that elaborate mechanisms are needed to transmit the load to
independent piezoelectric crystals.
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
Page 3


Objectives_template
file:///G|/optical_measurement/lecture7/7_1.htm[5/7/2012 11:54:06 AM]
 Module 2: Review of Probes and Transducers
 Lecture 7: Pressure measurement
 
The Lecture Contains:
Review of Probes and Transducers
Piezoelectric Pressure Transducers
Static Tube
Pitot Tube
Manometers
Velocity Measurement Using Pitot Tube
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
Objectives_template
file:///G|/optical_measurement/lecture7/7_2.htm[5/7/2012 11:54:06 AM]
 Module 2: Review of Probes and Transducers
 Lecture 7: Pressure measurement
 
Piezoelectric Pressure Transducers
Instantaneous fluid pressure acting on a surface can be measured using pressure transducers. These
consist of piezoelectric crystals that get charged when their faces are deformed. The electrical signal
is suitably amplified before the local pressure is measured as voltage. Pressure transducers are
primarily suitable for measurement of pressure fluctuations. Their operating characteristics deteriorate
as the forcing frequency approaches zero. Pressure acting on a surface can also be estimated by
installing strain gauges on it. This is accomplished by determining the change in resistance of the wire
due to longitudinal strain. Fluid pressure can also be measured relative to atmospheric pressure using
a capacitance pick-up. Here the differential pressure changes the capacitor spacing and hence the
capacitance. When this element is placed in an electrical circuit the change in capacitance is
observed as a change in output voltage. The pressure difference across the faces of the capacitor is
developed commonly by a pitot or a static tube. Capacitor pick-ups vary significantly in construction
depending on the method used to fabricate a capacitor element. This could be a thin metal diaphragm
fixed on its boundary or a thin deformable layer on a semi-conductor substrate that forms a part of an
amplifier circuit. Piezoelectric transducers are also used in `lift-drag-moment' balance that directly
measures forces and moments on objects placed in wind tunnels. The design of the balance is
usually complicated by the fact that elaborate mechanisms are needed to transmit the load to
independent piezoelectric crystals.
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
Objectives_template
file:///G|/optical_measurement/lecture7/7_3.htm[5/7/2012 11:54:06 AM]
 Module 2: Review of Probes and Transducers
 Lecture 7: Pressure measurement
 
Static Tube
The static tube senses the local static pressure in the flow and is schematically shown in Figure
2.1.The holes on the periphery of the tube should be sufficiently far away from the tip facing the flow
so that the effect of distortion of flow by the probe on the static pressure being measured is
minimized. This distance  is preferably - , where  is the probe diameter
Figure 2.1: Static Tube.
The nose of a static tube is usually conical to streamline its surface and avoid flow separation. Three
equally spaced holes are located on the periphery of the tube and the pressure recorded by the tube
is an average over its circumference.
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
Page 4


Objectives_template
file:///G|/optical_measurement/lecture7/7_1.htm[5/7/2012 11:54:06 AM]
 Module 2: Review of Probes and Transducers
 Lecture 7: Pressure measurement
 
The Lecture Contains:
Review of Probes and Transducers
Piezoelectric Pressure Transducers
Static Tube
Pitot Tube
Manometers
Velocity Measurement Using Pitot Tube
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
Objectives_template
file:///G|/optical_measurement/lecture7/7_2.htm[5/7/2012 11:54:06 AM]
 Module 2: Review of Probes and Transducers
 Lecture 7: Pressure measurement
 
Piezoelectric Pressure Transducers
Instantaneous fluid pressure acting on a surface can be measured using pressure transducers. These
consist of piezoelectric crystals that get charged when their faces are deformed. The electrical signal
is suitably amplified before the local pressure is measured as voltage. Pressure transducers are
primarily suitable for measurement of pressure fluctuations. Their operating characteristics deteriorate
as the forcing frequency approaches zero. Pressure acting on a surface can also be estimated by
installing strain gauges on it. This is accomplished by determining the change in resistance of the wire
due to longitudinal strain. Fluid pressure can also be measured relative to atmospheric pressure using
a capacitance pick-up. Here the differential pressure changes the capacitor spacing and hence the
capacitance. When this element is placed in an electrical circuit the change in capacitance is
observed as a change in output voltage. The pressure difference across the faces of the capacitor is
developed commonly by a pitot or a static tube. Capacitor pick-ups vary significantly in construction
depending on the method used to fabricate a capacitor element. This could be a thin metal diaphragm
fixed on its boundary or a thin deformable layer on a semi-conductor substrate that forms a part of an
amplifier circuit. Piezoelectric transducers are also used in `lift-drag-moment' balance that directly
measures forces and moments on objects placed in wind tunnels. The design of the balance is
usually complicated by the fact that elaborate mechanisms are needed to transmit the load to
independent piezoelectric crystals.
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
Objectives_template
file:///G|/optical_measurement/lecture7/7_3.htm[5/7/2012 11:54:06 AM]
 Module 2: Review of Probes and Transducers
 Lecture 7: Pressure measurement
 
Static Tube
The static tube senses the local static pressure in the flow and is schematically shown in Figure
2.1.The holes on the periphery of the tube should be sufficiently far away from the tip facing the flow
so that the effect of distortion of flow by the probe on the static pressure being measured is
minimized. This distance  is preferably - , where  is the probe diameter
Figure 2.1: Static Tube.
The nose of a static tube is usually conical to streamline its surface and avoid flow separation. Three
equally spaced holes are located on the periphery of the tube and the pressure recorded by the tube
is an average over its circumference.
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
Objectives_template
file:///G|/optical_measurement/lecture7/7_4.htm[5/7/2012 11:54:07 AM]
 Module 2: Review of Probes and Transducers
 Lecture 7: Pressure measurement
 
Pitot Tube
A rapid way to measure gas velocity is to use a pitot tube inserted in flow. The pitot tube (also called
a stagnation tube) consists of a thin tube held parallel to the flow and at the mouth of which fluid
particles come to rest. This tube is typically of the size of a hypodermic needle and does not
significantly alter the flow. In incompressible flow the pitot tube records the total pressure
A second probe with a tap whose axis is normal to the flow measures the static pressure . The
difference between  and  is a measure of velocity. This arrangement is shown in Figure 2.2
Figure 2.2: Total and Static Pressure Measurement.
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
Page 5


Objectives_template
file:///G|/optical_measurement/lecture7/7_1.htm[5/7/2012 11:54:06 AM]
 Module 2: Review of Probes and Transducers
 Lecture 7: Pressure measurement
 
The Lecture Contains:
Review of Probes and Transducers
Piezoelectric Pressure Transducers
Static Tube
Pitot Tube
Manometers
Velocity Measurement Using Pitot Tube
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
Objectives_template
file:///G|/optical_measurement/lecture7/7_2.htm[5/7/2012 11:54:06 AM]
 Module 2: Review of Probes and Transducers
 Lecture 7: Pressure measurement
 
Piezoelectric Pressure Transducers
Instantaneous fluid pressure acting on a surface can be measured using pressure transducers. These
consist of piezoelectric crystals that get charged when their faces are deformed. The electrical signal
is suitably amplified before the local pressure is measured as voltage. Pressure transducers are
primarily suitable for measurement of pressure fluctuations. Their operating characteristics deteriorate
as the forcing frequency approaches zero. Pressure acting on a surface can also be estimated by
installing strain gauges on it. This is accomplished by determining the change in resistance of the wire
due to longitudinal strain. Fluid pressure can also be measured relative to atmospheric pressure using
a capacitance pick-up. Here the differential pressure changes the capacitor spacing and hence the
capacitance. When this element is placed in an electrical circuit the change in capacitance is
observed as a change in output voltage. The pressure difference across the faces of the capacitor is
developed commonly by a pitot or a static tube. Capacitor pick-ups vary significantly in construction
depending on the method used to fabricate a capacitor element. This could be a thin metal diaphragm
fixed on its boundary or a thin deformable layer on a semi-conductor substrate that forms a part of an
amplifier circuit. Piezoelectric transducers are also used in `lift-drag-moment' balance that directly
measures forces and moments on objects placed in wind tunnels. The design of the balance is
usually complicated by the fact that elaborate mechanisms are needed to transmit the load to
independent piezoelectric crystals.
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
Objectives_template
file:///G|/optical_measurement/lecture7/7_3.htm[5/7/2012 11:54:06 AM]
 Module 2: Review of Probes and Transducers
 Lecture 7: Pressure measurement
 
Static Tube
The static tube senses the local static pressure in the flow and is schematically shown in Figure
2.1.The holes on the periphery of the tube should be sufficiently far away from the tip facing the flow
so that the effect of distortion of flow by the probe on the static pressure being measured is
minimized. This distance  is preferably - , where  is the probe diameter
Figure 2.1: Static Tube.
The nose of a static tube is usually conical to streamline its surface and avoid flow separation. Three
equally spaced holes are located on the periphery of the tube and the pressure recorded by the tube
is an average over its circumference.
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
Objectives_template
file:///G|/optical_measurement/lecture7/7_4.htm[5/7/2012 11:54:07 AM]
 Module 2: Review of Probes and Transducers
 Lecture 7: Pressure measurement
 
Pitot Tube
A rapid way to measure gas velocity is to use a pitot tube inserted in flow. The pitot tube (also called
a stagnation tube) consists of a thin tube held parallel to the flow and at the mouth of which fluid
particles come to rest. This tube is typically of the size of a hypodermic needle and does not
significantly alter the flow. In incompressible flow the pitot tube records the total pressure
A second probe with a tap whose axis is normal to the flow measures the static pressure . The
difference between  and  is a measure of velocity. This arrangement is shown in Figure 2.2
Figure 2.2: Total and Static Pressure Measurement.
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
Objectives_template
file:///G|/optical_measurement/lecture7/7_5.htm[5/7/2012 11:54:07 AM]
 Module 2: Review of Probes and Transducers
 Lecture 7: Pressure measurement
 
In applications where  can vary substantially from one location to another,  and  must be
measured close to each other. This is accomplished using a pitot static tube (Figure 2.3). A probe
with a spherical head in which five holes are present at right angles to it is used in the measurement
of three dimensional velocity field (Figure 2.4). This is sometimes called a five-hole probe and can be
used as follows: The probe is rotated along two independent axes till pressure  becomes equal to 
 and  equal to , i.e., the manometer shows a null reading for the pairs of holes  and 
. Under these conditions  is the stagnation pressure of the flow and the probe axis is
parallel to the flow direction. In applications where the probe cannot be rotated, it is common to
calibrate  and  as functions of the magnitude of , local flow velocity and two
angles. Calibration charts for prescribed probe dimensions and hole locations are available and need
not be repeatedly generated. A simpler route is to employ the potential flow solution for flow over the
surface of the sphere, namely
where  is angular position of the point under consideration from the flow axis and is the
stagnation pressure.
Figure 2.3: Pitot-Static Tube.
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
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