E1C09 09/14/2010
15:4:52 Page 375
Chapter 9
Pressure and Velocity Measurements
9.1 INTRODUCTION
This chapter introduces methods to measure the pressure and the velocity within fluids. Instruments
and procedures for establishing known values of pressure for calibration purposes, as well as various
types of transducers for pressure measurement, are discussed. Pressure is measured in static systems
and in moving fluid systems. We also discuss well-established methods measuring the local and fullfield velocity within a moving fluid. Finally, we present practical considerations, including common
error sources, for pressure and velocity measurements. Although there are various practical test
standards for pressure, many of which are applied to a specific application or measuring device, the
American Society of Mechanical Engineers’ Performance Test Code (ASME PTC) 19.2 provides an
overview of basic pressure concepts and measuring instruments that has become the accepted
standard (1).
Upon completion of this chapter, the reader will be able to
explain absolute and gauge pressure concepts and describe the working standards that
measure pressure directly,
explain the physical principles underlying mechanical pressure measurements and the various
types of transducers used to measure pressure,
explain pressure concepts related to static systems or with moving fluids,
analyze the dynamic behavior of pressure system response due to transmission line effects,
and
explain the physical principles underlying various velocity measurement methods and their
practical use.
9.2 PRESSURE CONCEPTS
Pressure represents a contact force per unit area. It acts inwardly, and normally to a surface. To better
understand the origin and nature of pressure, consider the measurement of pressure at the wall of a
vessel containing an ideal gas. As a gas molecule with some amount of momentum collides with this
solid boundary, it rebounds off in a different direction. From Newton’s second law, we know that this
change in linear momentum of the molecule produces an equal but opposite (normal, inward) force
on the boundary. It is the net effect of these collisions averaged over brief instants in time that yields
the pressure sensed at the boundary surface. Because there are so many molecules per unit volume
375
15:4:52 Page 375
Chapter 9
Pressure and Velocity Measurements
9.1 INTRODUCTION
This chapter introduces methods to measure the pressure and the velocity within fluids. Instruments
and procedures for establishing known values of pressure for calibration purposes, as well as various
types of transducers for pressure measurement, are discussed. Pressure is measured in static systems
and in moving fluid systems. We also discuss well-established methods measuring the local and fullfield velocity within a moving fluid. Finally, we present practical considerations, including common
error sources, for pressure and velocity measurements. Although there are various practical test
standards for pressure, many of which are applied to a specific application or measuring device, the
American Society of Mechanical Engineers’ Performance Test Code (ASME PTC) 19.2 provides an
overview of basic pressure concepts and measuring instruments that has become the accepted
standard (1).
Upon completion of this chapter, the reader will be able to
explain absolute and gauge pressure concepts and describe the working standards that
measure pressure directly,
explain the physical principles underlying mechanical pressure measurements and the various
types of transducers used to measure pressure,
explain pressure concepts related to static systems or with moving fluids,
analyze the dynamic behavior of pressure system response due to transmission line effects,
and
explain the physical principles underlying various velocity measurement methods and their
practical use.
9.2 PRESSURE CONCEPTS
Pressure represents a contact force per unit area. It acts inwardly, and normally to a surface. To better
understand the origin and nature of pressure, consider the measurement of pressure at the wall of a
vessel containing an ideal gas. As a gas molecule with some amount of momentum collides with this
solid boundary, it rebounds off in a different direction. From Newton’s second law, we know that this
change in linear momentum of the molecule produces an equal but opposite (normal, inward) force
on the boundary. It is the net effect of these collisions averaged over brief instants in time that yields
the pressure sensed at the boundary surface. Because there are so many molecules per unit volume
375