MPXM2102AS datasheet(511/670 Pages) MOTOROLA

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Motorola Sensor Device Data

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Figure 3. Continuous Fluid System

Figure 4. Pressure Measurement at a Depth in a Liquid

Dynamic Pressure Systems

Dynamic pressure systems are more complex than static systems and can be more difficult to measure. In a dynamic system,

pressure typically is defined using three different terms. The first pressure we can measure is

static pressure. This pressure is the

same as the static pressure that is measured in a static system. Static pressure is independent of the fluid movement or flow. As

with a static system the static pressure acts equally in all directions. The second type of pressure is what is referred to as the

dynamic pressure. This pressure term is associated with the velocity or the flow of the fluid. The third pressure is total pressure

and is simply the static pressure plus the dynamic pressure.

Steady–State Dynamic Systems

Care must be taken when measuring dynamic system pressures. For a dynamic system, under steady–state conditions,

accurate static pressures may be measured by tapping into the fluid stream perpendicular to the fluid flow. For a dynamic system,

steady–state conditions are defined as no change in the system flow conditions: pressure, flow rate, etc. Figure 5 illustrates a

dynamic system with a fluid flowing through a pipe or duct. In this example a static pressure tap is located in the duct wall at point

A. The tube inserted into the flow is called a Pitot tube. The Pitot tube measures the total pressure at point B in the system. The

total pressure measured at this point is referred to as the

stagnation pressure. The stagnation pressure is the value obtained when

a flowing fluid is decelerated to zero velocity in an isentropic (frictionless) process. This process converts all of the energy from

the flowing fluid into a pressure that can be measured. The stagnation or total pressure is the static pressure plus the dynamic

pressure. It is very difficult to accurately measure dynamic pressures. When dynamic pressure measurement is desired, the total

and static pressures are measured and then subtracted to obtain the dynamic pressure. Dynamic pressures can be used to

determine the fluid velocities and flow rates in dynamic systems.

When measuring dynamic system pressures, care must be taken to ensure accuracy. For static pressure measurements, the

pressure tap location should be chosen so that the measurement is not influenced by the fluid flow. Typically, taps are located

perpendicular to the flow field. In Figure 5, the static pressure tap at point A is in the wall of the duct and perpendicular to the flow

field. In Figures 6a and 6c the static taps (point A) in the pressure probes are also perpendicular to the flow field. These examples

show the most common type of static pressure taps, however there are many different static pressure tap options. For total or

stagnation pressure measurements, it is important that the Pitot or impact tube be aligned parallel to the flow field with the tip

of the tube pointing directly into the flow. In Figures 6b and 6c, the Pitot tube is aligned parallel with the flow, with the tube opening

pointing directly into the flow. Although the static pressure is independent of direction, the dynamic pressure is a vector quantity

which depends on both magnitude and direction for the total measured value. If the Pitot tube is misaligned with the flow, accuracy

of the total pressure measurement may suffer. In addition, for accurate pressure measurements the pressure tap holes and probes

must be smooth and free from any burrs or obstructions that could cause disturbances in the flow. The location of the pressure

taps and probes, static and total, must also be selected carefully. Any location in the system where the flow field may be disturbed

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