MPXM2102AS datasheet(512/670 Pages) MOTOROLA

512 / 670 page

AN1573

3–366

Motorola Sensor Device Data

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should be avoided, both upstream and downstream. These locations include any obstruction or change such as valves, elbows,

flow splits, pumps, fans, etc. To increase the accuracy of pressure measurement in a dynamic system, allow at least 10 pipe /

duct diameters downstream of any change or obstruction and at least 2 pipe / duct diameters upstream. In addition the pipe / duct

diameter should be much larger than the diameter of the Pitot tube. The pipe / duct diameter should be at least 30 times the Pitot

tube diameter. Flow straighteners can also be used to minimize any variations in the direction of the flow. Also, when using a Pitot

tube, it is recommended that the static pressure tap be aligned in the same plane as the total pressure tap. On the Pitot–static tube,

the difference in location is assumed to be negligible.

Flow–through pipes and ducts will result in a velocity field and dynamic pressure field that are non–uniform. At the wall of any

duct or pipe there exists a no–slip boundary due to friction. This means that at the wall itself the velocity of the fluid is zero. Figure

5 shows an imaginary velocity distribution in a duct. The shape of the distribution will depend on the fluid conditions, system flow

and pressure. In order to accurately determine the average dynamic pressure across a duct section, a series of total pressure

readings must be taken across the duct. These pressure measurements should be taken at different radii and clock positions

across the cross section of a round duct or at various width and height locations for a rectangular duct. Once this characterization

has been performed for the duct , a correlation can be easily made between the total pressure measurement at the center of the

duct relative to the average duct total pressure. This technique is also used to determine the velocity profile within the duct.

Velocity

Distribution

Static Pressure Tap

Pitot Tube

(A) Static Pressure

Probe

(B) Total Pressure

Pitot Tube

and Total Pressure Pitot Tube

(Pitot–Static Tube)

Flow

Figure 5. Static and Total Pressure Measurements Within a Dynamic Fluid System.

Figure 6. Types of Pressure Probes

Freescale Semiconductor, Inc.

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Go to: www.freescale.com

MPXM2102AS Datasheet(PDF) 512 Page - Motorola, Inc