MPXM2102AS datasheet(509/670 Pages) MOTOROLA

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AN1573

Understanding Pressure and Pressure

Measurement

Prepared by: David Heeley

Systems and Applications Engineering

Motorola Semiconductor Products Sector

Sensor Products Division

Phoenix, Arizona

Introduction

Fluid systems, pressure and pressure measurements are extremely complex. The typical college curriculum for Mechanical

Engineers includes at least two semesters in fluid mechanics. This paper will define and explain the basic concepts of fluid

mechanics in terms that are easily understood while maintaining the necessary technical accuracy and level of detail.

Pressure and Pressure Measurement

What is fluid pressure? Fluid pressure can be defined as the measure of force per–unit–area exerted by a fluid, acting

perpendicularly to any surface it contacts (a fluid can be either a gas or a liquid, fluid and liquid are not synonymous). The standard

SI unit for pressure measurement is the Pascal (Pa) which is equivalent to one Newton per square meter (N/m2) or the KiloPascal

(kPa) where 1 kPa = 1000 Pa. In the English system, pressure is usually expressed in pounds per square inch (psi). Pressure can

be expressed in many different units including in terms of a height of a column of liquid. The table below lists commonly used units

of pressure measurement and the conversion between the units.

kPa

mm Hg

millibar

in H2O

PSI

1 atm

101.325

760.000

1013.25

406.795

14.6960

1 kPa

1.000

7.50062

10.000

4.01475

0.145038

1 mm Hg

0.133322

1.000

1.33322

0.535257

0.0193368

1 millibar

0.1000

0.750062

1.000

0.401475

0.0145038

1 in H2O

0.249081

1.86826

2.49081

1.000

0.0361

1 PSI

6.89473

51.7148

68.9473

27.6807

1.000

1 mm H2O

0.009806

0.07355

9.8 x 10–8

0.03937

0.0014223

Figure 1. Conversion Table for Common Units of Pressure

Pressure measurements can be divided into three different categories:

absolute pressure, gage pressure and differential

pressure. Absolute pressure refers to the absolute value of the force per–unit–area exerted on a surface by a fluid. Therefore the

absolute pressure is the difference between the pressure at a given point in a fluid and the absolute zero of pressure or a perfect

vacuum.

Gage pressure is the measurement of the difference between the absolute pressure and the local atmospheric pressure.

Local atmospheric pressure can vary depending on ambient temperature, altitude and local weather conditions. The U.S. standard

atmospheric pressure at sea level and 59

°F (20°C) is 14.696 pounds per square inch absolute (psia) or 101.325 kPa absolute

(abs). When referring to pressure measurement, it is critical to specify what reference the pressure is related to. In the English

system of units, measurement relating the pressure to a reference is accomplished by specifying pressure in terms of pounds per

square inch absolute (psia) or pounds per square inch gage (psig). For other units of measure it is important to specify gage or

absolute. The abbreviation ‘abs’ refers to an absolute measurement. A gage pressure by convention is always positive. A ‘negative’

gage pressure is defined as vacuum. Vacuum is the measurement of the amount by which the local atmospheric pressure exceeds

the absolute pressure. A perfect vacuum is zero absolute pressure. Figure 2 shows the relationship between absolute, gage

pressure and vacuum.

Differential pressure is simply the measurement of one unknown pressure with reference to another

unknown pressure. The pressure measured is the difference between the two unknown pressures. This type of pressure

measurement is commonly used to measure the pressure drop in a fluid system. Since a differential pressure is a measure of one

pressure referenced to another, it is not necessary to specify a pressure reference. For the English system of units this could simply

be psi and for the SI system it could be kPa.

MOTOROLA

SEMICONDUCTOR APPLICATION NOTE

Freescale Semiconductor, Inc.

For More Information On This Product,

Go to: www.freescale.com

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