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Motorola Sensor Device Data
For More Information On This Product,
Go to: www.freescale.com
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
9.8 x 10–8
27.6807
1.000
1 mm H2O
0.009806
0.07355
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 pressurerefers 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 pressureis 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.
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