MPXM2102AS datasheet(510/670 Pages) MOTOROLA

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Pressure

Local Atmospheric Pressure

Vacuum (Negative Gage)

Absolute

Atmospheric

Gage

Figure 2. Pressure Term Relationships

In addition to the three types of pressure measurement, there are different types of fluid systems and fluid pressures. There are

two types of fluid systems;

static systems and dynamic systems. As the names imply, a static system is one in which the fluid is

at rest and a dynamic system is on in which the fluid is moving.

Static Pressure Systems

The pressure measured in a static system is

static pressure. In the pressure system shown in Figure 3, a uniform static fluid

is continuously distributed with the pressure varying only with vertical distance. The pressure is the same at all points along the

same horizontal plane in the fluid and is independent of the shape of the container. The pressure increases with depth in the fluid

and acts equally in all directions. The increase in pressure at a deeper depth is essentially the effect of the weight of the fluid above

that depth. Figure 4 shows two containers with the same fluid exposed to the same external pressure – P . At any equal depth within

either tank the pressure will be the same . Note that the sides of the large tank are not vertical. The pressure is dependent only

on depth and has nothing to do with the shape of the container. If the working fluid is a gas, the pressure increase in the fluid due

to the height of the fluid is in most cases negligible since the density and therefore the weight of the fluid is much smaller than the

pressure being applied to the system. However, this may not remain true if the system is large enough or the pressures low enough.

One example considers how atmospheric pressure changes with altitude. At sea level the standard U.S. atmospheric pressure

is 14.696 psia (101.325 kPa). At an altitude of 10,000 ft (3048 m) above sea level the standard U.S. atmospheric pressure is 10.106

psia (69.698 kPA) and at 30,000 ft (9144 m), the standard U.S. atmospheric pressure is 4.365 psia (30.101 kPa).

The pressure in a static liquid can be easily calculated if the density of the liquid is known. The absolute pressure at a depth H

in a liquid is defined as:

Pabs = P + (ρ x g x H)

Where :

Pabs is the absolute pressure at depth H.

P is the external pressure at the top of the liquid. For most open systems this will be atmospheric pressure.

ρ is the density of the fluid.

g is the acceleration due to gravity (g = 32.174 ft/sec2 (9.81 m/sec2)).

H is the depth at which the pressure is desired.

Freescale Semiconductor, Inc.

For More Information On This Product,

Go to: www.freescale.com

MPXM2102AS Datasheet(PDF) 510 Page - Motorola, Inc