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TABLE OF CONTENTS Chapter 3 - Physical Properties of Fluids: Fluid Pressure
Pressure is defined as force divided by the area over which the force is evenly
distributed. Since liquids and gases are fluid, they exert pressure on their containers
evenly. The pressure of the flowing fluid becomes a very useful criterion when
defining the performance of a flowmetering system.
The SI system of units measures pressure in newtons (force) per square meter
(area), which has the special name "pascal" (Pa). The pascal turns out to be so
small that it is common to use kilopascals or megapascals, (kPa or MPa), which
are 1000 and 1,000,000 pascals, respectively. In countries that formerly used the
metric system, there is a tendency to remain with the older units of bars or kilograms/
centimeter2. In the United States the English units for pressure, lb/in.2 gage
(psig) and lb/in.2 absolute (psia) have remained very common.
Three distinct types of pressure must be described by the units, as follows:
- Absolute pressure is the actual pressure of the fluid with respect to a
perfect vacuum, regardless of the atmospheric pressure on the outside
of the container. - Gage pressure is the fluid pressure with respect to the atmospheric
pressure outside its container. - Differential pressure is the difference between two pressures. (Notice
that gage pressure is actually a differential pressure between fluid
pressure and atmospheric pressure.)
The SI system does not distinguish between absolute and gage pressure; therefore,
it is always necessary to spell it out as "kPa abs," "kPa gage," and perhaps
"kPa diff." The same is true when using units of bars or kg/cm2. In the English
system, the abbreviations "psig," "psia," and "psid" clearly describe the pressure
type intended.
The SI system differentiates between force units and mass units (newtons and
kilograms). In the English and the old metric systems the same name is used for
force as for mass, (pounds and kilograms, respectively). This leads to some confusion
and requires the inclusion of a unit converter, gc, which is numerically equal
to the acceleration of gravity at sea level.
| F = mgc |
Yet another unit for pressure is the height of a column of water or mercury that
would create the designated pressure at the base of the column. This unit is very
commonly used for differential pressure. Typical units are "inches of water," "millimeters
of mercury," etc. It is easily seen that the units of measurement for pressure
are not clear and straightforward. The following equivalents, along with
Figure 3-2, will help.
| 1 psi | = | 6.895 kPa | An absolute pressure transmitter is actually a differential pressure transmitter with one port completely evacuated and sealed. Therefore, it is more expensive than a gage pressure transmitter. The latter can be substituted for the former when measuring at high enough pressure so that variation in atmospheric pressure is negligible. If the pressure is above about 200 psig, changes in atmospheric pressure will usually have no significant effect on a gas flow computing system. | |
| 1 kPa | = | 0.1450 psi | ||
| 1 bar | = | 100 kPa | ||
| 1 bar | = | 14.50 psi | ||
| 1 MPa | = | 145.0 psi | ||
| 1 psi | = | 27.73 inches of water | ||
| 1 psi | = | 2.310 feet of water | ||
| 1 kPa | = | 101.97 mm of water | ||
| 1 kPa | = | 4.019 inches of water | ||
At sea level the atmospheric pressure will vary with the weather conditions. It
is approximately 14.7 ± 0.5 psia (101 ± 3 kPa abs). If gage pressure is less than
atmospheric, it is sometimes reported as "vacuum." This can be misleading
because it means "the amount below atmospheric" but is shown with a positive
sign rather than a negative one. Modern usage avoids "vacuum" as a unit of pressure.
Instead, it should simply be called a "negative gage pressure"; for example,
"- 3 psig," not "3 psi vacuum."
Fluid Density
Density is defined as the mass of the fluid per unit volume (? = m/V). In the
English system, density is usually expressed in pounds per cubic foot, where the
pounds represent mass rather than force. In the metric system density is often
expressed either in kilograms per cubic meter or kilograms per liter. Several
equivalence formulas are:
| 1 lb/ft3 = 16.026 kg/m3 | |
| 1 lb/ft3 = 0.016026 kg/l | |
| 1 kg/l = 0.0624 lb/ft3 |
