Page 16 - ebook fluid mechanics_finalize
P. 16
FLUID MECHANICS
UNITS :
-1 -2
-2
Newton’s per square metre, Nm , kgm s . (The same unit also known as Pascal,
Pa; 1 Pa = 1Nm )
-2
-2
5
Also frequently used is the alternative SI unit the bar, where 1 bar = 10 Nm
1.2.1 Atmospheric Pressure, Patm or Po
The surface of the earth is at the bottom of an atmospheric sea. The standard
atmospheric pressure is measured in various units:
Atmospheric pressure at sea level is about 101.3 kPa, which is equivalent to a
head of 10.35 m of water or 760 mm of mercury approximately, and it
decreases with altitude.
The fundamental SI unit of pressure is the Pascal (Pa), but it is a small unit so kPa is
the most common direct pressure unit for atmospheric pressure. Since the static
fluid pressure is dependent only upon density and depth, choosing a liquid of
standard density like mercury or water allows you to express the pressure in units of
height or depth, e.g., mm of Hg or inches of water.
The mercury barometer is the standard instrument for atmospheric pressure
measurement in weather reporting.
Patm = 101kPa, Read in Gauge Pressure, PG OR Patm = 0 Pa, Read in Absolute
Pressure, PABS
5
