Basic Documentation

Room Pressurization Control Application Guide
Pascals (Pa)
SI unit to express relatively low air pressure values as is the case in ventilation system
applications. One Pascal is much less than 1 inch w.c. (I/P unit for low pressure values) since
248.83788
22
Pascals equal 1 inch w.c. In clean room pressure applications a typical room
static pressure level of 0.050 inches w.c. would equate to approximately 12.4 Pascals. For
higher pressures the Kilopascal (KPa) unit, which equals 1000 Pascals, is more commonly
used. One psi is equal to approximately 6.895 KPa.
Pitot Tube
Standard design for an air pressure measurement probe used to measure total and static
pressures of a moving air stream. For pressure measurement in ventilation system ducts, a
pitot tube provides two pressure measurement outputs. One output is the full forward
pressure of a moving air stream and is termed total pressure. The outer output is the air
stream that is perpendicular (crosswise) to the direction of airflow and is termed the static
pressure of the air stream. By connecting a differential pressure gauge (a manometer) to the
two pitot tube measurement outputs, the difference between them, termed the velocity
pressure, is obtained. Velocity pressure is mathematically related to the velocity of a moving
air stream and provides a convenient way to determine the airflow velocity in FPM (I/P units)
or m/s (SI units).
Pressure Conversion Factors
Table 5. Pressure Conversion Factors.
1 Psi 6894.76 Pascals (Pa) 1 Pascal 0.000145038 Pounds/Sq. In.
(PSI)
1 Psi 6.89476 Kilopascals (kPa) 1 Kilopascal 0.145038 Pounds/Sq. In. (PSI)
1 Psi
2.3114155 Feet of Water at 60°F
1 Pascal 0.004026653 Inches of Water
(AMCA)
1 Psi 27.7369861 Inches of Water at
60°F
1 Kilopascal 4.026653 Inches of Water
(AMCA)
1 Inches of Water 248.83788 Pascals (Pa)
(ASHRAE)
1 mm of Hg 0.535245108 Inches of Water
at16°C
1 Inches of Water 0.24883788 Kilopascals (kPa) 1 mm of Hg 0.0193368 Pounds/Sq. In. (PSI)
1 Atmosphere
33.967626 Feet of Water at 60°F
1 Atmosphere
760.000 mm of Hg at 0°C
1 Atmosphere 14.695595 Pounds/Sq. In. (PSI) 1 Atmosphere 101.3226006 Kilopascals (kPa)
Standard Air (Standard Conditions)
Standard air is considered to be at a barometric pressure of 29.92 inches of mercury (Hg)
and at either 0% RH and a temperature of 69.8°F, or at 50% RH and a temperature of
68.0°F. It is a reference point that enables establishing other physical characteristics of as
density, specific heat, etc.
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ASHRAE conversion factor.
2-Glossary Siemens Building Technologies, Inc.