# Pressure Scales – Pressure Units

## Pascal – Unit of Pressure

As was discussed, the SI unit of pressure and stress is the pascal.

• 1 pascal  1 N/m2 = 1 kg / (m.s2)

Pascal is defined as one newton per square meter. However, it is a fairly small unit for most engineering problems, so it is convenient to work with multiples of the pascal: the kPa, the bar, and the MPa.

• 1 MPa  106 N/m2
• 1 bar    105 N/m2
• 1 kPa   103 N/m2

The unit of measurement called standard atmosphere (atm) is defined as:

• 1 atm = 101.33 kPa

The standard atmosphere approximates the average pressure at sea level at the latitude of 45° N. Note that there is a difference between the standard atmosphere (atm) and the technical atmosphere (at).

A technical atmosphere is a non-SI unit of pressure equal to one kilogram-force per square centimeter.

• 1 at = 98.67 kPa

## Pounds per square inch – psi

The standard unit in the English system is the pound-force per square inch (psi). It is the pressure resulting from a force of one pound-force applied to an area of one square inch.

• 1 psi  1 lbf/in2 = 4.45 N / (0.0254 m)2 ≈ 6895 kg/m2

Therefore, one pound per square inch is approximately 6895 Pa.

The unit of measurement called standard atmosphere (atm) is defined as:

• 1 atm = 14.7 psi

The standard atmosphere approximates the average pressure at sea level at the latitude of 45° N. Note that there is a difference between the standard atmosphere (atm) and the technical atmosphere (at).

A technical atmosphere is a non-SI unit of pressure equal to one kilogram-force per square centimeter.

• 1 at = 14.2 psi

## Bar – Unit of Pressure

A bar is a metric unit of pressure. It is not part of the International System of Units (SI). The bar is commonly used in the industry and meteorology, and an instrument used in meteorology to measure atmospheric pressure is called a barometer.

One bar is exactly equal to 100 000 Pa and is slightly less than the average atmospheric pressure on Earth at sea level (1 bar = 0.9869 atm). Atmospheric pressure is often given in millibars, where standard sea level pressure is defined as 1013 mbar, 1.013 bar, or 101.3 (kPa).

Sometimes, “Bar(a)” and “bara” are used to indicate absolute pressures, and “bar(g)” and “barg” for gauge pressures.

## Typical Pressures in Engineering – Examples

The pascal (Pa) as a unit of pressure measurement is widely used worldwide. It has largely replaced the pounds per square inch (psi) unit, except in some countries that still use the Imperial measurement system, including the United States. For most engineering problems, pascal (Pa) is a fairly small unit, so it is convenient to work with multiples of the pascal: the kPa, the MPa, or the bar. The following list summarizes a few examples:

• Typically most nuclear power plants operate multi-stage condensing steam turbines. These turbines exhaust steam at a pressure well below atmospheric (e.g.,, at 0.08 bar or 8 kPa or 1.16 psia) and in a partially condensed state. In relative units, it is a negative gauge pressure of about – 0.92 bar, – 92 kPa, or – 13.54 psig.
• The Standard Atmospheric Pressure approximates to the average pressure at sea-level at the latitude 45° N.  The Standard Atmospheric Pressure is defined at sea-level at 273oK (0oC) and is:
• 101325 Pa
• 1.01325 bar
• 14.696 psi
• 760 mmHg
• 760 torr
• Car tire overpressure is about 2.5 bar, 0.25 MPa, or 36 psig.
• Steam locomotive fire tube boiler: 150–250 psig
• A high-pressure stage of condensing steam turbine at nuclear power plant operates at steady state with inlet conditions of  6 MPa (60 bar, or 870 psig) t = 275.6°C, x = 1
• A boiling water reactor is cooled and moderated by water like a PWR, but at a lower pressure (e.g.,, 7MPa, 70 bar, or 1015 psig), which allows the water to boil inside the pressure vessel producing the steam that runs the turbines.
• Pressurized water reactors are cooled and moderated by high-pressure liquid water (e.g.,, 16MPa, 160 bar, or 2320 psig). At this pressure, water boils at approximately 350°C (662°F), which provides a subcooling margin of about 25°C.
• The supercritical water reactor (SCWR) is operated at supercritical pressure. The term supercritical in this context refers to the thermodynamic critical point of water (TCR = 374 °C;  pCR = 22.1 MPa)
• Common rail direct fuel injection: It features a high-pressure (over 1 000 bar or 100 MPa or 14500 psi) fuel rail on diesel engines.

References:
Reactor Physics and Thermal Hydraulics:
1. J. R. Lamarsh, Introduction to Nuclear Reactor Theory, 2nd ed., Addison-Wesley, Reading, MA (1983).
2. J. R. Lamarsh, A. J. Baratta, Introduction to Nuclear Engineering, 3d ed., Prentice-Hall, 2001, ISBN: 0-201-82498-1.
3. W. M. Stacey, Nuclear Reactor Physics, John Wiley & Sons, 2001, ISBN: 0- 471-39127-1.
4. Glasstone, Sesonske. Nuclear Reactor Engineering: Reactor Systems Engineering, Springer; 4th edition, 1994, ISBN: 978-0412985317
5. Todreas Neil E., Kazimi Mujid S. Nuclear Systems Volume I: Thermal Hydraulic Fundamentals, Second Edition. CRC Press; 2 edition, 2012, ISBN: 978-0415802871
6. Zohuri B., McDaniel P. Thermodynamics in Nuclear Power Plant Systems. Springer; 2015, ISBN: 978-3-319-13419-2
7. Moran Michal J., Shapiro Howard N. Fundamentals of Engineering Thermodynamics, Fifth Edition, John Wiley & Sons, 2006, ISBN: 978-0-470-03037-0
8. Kleinstreuer C. Modern Fluid Dynamics. Springer, 2010, ISBN 978-1-4020-8670-0.
9. U.S. Department of Energy, THERMODYNAMICS, HEAT TRANSFER, AND FLUID FLOW. DOE Fundamentals Handbook, Volume 1, 2, and 3. June 1992.

Pressure