Fourth Class Notes on Pressure (Principles of Heat and Fluids)

Class note on Pressure, Energy, Power


First Quiz

Global Oil Usage for oil is 1.25 Cubic Miles per year. Convert the 1.25 Cubic miles per year to liters per seconds.

Quiz \hspace{0.25cm} question:
1.25\frac { { mi }^{ 3 } }{ yr } \times \frac { 1yr }{ 365d } \times \frac { 1d }{ 1440mins } \times \frac { 1min }{ 60sec } \times \frac { 5280 }{ 1mi } \times \frac { 5280 }{ 1mi } \times \frac { 5280 }{ 1mi } \times \frac { 1L }{ { 0.0353 }ft^{ 3 } }


Class Notes


Pressure (p) is the force (F) exerted or distributed uniformly over an area (A) which is perpendicular to that force.

Pressure\quad =\quad Force\quad /\quad Area

The British unit for pressure is pounds per square inch (psi) or pounds per square per square foot (psf). The SI unit for pressure is newton per square meter (N/m^2) or Pascal


  • pounds per square inch (psi) or
  • pounds per square per square foot (psf)

SI Unit:

  • Newton per square meter (N/m^2) or
  • Pascal (Pa)


Types of pressures

  • Atmospheric pressure: Atmospheric pressure is the pressure exerted by the air on earth which is equal to 14.696 lbf/in^2 rounded to 14.7 psi at sea level under normal conditions. Atmospheric pressure can also be defined as the pressure produced by a column of mercury exactly 29.92 inches in length, therefore:

14.7 psi = 29.92 in. Hg = 760 mm Hg

1 in. Hg = 0.492 psi

1 mm Hg = 133.322 Pa

  • Gage pressure: is fluid pressure in a closed vessel which is measured by two types of pressure gages; 1. Manometer, 2. Bourdon tube


Bourdon Tube:

  • Absolute pressure: Absolute pressure is the total or true pressure of a fluid whereas gage presssure is the pressure as indicated by a gage. Gages are calibrated to read zero at atmospheric pressure and neither the manometer nor the bourdon tube measures the total pressure of the fluid in a vessel; both measure only the difference in pressure between the total pressure of the fluid in the vessel and the atmospheric pressure. The absolute pressure of a fluid in a vessel is determined by adding the atmospheric pressure to the gage pressure, and when the fluid pressure is less than atmospheric, the absolute pressure is found by subtracting the gage pressure from the atmospheric pressure.

Absolute\quad pressure\quad =\quad atmospheric\quad pressure\quad +\quad gage\quad pressure

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