Question

Differentiate amongst gauge pressure, atmospheric pressure and absolute pressure. Also give the value of atmospheric pressure in bar and mm of Hg.

Answer 1

While working in a system, the thermodynamic medium exerts a force on boundaries of the vessel in which it is contained. The vessel may be a container, or an engine cylinder with a piston etc. The exerted force F per unit area A on a surface, which is normal to the force, is called intensity of pressure or simply pressure p. Thus

P = F/A= ρ.g.h

 It is expressed in Pascal (1 Pa = 1 Nm²), 

bar (1 bar = 10⁵ Pa), 

standard atmosphere (1 atm =1.0132 bar),

 or

 technical atmosphere (1 kg/cm² or 1 atm).

 1 atm means

 1 atmospheric absolute.

 The pressure is generally represented in following terms.

 1. Atmospheric pressure 

2. Gauge pressure 

3. Vacuum (or vacuum pressure)

 4. Absolute pressure

Atmospheric Pressure (P_^atm) 

It is the pressure exerted by atmospheric air on any surface. It is measured by a barometer. Its standard values are; 

1 P_atm = 760 mm of Hg i.e. column or height of mercury

 = ρ.g.h. = 13.6 × 10³ × 9.81 × 760/1000

= 101.325 kN/m² = 101.325 kPa

 = 1.01325 bar

 when the density of mercury is taken as 13.595 kg/m3 and acceleration due to gravity as 9.8066 m/s² 

Gauge Pressure (P_gauge)

 It is the pressure of a fluid contained in a closed vessel. It is always more than atmospheric pressure. It is measured by an instrument called pressure gauge (such as Bourden’s pressure gauge). The gauge measures pressure of the fluid (liquid and gas) flowing through a pipe or duct, boiler etc. irrespective of prevailing atmospheric pressure.

 Vacuum (Or Vacuum pressure) (P_vacc)

 It is the pressure of a fluid, which is always less than atmospheric pressure. Pressure (i.e. vacuum) in a steam condenser is one such example. It is also measured by a pressure gauge but the gauge reads on negative side of atmospheric pressure on dial. The vacuum represents a difference between absolute and atmospheric pressures.

 Absolute Pressure (P_abs)

 It is that pressure of a fluid, which is measured with respect to absolute zero pressure as the reference. Absolute zero pressure can occur only if the molecular momentum is zero, and this condition arises when there is a perfect vacuum. Absolute pressure of a fluid may be more or less than atmospheric depending upon, whether the gauge pressure is expressed as absolute pressure or the vacuum pressure. 

Inter–relation between different types of pressure representations. It is depicted in Fig. which can be expressed as follows.

p_abs = p_atm + p_gauge

 p_abs = p_atm – p_vace 

Depiction of atmospheric, gauge, vacuum, and absolute pressures and their interrelationship. 

Hydrostatic Pressure

 Also called Pressure due to Depth of a Fluid. It is required to determine the pressure exerted by a static fluid column on a surface, which is drowned under it. Such situations arise in water filled boilers, petrol or diesel filled tank in IC engines, aviation fuel stored in containers of gas turbines etc. This pressure is also called ‘hydrostatic pressure’ as it is caused due to static fluid. The hydrostatic pressure acts equally in all directions on lateral surface of the tank. Above formula holds good for gases also. But due to a very small value of p (and w), its effect is rarely felt. Hence, it is generally neglected in thermodynamic calculations. One such tank is shown in Fig. It contains a homogeneous liquid of weight density w. The pressure p exerted by it at a depth h will be given by

Fig Pressure under depth of a fluid increases with increase in depth.