Difference between Absolute Pressure and Atmospheric Pressure

Pressure refers to the Physical force exerted on an object per unit area. It can alternatively be described as the force-to-area ratio (over which the force is acting). F/A is the basic pressure formula (Force per unit area). Pascals is a unit of pressure (Pa). Absolute, atmospheric, differential, and gauge pressures are examples of pressures. Have you ever noticed that when you drink from a straw, you actually suck the air out of it? You’re actually applying ‘Pressure’ while sipping your beverage.

What is Absolute Pressure?

Absolute pressure refers to the pressure in relation to the zero pressure existing, that is, the pressure in the empty, free space. In a vacuum, there is no pressure. P_abs is the abbreviation for absolute pressure. It is equal to measuring pressure plus ambient pressure and is measured with a barometer. The pressure measured in proportion to absolute zero pressure in a vacuum is known as absolute pressure. A measurement of pressure below atmospheric pressure is known as negative pressure or vacuum pressure.

Formula For Absolute Pressure

P_abs = P_atm+P_gauge

Where,

P_atm is atmospheric pressure

P_gauge is negative pressure

What is Atmospheric Pressure?

Air is present above sea level, and there are so many layers that the higher you go, the more pressure air exerts, and atmospheric pressure is the pressure exerted by air in the atmosphere. The atm unit is used to measure atmospheric pressure. A barometer is an instrument that is used to measure atmospheric pressure. The barometer is filled with Mercury and vacuumed within. The standard atmosphere (symbol: atm) is a pressure unit with a value of 101,325 Pa.

Formula For Atmospheric Pressure

P_{atm =} p_gh

Where,

p is the density of air

g is acceleration due to gravity

h is height of the mercury

Difference Between Absolute Pressure and Atmospheric Pressure

	Absolute Pressure	Atmospheric Pressure
1.	The pressure in respect to the zero pressure, that is, the pressure in empty, free space, is referred to as absolute pressure. There is no pressure in a vacuum.	Atmospheric pressure at that spot is the force acting on a unit area around a location as a result of the full height of the air column of the atmosphere above it.
2.	Absolute pressure is the pressure measured in proportion to absolute zero pressure in a vacuum. Negative pressure, often known as vacuum pressure, is a measurement of pressure below atmospheric pressure.	The hydrostatic pressure created by the weight of air above the measurement point is very close to atmospheric pressure. Because the overlaying air mass decreases as elevation increases, atmospheric pressure decreases.
3.	It is denoted by P abs	It is denoted by P atm
4.	Formula for absolute pressure is,  Pabs = Patm+Pgauge	Formula for atmospheric pressure is,  Patm = pgh
5.	This pressure is determined in psi (pounds per square inch).	Its unit is newton per square meter Nms-1.
6.	Absolute pressure is also called as sealed pressure.	Atmospheric pressure is also known as barometric pressure.
7.	At sea level it is 14.7 psi.	At sea level it is 1 atm.
8.	For Example, Using an altimeter to determine the height of an aircraft. Because air pressure changes with altitude, using it as a reference is not a good idea, hence absolute pressure is employed to ensure precision in measurements, which is critical for flight safety.	For Example, When the bulb is squeezed, air from the tube and bulb exits as bubbles. However, there is air pressure on the liquid’s surface. The water inside the tube moves when we release the bulb.

Sample Problems

Problem 1: When a dolphin glides through the air, it is subjected to a 0.75 m of mercury external pressure. When a dolphin is 5 m below the free surface of the water, the absolute pressure is?

Solution:

h= 5 m

P_atm = 0.75 m of mercury = P_Hg × g × 0.75 = 13600 x 9.81 x 0.75 = 100062 Pa

P _gauge = P_water × g x h

P_gauge = 1000 x 9.81 x 5 = 49050 Pa

P_abs = P _gauge + P_atm

P_abs = 49050 + 100062

P_abs = 149112 Pa = 149112 N/m₂

= 0.149 N/mm² 

= 0.15 N/mm²

Problem 2: A vacuum gauge attached to a chamber indicates a pressure of 40 kPa. At the location, the atmospheric pressure is 100 kPa. The absolute pressure in the chamber is then measured.

Solution:

We know, Absolute Pressure = Vacuum Pressure – Atmospheric pressure

Vacuum pressure = 40 kPa, Atmospheric pressure = 100 kPa

Absolute pressure = 100 – 40 = 60 kPa

Problem 3: If the atmospheric pressure is equivalent to 750 mm of mercury, what is the absolute pressure at a location 3 m below the free surface of a liquid with a density of 1.53 103 kg/m³ ? Mercury has a specific gravity of 13.6 while water has a density of 1000 kg/m³.

Solution:

h= 3 m, Pliquid = 1.53 x 103 kg/m²

h_atm = 750 mm of mercury (Hg) = 0.75 m,

specific gravity ,S_Hg = 13.6, p = 13.6 x 1000 kg/m²

P_atm = pgh_atm = 13.6 x 1000 x 9.81 x 0.75 = 100,062 N/m²

P_abs = P_atm + P_gauge

P_gauge = (pgh)_liquid = 1.53 x 10ªx 9.81 x 3 = 45027.9 N/m2

P_abs = P_atm + P_gauge = 100062 + 45027.9 = 145090 N/m2

Problem 4: If the air in a cylinder has a gauge pressure of 250 kPa, the absolute pressure is?

Solution:

P_g = 250 kPa

P_abs= P_g + P_atm

P_abs = 250 + 101.3

P_abs = 351.3 kPa = 350 kPa.

Problem 5: In a closed vessel, the volume and temperature of air (assumed to be an ideal gas) are 2.87m³ and 300 K, respectively. A manometer mounted on the vessel’s wall indicates a gauge pressure of 0.5 bar. The mass of air (in kilograms) in the vessel is if the gas constant of air is R = 287 J/kg K and the atmospheric pressure is 1 bar.

Solution:

V = 2.87 m³ ; T= 300 K

P_gauge = 0.5 bar; P_atm = 1 bar; R = 287 j/Kg-K

PV = mRT (P is absolute pressure)

P_abs = P_atm + P_gauge = 0.5 + 1 = 1.5 bar = 150 kPa

PV = mRT

m=PV/RT

=150 x 2.87 / 0.287 x 300

m= 5 Kg