Feel the pressure: Understanding Absolute Pressure, Atmospheric Pressure, Gauge Pressure and Vacuum Pressure

Pressure is a physical quantity that measures the amount of force per unit area applied to an object or a surface. It is a scalar quantity. There are several different units used to express pressure, and they can be converted from one unit to another using conversion factors. Here are some common units of pressure and their relationships: 

1 atmosphere (atm) = 101,325 pascals (Pa) = 14.7 pounds per square inch (psi)

1 bar = 100,000 pascals (Pa) = 14.5 pounds per square inch (psi)

1 pound per square inch (psi) = 6,894.8 pascals (Pa) = 0.0689 atmospheres (atm)

1 torr = 1/760 atmosphere (atm) = 133.3 pascals (Pa) = 0.0193  psi

1 millibar (mbar) = 100 pascals (Pa) = 0.001 bar

1 Pascal (Pa) = 1 Newton per square meter (N/m²)

1 Millimeter of mercury (mmHg) = 133.322 Pa = 0.0013158 bar

1 Inch of mercury (inHg) = 3386.39 Pa = 0.0334211 bar

1 Kilogram-force per square centimeter (kgf/cm²) = 98,066.5 Pa = 0.981 bar

1 Poundal per square foot (pdl/ft²) = 1.48816 Pa

When we talk about pressure, there are several different types of pressure that we need to consider. These include absolute pressure, atmospheric pressure, gauge pressure, and vacuum pressure. In this post, we will explain the differences between these types of pressure and how they are related to each other.

Absolute Pressure

Absolute pressure is the total pressure exerted by a fluid, including both the atmospheric pressure and the pressure exerted by the fluid itself. For example, if the atmospheric pressure is 1 bar (100 kPa) and the pressure of the fluid is 2 bar (200 kPa), the absolute pressure would be 3 bar (300 kPa) because it includes both the atmospheric pressure and the pressure of the fluid. It is usually measured relative to a perfect vacuum, where the pressure is zero. Absolute pressure is often used in scientific and engineering applications, where precise pressure measurements are required. The symbol for absolute pressure is P(abs).

Atmospheric Pressure

Atmospheric pressure is the pressure exerted by the weight of the atmosphere. It is essentially the ambient pressure or the pressure around us. Atmospheric pressure is measured using a barometer and is usually expressed in units of pressure such as pounds per square inch (psi), kilopascals (kPa), or atmospheres (atm). The symbol for atmospheric pressure is P(atm) and it is almost 1.013 bar. 

Gauge Pressure

Gauge pressure is the pressure measured relative to the atmospheric pressure. It only takes into account the pressure exerted by the fluid itself, and not the atmospheric pressure. For example, if the atmospheric pressure is 1 bar (100 kPa) and the pressure of the fluid is 2 bar (200 kPa), the gauge pressure would be 1 bar (100 kPa) because it only considers the pressure of the fluid and not the atmospheric pressure. Gauge pressure is often used in industrial and mechanical applications, where pressure measurements are required to determine the pressure difference between two points. The symbol for gauge pressure is P(gauge).

Vacuum Pressure

Vacuum pressure is the pressure below atmospheric pressure. It is often used in scientific and industrial applications, such as in the production of vacuum tubes or vacuum pumps. Vacuum pressure is measured relative to atmospheric pressure and is expressed in units such as pounds per square inch (psi), kilopascals (kPa), or millibars (mbar). The symbol for vacuum pressure is P(vac).

The relationship between absolute pressure, atmospheric pressure, gauge pressure, and vacuum pressure can be expressed using the following equations:

P(abs) = P(gauge) + P(atm)

P(vac) = P(atm) - P(abs)   or P(vac) = -P(gauge)

These equations show that absolute pressure is equal to the sum of gauge pressure and atmospheric pressure, while vacuum pressure is equal to the difference between atmospheric pressure and absolute pressure. Alternatively, vacuum pressure can also be expressed as the negative of gauge pressure.

In conclusion, understanding the concepts of absolute pressure, atmospheric pressure, gauge pressure, and vacuum pressure is important in many fields of science, engineering, and industry. By knowing the differences between these types of pressure and how they are related to each other, we can make accurate pressure measurements and design effective systems that rely on pressure measurements.