Define absolute, gauge, and vacuum pressure.

Short Answer:

Absolute pressure is the total pressure measured from an absolute vacuum (zero pressure). It includes both the atmospheric pressure and the pressure of the fluid. Gauge pressure is the pressure measured above or below the atmospheric pressure by using a pressure gauge. Vacuum pressure is the pressure that is lower than the atmospheric pressure.

In simple words, absolute pressure = gauge pressure + atmospheric pressure, while vacuum pressure is the difference between atmospheric pressure and the pressure in the system when it is below the atmospheric level. These three pressures are used in fluid mechanics to measure and compare fluid pressures accurately.

Detailed Explanation :

Absolute, Gauge, and Vacuum Pressure

Pressure is one of the most important physical quantities in fluid mechanics. It represents the force exerted per unit area by a fluid and is measured in different ways depending on the reference point used. The three most common types of pressure are absolute pressure, gauge pressure, and vacuum pressure.

Each of these types of pressure is used for different applications in engineering, and understanding the relationship between them is essential for accurate measurement and system design.

  1. Absolute Pressure

Definition:
Absolute pressure is the total pressure exerted by a fluid, measured relative to a perfect vacuum (zero pressure). It includes both the gauge pressure and the atmospheric pressure.

Mathematically,

Here,

  • : Absolute pressure
  • : Gauge pressure
  • : Atmospheric pressure

Absolute pressure is always positive because it is measured from a perfect vacuum. For example, if the gauge pressure of a gas tank is 2 bar and the atmospheric pressure is 1.013 bar, then:

Explanation:
Absolute pressure represents the actual physical pressure within a system, considering both the external atmospheric pressure and any additional pressure within the system. It is essential in thermodynamic and gas law calculations because gases and vapors always respond to total (absolute) pressure, not just the excess pressure above the atmosphere.

Applications:

  • Used in thermodynamics and gas law equations (e.g.,  ).
  • Measurement of pressures in vacuum systems.
  • Calibration of pressure measuring instruments.
  • Analysis of air and gas flow in closed systems.
  1. Gauge Pressure

Definition:
Gauge pressure is the pressure measured relative to atmospheric pressure. It indicates whether the pressure inside a system is greater or less than the surrounding atmospheric pressure.

Mathematically,

Gauge pressure can be positive or negative depending on whether the system pressure is above or below atmospheric pressure.

Example:
If the absolute pressure inside a car tire is 3.013 bar and atmospheric pressure is 1.013 bar,

This means the tire has a gauge pressure of 2 bar, which is what is usually indicated by a tire pressure gauge.

Explanation:
Gauge pressure is what most instruments, like Bourdon gauges or pressure transducers, measure. Since these instruments are open to the atmosphere on one side, they automatically account for atmospheric pressure.

Applications:

  • Used in automotive systems to measure tire, oil, and fuel pressure.
  • Hydraulic and pneumatic systems for controlling operating pressures.
  • Industrial pressure monitoring (boilers, compressors, pipelines).
  • Used wherever the working pressure above atmospheric pressure is important.
  1. Vacuum Pressure

Definition:
Vacuum pressure is the pressure below atmospheric pressure. It is the difference between the atmospheric pressure and the absolute pressure when the latter is less than the atmospheric pressure.

Mathematically,

This means vacuum pressure is a negative gauge pressure, indicating suction or partial vacuum conditions.

Example:
If the absolute pressure inside a vacuum chamber is 0.3 bar and the atmospheric pressure is 1.013 bar,

Thus, the vacuum pressure is 0.713 bar below atmospheric pressure.

Explanation:
Vacuum pressure is often used to describe suction systems, vacuum pumps, and other equipment that operates below atmospheric pressure. The higher the vacuum pressure, the lower the actual (absolute) pressure inside the system.

Applications:

  • Vacuum pumps and suction devices.
  • Air conditioning and refrigeration systems.
  • Laboratory vacuum chambers.
  • Industrial applications such as packaging and drying under reduced pressure.

Relationship Between the Three Pressures

The three types of pressures are related by the following simple relationships:

  1. When pressure is above atmospheric pressure:
  1. When pressure is below atmospheric pressure (vacuum condition):

These relationships show that absolute pressure is the true measure of how much pressure exists, whether the system is under compression or suction.

Atmospheric Pressure (Reference Pressure)

Atmospheric pressure acts as a reference for both gauge and vacuum pressures. It is the pressure exerted by the weight of the Earth’s atmosphere and varies slightly with altitude and weather conditions.
At sea level,

All practical pressure measurements are compared with this atmospheric pressure as the base level.

Measurement of Pressures

  • Absolute pressure is measured using instruments like an absolute pressure gauge or barometer (which measures total atmospheric pressure).
  • Gauge pressure is measured using a Bourdon pressure gauge, which is common in mechanical systems.
  • Vacuum pressure is measured using a vacuum gauge or manometer.

Practical Examples

  1. Car Tire:
    The pressure indicated by a tire gauge (around 30 psi) is gauge pressure. The absolute pressure inside the tire is 30 psi + atmospheric pressure (14.7 psi) = 44.7 psi.
  2. Vacuum Pump:
    A vacuum system showing a pressure of 0.2 bar absolute has a vacuum pressure of 1.013 − 0.2 = 0.813 bar.
  3. Boiler System:
    A steam boiler operating at 6 bar gauge pressure actually has an absolute pressure of 6 + 1.013 = 7.013 bar.
Conclusion

In conclusion, absolute, gauge, and vacuum pressure are three important ways of expressing fluid pressure, depending on the reference level used. Absolute pressure is measured from an absolute vacuum, gauge pressure is measured relative to atmospheric pressure, and vacuum pressure is measured below atmospheric pressure. These pressures are interrelated and widely used in engineering systems such as hydraulics, pneumatics, and vacuum applications. Understanding these differences is essential for accurate pressure measurement and the safe design of mechanical systems.