Difference Between Gauge Pressure and Atmospheric Pressure

Gauge Pressure vs Atmospheric Pressure

Atmosphere pressure and gauge pressure are two important concepts in pressure and thermodynamics. It is vital to have a clear understanding in these concepts in order to excel in such fields. This article will discuss what pressure, atmospheric pressure and gauge pressure are, their similarities, definitions and the differences between atmospheric pressure and gauge pressure.

What is Atmospheric Pressure?

An understanding on the concept of pressure is required to understand atmospheric pressure. Pressure is defined as the force per unit area applied in a direction perpendicular to the object. The pressure of a static fluid is equal to the weight of the fluid column above the point the pressure is measured. Therefore, the pressure of a static (non-flowing) fluid is dependent only of the density of the fluid, the gravitational acceleration, the atmospheric pressure and the height of the liquid above the point the pressure is measured. The pressure can also be defined as the force exerted by the collisions of particles. In this sense, the pressure can be calculated using the molecular kinetic theory of gasses and the gas equation. Atmospheric pressure is defined as the force per unit area exerted against a surface by the weight of air above that surface in the Earth’s atmosphere. When going to the high altitudes, the air mass above the point decreases, thereby reducing the atmospheric pressure. The atmospheric pressure at the mean sea level is taken as the standard atmospheric pressure. The pressure is measured in Pascal (unit P). This unit is also equivalent to Newton per square meter. Other widely used units are the Hgmm or Hgcm, which means the equivalent mass of the mercury column the air pressure can support. The atmospheric pressure at the mean sea level is taken as 101.325 kPa or sometimes as 100 kPa.

What is Gauge Pressure?

Gauge pressure is the pressure difference between the absolute pressure and the atmospheric pressure, or in other words, the ambient pressure, around the point the pressure is measured. There are two methods used in measuring the gauge pressure. One is measuring the pressure relative to the ambient pressure, and the other one is to measure the pressure relative to a fixed pressure. There are two devices designed to measure the gauge pressure built upon these two methods. A vented gauge uses two open ends placed in two different pressures, to measure the pressure difference. It is obvious that a vented gauge placed in the open air will produce zero as the pressure difference. The sealed gauge uses only one end to measure the pressure with respect to a pre-defined pressure at the other end. A sealed gauge placed in the open air does not necessarily have to produce zero, but it will produce zero when the pressure at the other end is equal to the calibrated pressure.

What is the difference between Atmospheric Pressure and Gauge Pressure?

• Atmospheric pressure is an absolute pressure.

• Gauge pressure is the pressure above the atmospheric pressure; therefore, it is a relative pressure.

• The atmospheric pressure at the mean sea level is a constant.