【Y2025】Understand gauge pressure, absolute pressure, relative pressure, and vacuum in one diagram
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This post was last edited by zhoudingshengs on 2025-1-12 at 11:33. The attachment is: Understanding gauge pressure, absolute pressure, relative pressure, and vacuum in one diagram.
Atmospheric pressure is generally 101325 Pa, approximately 0.1 MPa or 100 kPa; it is also referred to as 1 bar in engineering contexts.
Relative pressure generally refers to the pressure relative to the local atmosphere; it can also be called gauge pressure, which is the pressure measured by a pressure gauge. For example, a gauge pressure of 0.2 MPa means a relative pressure of 0.2 MPa, that is, the difference between the pressure at that location and atmospheric pressure is 0.2 MPa.
Gauge pressure is absolute pressure, and its value equals the sum of atmospheric pressure and relative pressure (gauge pressure). For example, if the gauge pressure is 0.2 MPa, then the relative pressure is also 0.2 MPa, while the absolute pressure is 0.3 MPa.
Vacuum level refers to the degree of vacuum; it generally indicates by how much the pressure is lower than atmospheric pressure, and can also be considered a type of relative pressure. It is most commonly used in environments involving pump products. For example, a vacuum level of 0.02 MPa means that the pressure is 0.02 MPa lower than atmospheric pressure; in this case, the absolute pressure is 0.08 MPa. Of course, the atmospheric pressure is only 0.1 MPa in total; it’s impossible to achieve a vacuum level of 0.2 MPa, so there’s no way to reduce the absolute pressure.
Static pressure refers to the pressure exerted on the walls of a container or the surface of an object by the fluid itself, due to its own gravity and intermolecular forces, when the fluid is at rest or moving in a uniform straight line. In the case of fluid flow, static pressure refers to the pressure component perpendicular to the direction of fluid flow. Its value is an absolute pressure value, but in the Fluent software it is presented as a relative value with respect to the operating pressure.
Dynamic pressure refers to the pressure generated due to fluid flow; it is related to velocity and its value is 0.5 × ρ × v, as derived from Bernoulli’s equation.
Total pressure is the sum of static pressure and dynamic pressure. In FLUENT’s pressure inlet, users are often required to enter the total pressure value; in fact, both pressure and velocity are taken into account here. Therefore, it is incorrect to simply input the pressure gauge reading at the pressure inlet location, as speed must also be taken into account. Of course, if the velocity here is 0, the total pressure is equal to the static pressure.