- In a gas, the net rate of transfer of momentum in the direction of the positive normal to an imaginary plane surface of specified area located in a specified position in the gas by molecules crossing the surface in both directions, momentum transmitted in the opposite direction being counted as negative, divided by the area of the surface.
In general, it is assumed that the area of the imaginary plane surface is small enough so that the pressure with respect to any part of the surface is equal (within narrow limits) to the pressure based on the whole surface. Different kinds of pressure (static, dynamic, partial, total, vapor, etc.) are distinguished by the orientation of the surface with respect to mass-flow velocity vectors or by the restriction to a specified set of molecular species crossing the imaginary surface.
- On a boundary surface, the force applied per unit area and equal to the pressure in the gas as determined by molecules crossing an imaginary surface located at a fixed distance of molecular magnitude in front of the real surface, the positive normal being drawn from the imaginary surface toward the real surface.
The term pressure when used alone can be assumed to refer to the total pressure in a gas at rest or else to refer to the static pressure in a gas flowing under steady-state conditions.
- = atmospheric pressure.
- As measured in a vacuum system, the quantity measured at a specified time by a so-called vacuum gage, whose sensing element is located in a cavity (gage tube) with an opening oriented in a specified direction at a specified point within the system, assuming a specified calibration factor.
The sensitivity of the sensing element is, in general, not the same for all molecular species, but the gage reading is frequently reported using the calibration factor for air regardless of the composition of the gas. The opening to the gage tube is often carelessly oriented with respect to mass-flow vectors in the gas (which is seldom at rest), and errors due to variations in wall temperatures of tube and system are frequently neglected. The actual total pressure in a high-vacuum system cannot usually be measured by a single gage, but in vacuum technology the term total pressure is sometimes used to refer to the reading of a single untrapped gage which responds to condensable vapors as well as permanent gases.
This article is based on NASA's Dictionary of Technical Terms for Aerospace Use