It represents any pressure which takes a gas according to its fraction in a gas mixture and is calculated from its proportion by volume and the total pressure which the gas mixture is under. This relationship is described in more detail in Dalton’s law (Eq. 1) of partial pressures.
Dalton’s law (also called Dalton’s law of partial pressures) states that the total ptotal of a gas mixture is equal to the sum of all partial pressures pI of the individual gases (for ideal gases).
Transformation results in Eq. 2:
The partial pressure of a gas can be reduced by decreasing either the total pressure or its volume content. This is done in melt treatments, for example, in order to remove gas which is dissolved in liquid metal. In practice, a reduction of the total pressure means a melt treatment or casting in a vacuum (low vacuum).