Partial pressure pi
The partial pressure of a certain gas or vapor is the pressure which that gas
or vapor would exert if it alone were present in the vessel.
Important note: Particularly in rough vacuum technology, partial pressure in
a mix of gas and vapor is often understood to be the sum of the partial
pressures for all the non-condensable components present in the mix – in
case of the “partial ultimate pressure” at a rotary vane pump, for example.