A pump's ultimate pressure, often called its ultimate vacuum, is a
value measured when the pump's design is finalized and an example
built. In general terms, it is measured by blanking the pump's inlet
with a pressure gauge, operating the pump for some time, recording
the pressure achieved and calling that the ultimate pressure. Because
there are many pressure units, you will see ultimate pressures quoted
in Torr, millibar, Pascal, inches of mercury, etc.
Pumping speed is formally defined as the ratio of the throughput of
a given gas to the partial pressure of that gas at a specific point near
the inlet port of the pump. With less formality, but more clarity, it is
the volume of gas (at any pressure) that is removed from the system
by the pump in unit time. In short, pumping speed is a measure of the
pump's capacity to remove gas from the chamber. It is measured in
liters per second (L/sec.), cubic feet per minute (cfm), or cubic meters
per hour (m 3/hr.).
There are several points to note when comparing pumps based on
their rated pumping speed. First, pumping speed is measured under
the same ideal conditions used to measure ultimate pressure
minimum volume, right at the pump inlet, lowest possible outgassing
rate, ideal conditions, etc. Second, pumping speed is specified by the
manufacturer as the highest value over the entire operating pressure
range of the pump. The pumping speed curve shown below illustrates
the characteristic differences between roots pumps, mechanical
pumps, and high vacuum pumps.