Editing Vacuum pump (section)

== Performance measures ==
Pumping speed refers to the volume flow rate of a pump at its inlet, often measured in volume per unit of time.  Momentum transfer and entrapment pumps are more effective on some gases than others, so the pumping rate can be different for each of the gases being pumped, and the average volume flow rate of the pump will vary depending on the chemical composition of the gases remaining in the chamber.<ref name=":4">{{Cite book |last=Hablanian |first=M. H. |chapter-url=https://books.google.com/books?id=4FMPEAAAQBAJ&pg=PA41 |title=High-vacuum technology : a practical guide |date=1997 |publisher=Marcel Dekker |isbn=0-585-13875-3 |edition=2nd ed., rev. and expanded |location=New York |pages=41–66 |chapter=Chapter 3: Fluid Flow and Pumping Concepts |oclc=44959885}}</ref>

Throughput refers to the pumping speed multiplied by the gas pressure at the inlet, and is measured in units of pressure·volume/unit time. At a constant temperature, throughput is proportional to the number of molecules being pumped per unit time, and therefore to the [[mass flow rate]] of the pump. When discussing a leak in the system or backstreaming through the pump, throughput refers to the volume leak rate multiplied by the pressure at the vacuum side of the leak, so the leak throughput can be compared to the pump throughput.<ref name=":4" />

Positive displacement and momentum transfer pumps have a constant volume flow rate (pumping speed), but as the chamber's [[pressure]] drops, this volume contains less and less mass. So although the pumping speed remains constant, the throughput and [[mass flow rate]] drop exponentially. Meanwhile, the leakage, [[evaporation]], [[Sublimation (phase transition)|sublimation]] and backstreaming rates continue to produce a constant throughput into the system.<ref name=":4" />