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Pumping Speed

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 perhaps more clarity, pumping speed is, essentially, a measure of the pump's ability to permanently remove gas from the chamber. It is measured in units of volume per unit time: liters per second (L/sec), cubic feet per minute (cfm) or cubic meters per hour (m3/hr). Notice, pressure is never mentioned because pumping speed is a volumetric flow, not a quantity or mass flow. A graph of pumping speed (L/sec) vs pressure (torr) for a (fictitious) mechanical pump looks like this.

Pumping Speed Curve

Pumping speed is one of two numbers given by a pump manufacturer to describe a pump's qualities or capabilities. The other number is ultimate pressure. Given these two, you can begin to calculate whether the pump is adequate for the particular vacuum task you want perform.

Various standards authorities (American Vacuum Society, Pneurop, and possibly Japanese standards organizations) suggest methods of measuring pumping speed. As far as I'm aware, all methods suggest capping the pump with a dome at its inlet port and monitoring the pressure a a sequence of different gas flow rate are injected into the dome from a calibrated mass flow controller.

Measuring Pumping Speed and Ultimate Pressure

Vacuum pump manufacturers gives two specifications: pumping speed and ultimate pressure (also called ultimate vacuum). The ultimate pressure is measured by capping the pump's inlet and finding the equilibrium pressure after operating the pump for many hours. Because it is measured under "ideal" circumstances, it is crucial to remember — a chamber connected to this pump will never reach the quoted ultimate pressure!

Perhaps worse, pump manufacturers measure the ultimate pressure of mechanical pumps using a McLeod gauge that cannot measure vapors such as pump oil and water. Consequently, the so-called ultimate (partial) pressure of a rotary vane pump may be quoted in the 10-5 Torr range, causing much confusion when the practical ultimate pressure (using a gauge that responds to oil and water vapor) is two decades higher.