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Which of the following figures depicts the practical pumping chart of a Rotary pump having a pumping speed of 100 L/min ?

  • A.Pumping speed against falling pressure: a rising curve with a peak at 10^-3 T that falls back to zero
  • B.Pumping speed against falling pressure: a straight line rising through the 100 L/min level at 10^-3 T
  • C.Pumping speed against falling pressure: a rise to 100 L/min, a flat stretch to 10^-6 T, then a fall to zero
  • D.Pumping speed against falling pressure: a rise to 100 L/min followed by a flat line, with 10^-6 T marked

Answer: A

The keyed answer is chart (A), the only one in which the pumping speed falls to zero near a pressure of about 10-3 torr.

A rotary (oil-sealed) pump cannot pump below its ultimate pressure, which is of the order of 10-3 torr. Its speed therefore has to die away as the pressure approaches that value. Charts (C) and (D) carry the full speed all the way to 10-6 torr, a range that a rotary pump cannot reach (it is the range of diffusion or turbomolecular pumps backed by a rotary pump), and chart (D) never falls at all. Chart (B) is a straight line that keeps rising past the rated 100 L/min, which no pump does.

What could not be confirmed is the shape of the rising part: in practice a rotary pump holds close to its rated speed over most of the range from atmosphere downward, and chart (A) shows it climbing from zero at one atmosphere. So (A) is the best of the four by elimination and by its fall to zero near the ultimate pressure, not a faithful picture of a measured curve.

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