Vacuum science and technology

The meaning of the word, ‘vacuum’ is complete emptiness. Thus we can conclude that vacuum means space in which air and other gases are absent. Technically vacuum is significant only when the gas pressure becomes less than 760 torr. The most common example of creating vacuum is the suction in an ordinary drinking straw. Many devices require vacuum conditions for their operations. In dewar flask, television tube, electric bulb, vacuum cleaner etc. we use vacuum conditions. The boiling temperature of a liquid is reduced in vacuum. Transmission of sound is difficult through a region having high degree of vacuum. The advance in technology is also due to vacuum technology. X-ray technology, microelectronic technology, computer technology, metallurgical advances etc. are mainly due to vacuum technology.

Production of vacuum

A pump is required to produce vacuum. In mechanical pumps gas is initially trapped and compressed. Later gas is removed from the low pressure to the high pressure side of the pump, where it is expelled to the open air either directly or through a second mechanical ‘backing pump’. In diffusion pump, gas molecules are forced to interact with heavy molecules of pump fluid. In chemical pumps, gas molecules are allowed to combine chemically with highly reactive substances like titanium. Sorption pumps use charcoal or zeolites. They absorb gas molecules. Cryo pumps are vessels with chemically nonreactive internal parts. Another method is ionization of the gas where accelerating the ions in a high voltage field, the gas molecules are forced to move into a wall or target electrode.

Measurement of vacuum

In Barometric type of measurement, we use mercury or oil. In mechanical type, diaphragm gauge is used in which the pressure causes mechanical movement of a septum forming one wall of an evacuated enclosure. This is similar to that of an aneroid barometer. In Mcleod gauge, assumption followed is that Boyle’s law is valid. Manometric liquid used in Mcleod gauge is mercury. Thermodynamic principles are utilized in thermal conductivity gauges. Hot-cathode ionization gauges and cold-cathode ionization gauges are other examples. Knudsen gauges depends on the recoil force on a warm surface when a molecule rebounds from it. Pirani gauge is based on the property of variation of thermal conductivity with pressure.