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TIP OF THE MONTH 12 : Pump out need

Why do I suddenly need so long to pump out?

TIP OF THE MONTH 12 : Pump out need

QUESTION: I have a sliding-vane rotary pump on a vacuum air-lock and during the past few weeks the lock operating times have become longer and longer – will my pump last for some time yet or must I send it to be serviced soon?

ANSWER: Probably your pump doesn’t need servicing at all, it is just battling against the high humidity in the summer.

BACKGROUND: Pumping out from atmospheric pressure to the transfer pressure takes place repeatedly in a vacuum lock. At the start of the pump-down process, the pressure reduction simply depends only on the compartment volume and the suction capacity of the pump being used. The reduction in pressure may slow down when the measuring tube indicates a pressure between 0.1 and 10 mbar (more likely at lower pressures with large pumps and at higher pressures with small pumps). This is caused by water vapour released (desorbed) from the compartment walls.

Water is brought into the vacuum system through the repeated venting with atmospheric air. The amount of water that is introduced depends on

  • the relative humidity of the air
  • the duration of venting, and
  • the nature of the compartment surface.

One cubic metre of air can absorb about 9.5 grams of water at atmospheric pressure and a temperature of 10°C. However, this is already more than 30 grams at 30°C. Therefore if

  • a dust-laden compartment with rough walls
  • is exposed to atmospheric air for a prolonged time
  • at high temperatures in summer,

more water is introduced into the compartment than would happen in winter and/or with clean compartment walls.

In a vacuum system the water condenses on the surfaces instead of remaining in the form of vapour. This means that during pumping down it must first of all be released from the surface before it can be pumped out. This is a slower process than the pumping operation itself, therefore it lengthens the pump-down time.

You can recognise moisture in your vacuum vessel if the pumping process slows down at a pressure of about 2 to 10 mbar (Pirani measuring instrument display) or if a whitish, milky emulsion is visible in the sliding-vane rotary pump’s oil sight glass.

Glossary

Condensation: Condensation is the transition of a substance from the gaseous to the liquid state.

Desorption: describes the process during which atoms or molecules leave the surface of a solid object. The reverse process, i.e. the adhesion of atoms or molecules to the surface of a solid object, is called Adsorption.

Pirani vacuum meter: Vacuum meter frequently named after its inventor, Berlin physicist Marcello Pirani (1880-1964). It works within the pressure range from around 10-4mbar to 1000 mbar and uses the dependency of pressure on heat conductivity as an indirect measured variable for pressure. The change in heat conductivity is measured on the basis of the change in resistance of an electrically heated wire.

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