Mumbai University > Mechanical Engineering > Sem 5 > Mechanical measurements and control

Marks: 5M

Year: DEC 2014 ,DEC2015 ,May 2016

The ionization gauge consists of three distinct parts; the filament, the grid, and the collector.

The filament produces electrons by thermionic emission.

A positive charge on the grid attracts the electrons away from the filament; they circulate around the grid passing through the fine structure many times until eventually they collide with the grid.

• Gas molecules inside the grid may collide with circulating electrons.

• The collision can result in the gas molecules being ionized.

• The collector inside the grid is negatively charged and attracts these positively charged ions.

• Likewise they are repelled from the positive grid at the same time.

• The number of ions collected by the collector is directly proportional to the number of molecules inside the vacuum system.

• The cathode is thus a positive ion collector and the anode is an electron collector.

• By this method, measuring the collected ion current gives a direct reading of pressure.

• The sensitivity of ionization gauge is given by, S =Ip/Pig Where, Ip = Ion current Ig = Electron current P = Gas pressure

• The sensitivity equation can be rearranged for gas pressure, P = Ip/Sig

Advantages

• Wide pressure range from 〖10〗^(-8) to 〖10〗^(-11) torr.

• Sensitivity is constant for wide pressure range.

• Process control and remote indication is possible.

• Response of the system is fast.

Disadvantages

• A cathode filament can burn quickly if exposed to air. Hence, it is necessary to ensure enough low vacuums.

• Some gases may get decomposed due to hot cathode filament.

• The gas whose pressure is to be measured gets contaminated.

• Needs to be installed carefully.

• Electric circuit is complex.

• High initial cost.