Write short notes on Thermionic ionization gauge.
Answers
The ionization gauge consists of three distinct parts; the filament, the grid, and the collector.
The filament produces electrons by thermionic emission.
Apositive 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.