gres
Full Member level 4
Except pressure in vacuum unit, sometimes we need to know atmosphere composition.
Why?
Vacummeter shows onlypressure, can’t show kind of gas or vapour. In not ideal vacuum (you can’t reach ideal) are different kinds of gas, and it’s percent value is not always the same as air. Why? For example we have metal tank. We want to reduce atmospheric pressure from 1000hPa to 10^-5hPa using rotary pomp and turbo-molecular pomp.
As we know air consist different gases. Rotary pomp don’t chance composition, but molecular does. It pump heavy gas easier. That means composition is changing. There will be more light gasses like hydrogen, helium and of course steam.
To test what gasses are in vacuum unit we use mass spectrometers. Quadrupole is most common.
Similar as ion vacummeter it consist glowing cathode and electrodes that are ion source. Ions fall between 4 rods, that are in pair connection. Voltage goes into rods. Voltage is DC and AC V+Usin wt, so you can choose next masses.
U and V increase w is constant. Proportion between V/U is also constant.
Ions that doesn’t consist right proportion between mass and charge, ends it’s life at rods. Rest goes to faraday box. Ion current that is made by ions is the capacity of gas. That current is a few, or hundreds pikoampers. Proportion between V/U tells about resolving power. If the current is to low, sensitivity will increase but resolution decrease. That means that single signals fall in to one. So we need to adjust spectrometer to perfect work
Information about construction are here https://www.elektroda.pl/rtvforum/topic1423274.html
