Influence of several common gases on vacuum detection

- May 08, 2018 -

First, the influence of oxygen on the vacuum gauge


Oxygen can be measured by vacuum gauge, but when the partial pressure of oxygen is too high, the surface of mercury will oxidize, which will affect the surface tension of mercury and make the mercury move in the capillary inaccurate, resulting in measurement error.
The filament of common heat conduction ( resistance ) vacuum gauge will oxidize at high temperature, thus changing the surface state of hot wire, causing the change of the zero drift and sensitivity of the gauge. the gauge of antioxidant hot wire should be adopted. The filament of hot cathode ionization gauge is easily oxidized, which shortens its life.
As mentioned above, the gas environment with high partial pressure of oxygen can be measured by thin film vacuum gauge in the coarse and low vacuum range. Radiation vacuum gauge and cold cathode gauge can be used in high vacuum.


Second, the influence of water vapor on vacuum gauge


Steam is a condensable gas. when measuring it, the compressed vacuum gauge will compress the gas to be measured, which will condense the steam. Therefore, in general, the compressed vacuum gauge cannot be used to measure water vapor. Its influence on heat conduction vacuum gauge is the same as that of oxygen, and it will also change the zero drift and sensitivity of the gauge.
Generally speaking, the u - shaped manometer and diaphragm gauge, radiation vacuum gauge, viscosity gauge, knudsen vacuum gauge and the like can be used to measure water vapor. Cold cathode gauge tubes will not suffer from oxidation problems, and can also be used for measuring vacuum systems with high partial pressure of water vapor.
The filament of resistance vacuum gauge and tungsten cathode hot cathode ionization gauge is made of tungsten. the tungsten filament has high temperature, and the high temperature filament oxidizes with water vapor, which will be decomposed by the surface of the high temperature tungsten and react with the tungsten to generate tungsten oxide and atomic hydrogen, which will be attached to the glass wall of the gauge after evaporation. Atomic hydrogen takes oxygen from tungsten oxide on the tube wall and turns it into water vapor. In this way, water vapor plays a role in transporting tungsten. Resulting in continuous evaporation of tungsten cathode and rapid damage of filament. Especially when the partial pressure of water vapor is greater than 10 - 2pa, the life of the gauge is very short, so it is not suitable for measuring water vapor.


Third, the influence of oil vapor on vacuum gauge


When the pumping system with oil ( mechanical pump and oil diffusion pump ) is used for pumping, there are a lot of oil vapor and its splitters in the system. Their vapor pressures are relatively low, so they can't be measured by compressed vacuum gauge. For example, when measuring the limit vacuum degree of a mechanical pump with a compressed vacuum gauge, it is higher than the data measured with a heat conduction vacuum gauge by an order of magnitude. When oil vapor is measured by oil u - gauge, the working oil can dissolve oil vapor, so it can't be correctly indicated.
When oil vapor is measured by heat conduction vacuum gauge or ionization vacuum gauge, it will pollute the filament or decompose on the high-temperature cathode to generate hydrocarbons, which seriously pollutes the electrode and the wall of the tube, thus changing the heat conduction situation, and obviously changing the sensitivity and characteristics of the gauge and causing measurement error.
Therefore, in the system with high partial pressure of oil vapor, it is suggested to measure with thin film vacuum gauge when the vacuum is low. When the vacuum is high, the radiation vacuum gauge can be used for measurement.