Ultimate pressure of vacuum chamber

    The ultimate pressure (ultimate vacuum) of the vacuum device is the main parameter of the vacuum chamber of the device. Due to the wide variety of vacuum devices, the ultimate pressure is also called differently. For example, vacuum annealing furnaces call ultimate pressure "cold vacuum"; vacuum leak detectors call "background vacuum"; some space simulations The equipment is called "no-load ultimate vacuum" and so on. It is precisely because vacuum technology workers and non-vacuum companies or companies do not fully understand the meaning of ultimate pressure, which brings a lot of trouble in the development or use of vacuum devices. Even some companies do not understand the concept of the ultimate pressure of the vacuum device pumping system, which makes the purchased vacuum device unable to be used.

    What is ultimate pressure? From the vacuum system and related terms, it can be seen that the definition of ultimate pressure is: when the pump is working, the empty and dry vacuum container is gradually approaching the stable lowest pressure, and then it can be obtained: the pump refers to the main pump of the vacuum system, and the empty and dry vacuum container There are no working components in the vacuum container (ie, vacuum chamber), and the environment in the vacuum container is dry air or gas. It is a stable minimum pressure value that the vacuum container gradually approaches after a considerable time of pumping by the main pump. Obviously, the ultimate pressure reached by the vacuum system is not limited by time. The lower the ultimate pressure of the vacuum device, it signifies that the vacuum system design of the device is reasonable, and the choice of the main pump is ideal, which is one of the conditions for judging whether the vacuum device is advanced in technology. In the vacuum technology industry, the residual pressure is equal to the extreme root pressure in some cases. The residual pressure is the pressure of the gas or gas mixture in the vacuum container after a certain period of time of pumping or the end of the vacuum process. This is a problem that vacuum scientists should pay attention to when they fully understand the concept of ultimate pressure and residual pressure.

    The ultimate pressure of the vacuum chamber is always lower than the ultimate pressure of the vacuum pumping unit. Under the condition that the effective pumping speed of the pump near the suction port is constant, the ultimate pressure of the vacuum chamber is proportional to the leakage and outgassing of the vacuum chamber.

    If the vacuum chamber is formed with a metal welding structure, according to the current level of argon arc welding in my country, the weld leakage rate per unit length is generally 1.3×10^-4Pa·L/(s·cm). In addition, The total leakage rate caused by the leakage of each sealing ring of the vacuum chamber should also be considered.

    Outgassing mainly refers to the amount of outgassing from the surface of the vacuum chamber material, which is determined by the surface area of the vacuum chamber and the outgassing rate Pa*L/(s*cm) of different materials at room temperature.

    When designing a vacuum system, the amount of air leakage is mainly determined by the ultimate vacuum and working pressure of the vacuum chamber. Generally, it is selected to be less than one-tenth of the gas load (the amount of gas released during process production) under working conditions.

(huiputech)