Equilibrium pressure is reached when a normal vacuum system has been evacuated from atmospheric pressure for a specified amount of time. The chamber's dimensions, materials, seals, pre-treatments, venting technique, leak-tightness, and the type of vacuum pump employed all have a role in determining the pressure ( ~ 10-6 to 10-7 mbar).
Steps to Achieve the Desired Pressure
While every setup is slightly different, most systems reach equilibrium pressure when the outgassing gas load is proportional to the pumping speed. In high vacuum, the most significant source of the load is water vapor, which is preferentially coupled to the vacuum materials and can be considered a nearly endless source.
Systems are commonly heated for 24 to 48 hours at temperatures of 150 °C to 250 °C to obtain reduced (UHV and XHV) pressures. The pressure in the system will increase to about 105 mbar during baking. The machine will keep baking until the pressure drops significantly. The total
bake-out temperature shouldn't go above the maximum safe operating temperature of any part of the system.
Turbomolecular Pump Systems: 3 Factors to Consider When Backing
- Water Adsorption
The bottom blade stacks of the turbomolecular pump can be purged with an inert gas at a rate of 10-20 sccm to help reduce water adsorption. In addition, using gas to power the backup pump is recommended to avoid moisture buildup.
- Optimal Working Temperature
A turbomolecular pump's flange temperature is normally regulated to between 100 and 120 degrees Celsius. That requires the installation of specialized bake-out bands for the flange and the casing. Water vapor will be deposited onto the blades if they are not heated. These bands require temperature regulation to ensure they stay below the maximum safe levels. To be extra cautious, shield the turbomolecular pump inlet from radiant heat sources.
Using a turbomolecular pump with a capacity of 300 l/s, we find that the gas load at a bake-out pressure of 3x10-5 mbar is only about 0.01 mbar l/s for a 'typical' system containing around 200 liters and having a total surface area of around 2 square meters. That is roughly 0.03 grams per hour or 0.6 sccm.
This load is insignificant when paired with a scroll or oil-sealed rotary vane pump of suitable capacity. However, the type of load, in this case, water vapor, must be considered.
- Extra Gas Loads
At these low flows, the backing pump's capacity will quickly become 100% water, limiting the backing pressure if ballast is not present. During the baking process, some UHV specialists intermittently activate the ballast.
Although water has been the primary emphasis, it is also essential to identify any other potential gas loads that can be formed during the bake-out process. Hydrogen makes up a significant fraction of the residual gas during bake-out and is typically the most abundant gas once bake-out is complete. Even at low volumes, you must take precautions when working with hydrogen since it is combustible.
Trust Your Vacuum Pump to the Experts
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