In
the ultra high vacuum range (UHV) with pressures <10-8 mbar
cleanliness of the chamber and the flange components is critical. The
gas desorption rate is an important factor for achieving UHV conditions.
The manufacturing technique, surface treatment and cleaning processes
guarantees a low desorption rate. To reach the UHV range as quickly as
possible, baking the UHV system at 300 °C with
the vacuum pumps running is recommended. At these temperatures elastomer
gaskets are suitable for test purposes only. All flange connections are
sealed with OFHC copper gaskets (oxygen-free high conductivity copper).
CF-flanges are equivalent to Conflat®. The
following table shows the various standard dimensions for CH-UHV flanges.
CF
sealing principle
A flat copper gasket fits with a small clearance in the outer recess
of the flanges and thus assures centering of the flange connection.
The sealing of two opposite flanges is made by the knife edges of
the flanges which penetrate the flat copper gasket. The shear action
of the outer face of the cutting edge produces a yield pressure
on the copper gasket even at high temperatures. Since the stainless
steel knife edges penetrate deeply into the soft copper gasket the
gasket can only be used once.
For leak testing the grooves machined in a radial direction on the
flange surface and extended right up to the gasket are used. The
sniffer probe of the leak detector is in direct contact with the
gasket.
Components with two or more flanges have at least one rotary flange.
On adapter or intermediate parts the rotary flanges allow the mounting
of manipulators or current feed throughs etc. in the correct position.
For mounting of sight glasses and current feed-ins to chambers annealed
OFHC copper gaskets should be used which are softer and reduce material
tensions. Proper assembly is important.
