Pressures below 10
torr are termed ultra high vacuum.
To achieve pressures this low, cyropumps or ion pumps in series with
mechanical pumps are used. However, the system must be pumped to low
high vacuum pressure before these pumps can be operated, and their
pumping speed is comparatively low. Ultra high vacuum also requires
other specialized techniques, such as metal-metal connections and
high temperature baking of system components to eliminate adsorbed
residual gas.