UHV conditions are generally regarded as being in the region below 10 to the -9 millibar. Since atmospheric pressure is about 1 bar, this means that the number of atoms of gas in a UHV chamber is 1/1,000,000,000,000 that of air per unit volume. Frequently pressures are a factor 10 or more below this. UHV is needed for surface science as molecules in the air will land on a surface and change its properties. Even at a pressure of 10 to the -6 millibar, a layer of gas atoms will form on the surface in about 3 seconds (assuming every atom to strike the surface sticks to the surface). This is clearly not enough time to do an experiment. However, at UHV pressures, which are a factor 1000 and above lower in pressure, means that hours are needed before the sample is significantly degraded.

In order to achieve UHV, some special procedures are needed. Initially, the vacuum chamber will be pumped down to 10 to the -2 millibar using a rotary pump. Then the chamber will be pumped down to about 10 to the -6 millibar using a Turbomolecular pump. At this stage, the vacuum chamber is enclosed in heat resistant boards (known as ovens), and baked to a temperature of about 180 Celsius. After a day or so of baking, the ovens are removed, and the chamber allowed to cool down again. Once down at room temperature, the chamber should have a pressure in the UHV region. The process of baking removes gas atoms which are stuck to the chamber walls. These gas atoms slowly desorb from the chamber wall surfaces, and if the chamber was not baked, then months would have to pass before the chamber achieved UHV conditions.