Stainless steel with its unique mechanical properties is the most frequently used material for manufacturing of vacuum chambers and components. Its disadvantage is hydrogen outgassing which makes obtaining of ultrahigh vacuum (UHV) troublesome. Vacuum annealing, prolonged in situ bake out, air bake, low-permeable coatings, and other, usually expensive methods, are applied to reduce the outgassing rate and the concentration of hydrogen. Recombination limited outgassing was shown to explain the required duration and temperature of bake out better than diffusion limited outgassing.
In our experiments, thin wall stainless steel chambers (0.15 mm) were examined with requirement of minimising the outgassing rate. They were cleaned and baked out in situ at 540 K resulting in several ten diffusion time constants. Pressure rise was detected by spinning rotor gauge (SRG) after seal-off. Measurements were performed for thousands of hours to exclude transient phenomena. Pressure rise slope at room temperature at 10 -5 mbar of H 2 equivalent pressure gave q out 3 10 -15 mbar l/(s cm 2 ) and another sample measured at 10 -6 mbar gave q out 3 10 -17 mbar l/(s cm 2 ).
Numerical simulations of bake out with different values of recombination coefficient K L were made for comparison. Calculated and measured results are within limits showing that in thin wall stainless steel chambers extremely low outgassing rates can be obtained in moderate bake out procedure.