Laboratory of Pressure Metrology at the Institute of Metals and Technology is developing a static expansion system for calibrations of vacuum gauges. The gasses evolved from the chamber walls significantly affect the accuracy of generated calibration pressure at low pressures below 1mPa. The chambers of the expansion system were designed in accordance with established ultrahigh vacuum practice and are made from stainless steel. We studied the outgassing characteristics of the large calibration chamber by the gas accumulation method. The chamber was sealed-off from the pump and the rate of pressure rise was measured using the spinning rotor gauge (SRG). From this the total outgassing rate can be determined. At the end of the accumulation period the chamber was evacuated again. During evacuation the composition of the accumulated gas was dynamically analyzed with the quadrupole mass spectrometer. The method of dynamic gas analysis will be described and outgassing of unbaked and baked chamber will be compared. Bake-out temperature was gradually increased from 60 @degree@C to 250 @degree@C. By bake-out at 100 @degree@C for 50 hours we effectively removed water vapour and other adsorbed gases and the main outgassing component became H2 at a rate of 1x10-11 mbarls-1cm-2. Final outgassing rate after 160 hours of bake at 250 @degree@C was 2.4x10-13 mbarls-1cm-2. Measured H2 outgassing rate after bake-out at 100 @degree@C increased by a factor of 3 compared to the initial value of unbaked surface. This indicates the importance of the adsorbed layer on stainless steel surface on the kinetics of H2 evolution. The diffused hydrogen atoms from the bulk must recombine into H2 molecule in order to be de-sorbed. Other adsorbed gasses can hinder the recombination of hydrogen atoms.