The outgassing rate of a small test chamber made of type 304 stainless steel has been measured as a function of pumping speed in the range from 0.3 to 200 ℓ/s. The chamber was first exposed to air and then pumped with different pumping speeds for three days at 20 °C without baking. Typical outgassing rates of the chamber are determined from the measurement of steady pressures at the pumping time of 72 h. It is found, as a result, that measured outgassing rates q obey a set of power laws of q1=C1(S/A)m1 and q2=C2(S/A)m2, where S/A is the ratio of pumping speed S to the surface area A of the chamber, C1 and C2 are constants, the exponents m1 and m2 are 0.8282 for S/A⩾10−3 ℓ/s cm2 and 0.4655 for S/A⩽10−3 ℓ/s cm−2. It is shown that the observed power law can be derived from theoretical consideration and is useful to estimate the ultimate pressure of an unbaked vacuum system.

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