Experimental study on the scaling law of the outgassing rate with a pumping parameter

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.