Abstract
The conductance modulation method has three advantages useful for the measurement of pumping speed in ultrahigh vacuum region. First, the flow rate of gas for the speed measurement is not required if the rate is constant. Secondly, the absolute value of pressure in the test dome is also not necessary if the sensitivity of pressure gauge is unchanged during the measurement. Thirdly, the effects of the positions of pressure gauge and gas inlet in the test dome are relatively small.The orifice system composed of three discs is used as the variable conductance which has two definit values of conductance determined by the Monte Carlo calculation. One of the two values can be selected from outside of the ultrahigh vacuum chamber. The pumping speed of a combination pump made of a sputter ion pump and a titanium getter pump can be successfuly measured down to 10-7 Pa for nitrogen and hydrogen.
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