whenever I opened the valve of CVD for gas flowing, the vacuum chamber pressure gauge reduced to 1.7x10^0 torr from 9.1x10^-3 torr. How to maintain the pressure inside a tube 45torr or 300torr at the presence of gas flowing during the whole experiment.
45 - 300 Torr will not good for turbo, so you should try to control the pressure with only Rotary and gad inlet needle valve. Once you have achieved base pressure, close the turbo pump, inset gas with continues flow, control the pressure with out-let valve (connected with Rotary). I mostly need 70 - 100 mTorr so I control the pressure with gate-valve of turbo with continuous flow of gas.
To supplement Muhammad's answer, for such a "high" pressure you only really need to use a vane pump for achieving vaccuum. To prevent using too much gas you can reduce the pumping rate by having a needle valve in the pumping line to your vacuum pump, or very simply a section in the connecting tubes that has a small orrifice / hole in it (between sections) to reduce the pumping speed. An inlet mass flow controller or needle valve for you gas should also be used.
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