Limitations of a vacuum pump Can a vacuum pump create a vacuum with a pressure lower than the product of it's pressure ratio and the ambient pressure? If you have a pump evacuating a tank to the ambient, it seems like we wouldn't be able to go below the pressure ratio times the ambient pressure.
Also, what influences the pressure ratio?
 A: If the pressure ratio is a truly independent of the inlet pressure, then you should be able to get an arbitrarily low pressure as follows:


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*Pump from some volume A and exhaust to the atmosphere until the pressure in A is as low as the vacuum pump allows

*Pump from some volume B and exhaust to A until the pressure in B is as low as the vacuum pump allows

*Repeat with volume C and so on.


At each stage the pressure falls by less, but in the limit of repeating this infinitely you should theoretically be able to reach a true vacuum.
If your pump is a simple volume displacement pump (e.g. a piston), then (for ideal gases) the pressure ratio would depend on the volume ratio only, and would be independent of inlet pressure. Attempting the infinite process above would eventually reduce the inlet pressure to a point where the gas stopped behaving like an ideal continuum and started behaving like a collection of particles, at which point the pressure ratio would no longer be constant and the process could stall.
