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While creating a high vacuum for thin film deposition, we used rotary pump to reach about 0.001 torr and then used the diffusion pump to reach even higher vacuum. Why cant we use the diffusion pump from start? And why does the rotary pump unable to achieve higher vacuum?

Note- we used an oil diffusion pump and a rotary vane pump.

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  • $\begingroup$ I hate to be the one to say this, but en.wikipedia.org/wiki/Diffusion_pump hints pretty strongly at it. I think it's due to the the fact that a diffusion pump is relying on relatively low-mass vapor to entrain the remaining gas on the intake. $\endgroup$
    – William
    Commented Mar 15, 2017 at 14:14
  • $\begingroup$ I read that. But i wanted an answer from an practical point of view as if what would happen if i did discharge it into atmosphere. $\endgroup$
    – Siddhartha Dam
    Commented Mar 15, 2017 at 14:19
  • $\begingroup$ The two pumps operate in different regimes. The rotary pump does fine when the air is acting like a fluid, that is when the molecule's mean free path is very short. A diffusion pump only operates when the mean free path is quite long. The diffusion pump uses momentum transfer from the oil mist to preferentially direct air molecules down in to the pump and away from the vacuum system. This doesn't work unless the mean free path is on the order of the dimension of the pump. $\endgroup$
    – Jon Custer
    Commented Mar 15, 2017 at 14:35

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Diffusion pumps can't discharge directly into the atmosphere. The high vacuum they create would be difficult to maintain against outside pressure. They can function only over very low pressure ranges. If discharge pressure is too great, the oil can backstream into the vacuum chamber, which ruins the entire process. Also, direct contact of the heated oil with the outside atmosphere would cause the oil to burn.

Therefore, diffusion pumps discharge into an outlet pump that maintains a higher vacuum pressure, which then discharges into the atmosphere.

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  • $\begingroup$ Thanks for the answer. Can you answer the second part as well? $\endgroup$
    – Siddhartha Dam
    Commented Mar 15, 2017 at 14:18
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    $\begingroup$ @SiddharthaDam : Rotary vane pumps are capable of moving greater quantities of rough vapor at lower cost than diffusion pumps. But the moving parts of a Rotary vane pump would wear out much quicker, and might even be subject to catastrophic failure at the great speeds required to achieve the low vacuum pressures maintained by diffusion pumps. Also, required tolerances between moving parts to maintain ultra-low-pressure vacuum seal might be impossible to achieve. Here's a good summary of characteristics of various kinds of vacuum pumps: people.rit.edu/vwlsps/LabTech/Pumps.pdf. $\endgroup$
    – Ernie
    Commented Mar 15, 2017 at 14:30
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To answer the second part:

A rotary vane pump compresses a volume of air on vacuum side and transports it to the outlet. Once vacuum end pressure is achieved, the compression factor is such that outside pressure is achieved for the given volume. As the compression factor is finite only a finite end pressure can be achieved.

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