In this 2 Minute Tutorial John Screech describes the impact conductance limitations have on real (effective) pumping speed in a high or ultrahigh vacuum system.
(See the transcript below the video panel.)

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Video Transcript

This is a quick tutorial on the impact of Conductance on Pumping Speed for a High Vacuum System. To keep the math simple will show an aperture conductance limit between the high vacuum pump & the chamber, but any fittings between the pump and the Chamber are going to negatively affect pumping speed. The length and the diameter are both factors, and even elbows will reduce the pumping speed by about 10% per bend. The conductance formula for an aperture gives us a value of 82 liters per second for the restriction shown. The conductance formula shown here is simplified for Air and Nitrogen at room temperature. Manufacturers publish pump speeds based on industry standard protocols and fixtures, but what really matters is the Effective Pumping Speed that we calculate factoring in the conductance. Evacuating our chamber using a 250 liter per second pump through an 82 liter per second conductance will result in an Effective Pumping Speed of only 62 liters per second. We could double or triple the size of the pump here, but if we don't address the conductance limitation, we'll see negligible gains in our Effective Pumping Speed. Increasing the diameter of the opening by a factor of two should improve things since our conductance increases by a factor of 4. And the result is an improvement in effective pumping speed from 62 to 144 liters per second. But we're still giving up almost half our Pumping Speed to conductance limitations. Let's try a larger pump. Doubling the Turbo size to 500 liters per second increased our effective pumping speed to almost 200 liters per second, but now we're only utilizing 38% of our rated pump speed. Even if we installed a 1000 liter per second pump on this system, we'd still only get to an effective pumping speed of 250 liters per second - a quarter of the rated speed. With the correct size inlet for a 250 liter per second pump, the conductance of the opening is now over 900 liters per second, almost four times the pumping speed, which should allow us to realize about 80% of the rated speed. And sure enough, it does. Note to get 90% of the published speed requires a conductance eight times the pumping speed. Getting to 95% requires an opening with twenty times the conductance. You can see how we're approaching diminishing returns at this point, and by the way, to utilize 100% of the pump's rated speed, we would need an infinitely large connection between the pump and the Chamber. We can see the dramatic effect a system with poorly designed conductance can have on the performance of a vacuum system. I hope you found this explanation of conductance and Pumping Speed useful. For a simpler explanation of Conductance, Pumping Speed and Gas Load, check out "Vacuum Made Simple" at Agilent.com.