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When we want to get to really low pressures (which is most of the time here at LCLS) we use layered tiers of pumping: where we turn on one type, wait until the pressure stops dropping, then turn on the next, and again when the pressure stops dropping again we turn on a third type.
The first layer is good for pumping room pressure down through viscous flow and a bit into molecular flow. The second layer works best in molecular flow, and gets (Fun Fact: for similar reasons we have to use layered tiers of pressure gauges as well)
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