Correct: B) Gas molecules colliding frequently with each other, causing the gas to behave like a fluid pushed by pressure differences

Correct: C) The mean free path of gas molecules exceeds the dimensions of the system, so molecules travel independently from wall to wall without colliding with each other

Correct: C) Roughly 1 to 0.01 mbar (the transition region depends on system geometry)

Correct: B) At higher pressures, bulk gas in viscous flow is removed efficiently; below about 1 mbar, the remaining gas load comes from slow surface sources (water desorption, outgassing) and the flow transitions toward molecular regime where pumping is less efficient

The fast phase (950 to 1 mbar) is bulk gas removal in viscous flow — the pump sweeps gas out efficiently because molecules flow collectively. The slow phase (below ~1 mbar) involves two factors: (1) the gas load shifts to surface sources like water desorption, which release gas slowly, and (2) the flow regime transitions toward molecular flow, where pumping is inherently less efficient because molecules no longer respond to a collective pressure push. Both factors combine to produce the characteristic slowdown.

Correct: A) True

This is the characteristic shape of a normal pump-down curve. On a log-pressure vs time plot, the curve drops steeply at first (viscous flow, bulk gas removal) and then progressively flattens as the pressure decreases (transition to molecular flow, surface-dominated gas load). The flattening represents the system approaching equilibrium between the gas load and the pump's effective speed at that pressure. A curve that does not flatten — or one that flattens at a pressure much higher than expected — may indicate a problem.

Correct: C) How easily gas can flow through a component or passage — higher conductance means less restriction to gas flow

Conductance is a measure of how readily gas passes through a tube, valve, or other component. A wide, short tube has high conductance (gas flows through easily). A narrow, long tube has low conductance (gas flow is restricted). Conductance is the vacuum equivalent of "how wide is the road" — it determines how much gas can move through a passage for a given pressure difference. It is not an electrical property; the term is borrowed by analogy.

Correct: C) Conductance would decrease — a longer tube restricts gas flow more than a shorter one

Conductance decreases as tube length increases. A longer tube means gas molecules must travel further (and, in molecular flow, undergo more wall collisions) to get through the passage. In viscous flow, conductance is inversely proportional to length. In molecular flow, the effect is even more pronounced because each wall collision randomises the molecule's direction, so many molecules bounce back the way they came. A longer foreline means the pump has to work harder to remove gas from the chamber.

Correct: C) Increasing the diameter of the foreline

Diameter has an enormous effect on conductance. In molecular flow, conductance scales with the cube of the diameter — doubling the diameter increases conductance roughly eightfold. In viscous flow, the relationship is even stronger (fourth power). Diameter is the single most influential geometric factor in vacuum conductance. Polishing the surface has a negligible effect, adding a filter would reduce conductance, and increasing length makes conductance worse, not better.

Correct: C) A conductance bottleneck between the chamber and the pump — such as a restricted foreline or partially open isolation valve — is limiting the effective pumping speed at the chamber

When the pump can reach a lower pressure than the chamber achieves, and the system is clean and leak-free, the problem is between the pump and the chamber. The restriction reduces the effective pumping speed at the chamber — gas cannot travel from the chamber to the pump fast enough. The pump is doing its job, but the pipe or valve connecting it to the chamber is the bottleneck. This is why vacuum engineers care about conductance: the pump's rated performance only matters if the gas can actually reach the pump.

Correct: A) True

Components in series act like resistors in series — the total conductance is always lower than the individual conductance of any single element. The component with the lowest conductance dominates the restriction. This is why a single narrow section in an otherwise wide foreline can dramatically reduce pumping performance at the chamber. Finding and addressing the most restrictive element is the key to improving system conductance.

Correct: B) In molecular flow, gas molecules no longer respond to a collective pressure push — they move randomly, making the pump less effective at sweeping gas from the chamber, which contributes to slower pump-down at low pressures

In viscous flow, the pump creates a pressure difference and gas flows collectively toward the pump — like water flowing through a pipe. In molecular flow, molecules move independently and randomly — they must individually wander into the pump inlet. The pump can only capture molecules that happen to arrive at its inlet. This fundamental change in gas behaviour is one reason why pump-down slows at lower pressures. It is not the only reason (surface gas load also dominates at low pressure), but the flow regime change contributes significantly.

Correct: B) Increased gas load — the chamber surfaces have more adsorbed water or contamination than usual, so the slow phase of the pump-down takes longer

A pump-down curve that has the same general shape but takes longer suggests an increased gas load rather than a system fault. The viscous flow phase (950 to ~1 mbar) would still be normal, but the slow phase below 1 mbar — dominated by surface desorption — takes longer because there is more surface gas to remove. This is consistent with a chamber that was open longer than usual, was not cleaned, or was exposed to humidity. A pump failure would typically change the shape of the curve, not just stretch it.

Correct: C) In viscous flow, conductance depends on pressure (higher pressure means higher conductance); in molecular flow, conductance is independent of pressure and depends only on geometry

This is an important distinction. In viscous flow, the gas flows collectively and conductance increases with pressure — there are more molecules available to push through the passage. In molecular flow, each molecule moves independently, so the flow rate depends only on geometry (diameter, length) and molecular speed — not on how many other molecules are present. This means a tube that conducts gas well at atmospheric pressure may become a significant bottleneck at low pressure where molecular flow governs.

Correct: B) Whether a conductance bottleneck in the foreline or a restricted valve is preventing effective pumping speed from reaching the chamber in the molecular flow regime

At 0.05 mbar, the system is firmly in the molecular flow regime where conductance depends entirely on geometry. If the pump can reach lower pressures on its own (verified at the pump inlet), the restriction must be between the pump and the chamber. In a thin-film coating application, even small conductance losses matter because the process requires very low base pressures — a bottleneck that is invisible during rough pumping becomes the dominant limitation in the molecular flow range where thin-film processes operate.

Correct: C) Identify the single most likely leading bottleneck based on the available evidence — such as the pressure where pump-down stalls and whether the pump achieves its rated pressure when tested independently — rather than listing five equal causes

Effective troubleshooting means ranking hypotheses, not listing equal possibilities. A stall at 0.08 mbar on a clean system with a pump that reaches 0.01 mbar independently points to a conductance bottleneck — likely the foreline or isolation valve — as the leading cause. Testing the pump independently narrows the field immediately. The Week 3 rubric specifically rewards identifying one leading bottleneck over listing five equal causes. Evidence-based ranking is the skill.