Module 3 Scenario Cards: Flow Behaviour, Conductance & System Geometry

R1-A Component Reference

SC-M03-01: Slow Pump-Down Diagnosis — Normal vs Abnormal Below 1 mbar

Module: M03 Rig Config: R1-A Competency: M03-COMP-01 Indicators Assessed: M03-IND-01.01, M03-IND-01.02, M03-IND-01.03

System State

State Name: ROUGHING (extended) One-line description: R1-A is being roughed and has passed through the fast viscous-flow phase normally, but the pump-down below 1 mbar is taking longer than expected. The question is whether the slow pump-down is normal (first pump-down of the day with elevated surface water) or abnormal (indicating a problem).

Background Information (Provided to Students)

R1-A was left in VENTED state (both valves closed, pump off, chamber at ~950 mbar) overnight after being cleaned yesterday afternoon. The workshop environment is moderately humid.

This morning, the operator begins roughing. The fast phase from 950 to 1 mbar completes in the normal ~90 seconds. Below 1 mbar, the pump-down slows.

The operator records the pump-down data in the low-pressure region and compares it to the reference curve from the system logbook.

Valve Positions

Gauge Readings

Pump-Down Comparison Data

Pump Status

Student Prompt

Teaching Points (Facilitator Notes)

Expected student observations:

• The viscous flow phase (950 to 1 mbar) is identical — this confirms the pump and gross system conductance are normal
• The divergence occurs below 1 mbar, in the transition-to-molecular flow regime
• Today's curve eventually reaches the same base pressure — the system is not broken, it just takes longer
• The pump-down time is 18 minutes vs 8 minutes — more than double in the molecular flow phase

Key learning moments:

• Normal slow pump-down: After overnight exposure to atmosphere in a humid environment, chamber surfaces re-adsorb water. This water desorbs slowly under vacuum, creating an elevated gas load in the molecular flow regime. The pump is working normally; there is simply more surface gas to remove. This is expected on a "first pump-down of the day" and does not require intervention.
• Flow regime connection: Below 1 mbar, the gas transitions from viscous to molecular flow. In molecular flow, the pump is less efficient at capturing molecules and conductance drops. Combined with the elevated surface gas load, both factors contribute to the extended pump-down.
• Key diagnostic: base pressure is reached. The fact that the system eventually reaches 0.05 mbar confirms there is no leak and no permanent conductance problem. The extra time is attributable entirely to the additional surface gas.

Model status note: "R1-A first pump-down of the day: viscous phase normal (950 to 1 mbar in 90 seconds), molecular flow phase extended (1 to 0.05 mbar in 16.5 minutes vs reference 6.5 minutes) — consistent with elevated surface water after overnight atmospheric exposure. System reached normal base pressure of 0.05 mbar. No action required."

Escalation scenario (stalled at 0.20 mbar): If the system stalls at 0.20 mbar after 30 minutes, the assessment changes from "normal slow pump-down" to "abnormal — possible leak or severe contamination." The key additional information would come from a rate-of-rise test: isolating the system and monitoring R1-G-CH for 10 minutes. If the rate is constant, this indicates a leak. If the rate is decreasing, this indicates contamination that exceeds what overnight atmospheric exposure would normally produce.

Common student errors:

• Concluding the pump is defective because the pump-down is slow (the identical viscous phase rules this out)
• Not recognising that the same base pressure is eventually reached (this is the key indicator that the system is fundamentally sound)
• Failing to connect the flow regime change to the observed behaviour

SC-M03-02: Conductance Bottleneck Identification — Foreline Restriction

Module: M03 Rig Config: R1-A Competency: M03-COMP-02 Indicators Assessed: M03-IND-02.01, M03-IND-02.02, M03-IND-02.03

System State

State Name: ROUGHING (stalled at low pressure) One-line description: R1-A has been roughed and the pump is running normally, but the chamber pressure has stalled above the system's expected base pressure. Investigation reveals the problem is between the pump and the chamber, not in the chamber itself.

Background Information (Provided to Students)

R1-A was returned to service after a scheduled maintenance period. During maintenance, the foreline (R1-L-FL) was replaced. The technician who performed the replacement used the available stock: a tube with a smaller internal diameter (15 mm) than the original (25 mm), and a longer run (1.2 metres with one 90-degree bend, versus the original 0.6 metres straight).

The system is clean — the chamber was solvent-cleaned and dried during maintenance. No contamination is suspected. The first pump-down after maintenance is being monitored.

Valve Positions

Gauge Readings

Pump-Down Comparison

Additional Evidence

The pump was tested independently (foreline disconnected, gauge placed directly at pump inlet). The pump reached 0.008 mbar — well within its specification of 0.01 mbar ultimate pressure. The pump is not the problem.

A rate-of-rise test was performed (R1-V-ISO closed at 0.25 mbar). The rate of rise was 0.015 mbar/min, decreasing to 0.004 mbar/min over 10 minutes — consistent with normal outgassing, not a leak.

Pump Status

Student Prompt

Teaching Points (Facilitator Notes)

Expected student observations:

• The viscous flow phase (950 to 1 mbar) is only slightly slower — the restriction has a moderate effect in viscous flow
• The molecular flow phase (below 1 mbar) is catastrophically affected — the system stalls at 0.25 mbar vs the expected 0.05 mbar
• The pump is verified to meet specification
• The rate-of-rise test rules out a leak
• The only variable that changed is the foreline: narrower (15 mm vs 25 mm), longer (1.2 m vs 0.6 m), and with a bend

Key learning moments:

• Diameter dominates molecular flow conductance. Reducing the foreline diameter from 25 mm to 15 mm cuts the molecular flow conductance dramatically (approximately (15/25)^3 = 0.216, or about a five-fold reduction from diameter alone). Adding length (doubled) and a bend compounds the loss.
• Viscous vs molecular impact difference. The restriction has only a modest effect in viscous flow (5 extra seconds in the high-pressure phase) but a devastating effect in molecular flow (system cannot reach base pressure). This illustrates that conductance bottlenecks are pressure-regime-dependent — a tube that works fine at high pressure can cripple the system at low pressure.
• Identify the leading bottleneck. The student should name the foreline as the single leading cause — not list five equal possibilities. The evidence chain (pump verified, no leak, no contamination, only the foreline changed) eliminates all alternatives.

Model maintenance finding: "Post-maintenance pump-down on R1-A stalls at 0.25 mbar — expected base is 0.05 mbar. The replacement foreline (R1-L-FL) has a 15 mm bore (original: 25 mm), is 1.2 metres long with a 90-degree bend (original: 0.6 m straight).

In the molecular flow regime below 1 mbar, this reduced bore and increased length severely restrict gas conductance to R1-P-RP, preventing the pump's capability from reaching the chamber. This evidence indicates that restoring R1-L-FL to the original specification (25 mm bore, 0.6 m straight run) would be required to resolve the conductance limitation."

Thin-film coating impact: At 0.005 mbar, the conductance limitation would be even more severe. The system would stall at approximately 0.25 mbar — fifty times higher than the required coating pressure.

The thin-film coating trial would be impossible with the current foreline. Restoring the original foreline specification is a prerequisite for any low-pressure process work.

Common student errors:

• Blaming the pump despite the independent verification showing it meets spec
• Suggesting "check for leaks" despite the rate-of-rise test ruling this out
• Listing multiple equal causes instead of identifying the foreline as the single leading bottleneck
• Not connecting the diameter reduction to the molecular flow conductance impact
• Underestimating the severity — the diameter change seems small (10 mm difference) but the conductance effect is dramatic

SC-M03-03: Flow Regime Transition Observation — Comparing Pump-Down Phases

Module: M03 Rig Config: R1-A Competency: M03-COMP-01, M03-COMP-02 Indicators Assessed: M03-IND-01.01, M03-IND-01.02, M03-IND-01.03, M03-IND-02.03

System State

State Name: ROUGHING (full pump-down — start to base) One-line description: R1-A is being roughed from atmospheric to base pressure. The student observes the complete pump-down and identifies the distinct phases corresponding to different flow regimes and gas load sources.

Background Information (Provided to Students)

R1-A has been thoroughly cleaned and left in VENTED state for exactly one hour (both valves closed, pump off, ~950 mbar). The system is in known-good condition — no leaks, no contamination, fresh pump oil. This is the reference pump-down for the system, recorded to establish the baseline performance curve.

The facilitator has recorded detailed timing data throughout the pump-down.

Valve Positions

Gauge Readings — Full Pump-Down Record

Detailed Pump-Down Time Series

Pump Status

Student Prompt

Teaching Points (Facilitator Notes)

Expected student observations:

Phase A (950 to ~3 mbar) — Viscous Flow, Bulk Gas Removal:

• Flow regime: Viscous — molecules collide frequently, gas flows collectively
• Gas source: Bulk atmospheric gas (N2, O2, Ar, CO2, H2O)
• Characteristics: Very fast pressure drop, pump working at maximum efficiency
• Duration: ~90 seconds for the entire phase
• Observable clue: Pressure drops by orders of magnitude per minute

Phase B (~3 to ~0.1 mbar) — Transition Flow, Surface Gas Emerging:

• Flow regime: Transition — mean free path approaching system dimensions
• Gas source: Mixed — residual bulk gas plus emerging surface sources (water desorption)
• Characteristics: Dramatic slowdown in pressure drop rate (from hundreds of mbar/min to fractions)
• Duration: ~3.5 minutes
• Observable clue: The "knee" in the pump-down curve where the steep descent begins to flatten

Phase C (below ~0.1 mbar) — Molecular Flow, Surface-Dominated:

• Flow regime: Molecular — molecules travel wall-to-wall without intermolecular collisions
• Gas source: Almost entirely surface-related (water desorption, outgassing)
• Characteristics: Very slow, asymptotic approach to base pressure
• Duration: ~5 minutes
• Observable clue: Pressure changes by tiny increments; curve appears nearly flat on a linear scale

Key learning moments:

• The 100,000x rate change is explained by two compounding factors: (1) the gas source changes from an enormous bulk reservoir (10^19 molecules per cm^3 at atmosphere) to slow surface desorption (limited by diffusion), and (2) the flow regime changes from viscous (where the pump sweeps gas efficiently) to molecular (where molecules must randomly find the pump inlet). Neither factor alone explains the full magnitude — both contribute.
• Phase boundaries are gradual, not sharp. The transitions are continuous, not discrete switches. But the observable changes in the pressure drop rate provide practical markers for identifying which regime dominates at a given pressure.
• Conductance matters most in Phase C. Any restriction in the foreline or valve has its greatest impact in the molecular flow regime. A conductance bottleneck might be invisible during Phase A but becomes the dominant limitation in Phase C.

Model explanation for the new technician: "The pump is not slowing down — it is working just as hard as at the start. What has changed is the gas behaviour. At high pressure, there are enormous numbers of gas molecules packed together, flowing as a group toward the pump — the pump catches them easily.

At low pressure, the remaining molecules are spread far apart, moving randomly, and most of the gas is being slowly released from surfaces rather than from the air itself. The pump must wait for individual molecules to wander into its inlet, which takes much longer per molecule removed."

Commissioning report metrics: For thin-film coating commissioning, the report should highlight:

1. Base pressure achieved (0.050 mbar) and time to base (10 minutes) — establishes the system's baseline capability
2. Pump-down profile through the molecular flow regime (Phase C timing and shape) — this is where the coating process operates, so this phase determines practical cycle times
3. Comparison to the coating process target pressure — if the coating process requires 0.005 mbar, the report should note that R1-A's current base pressure (0.05 mbar) is 10x higher than required, and additional pumping capacity or system modifications may be needed

Common student errors:

• Describing only two phases (fast and slow) instead of recognising the three-phase structure with the transition region
• Attributing the slowdown entirely to "fewer molecules" without addressing the flow regime change
• Not connecting conductance to the molecular flow phase — missing the opportunity to link Phase C performance to foreline geometry
• Giving an overly technical explanation to the "new technician" prompt instead of using accessible language

End of Scenario Cards — Module 3