S1 Entry Ticket: Pre-Synchronous Session Diagnostic

Scenario: "The Foreline Swap"

R1-A has been operating reliably for several weeks. The system consistently reaches a base pressure of 0.05 mbar in approximately 8 minutes from atmospheric (~950 mbar). The pump (R1-P-RP) was last serviced one month ago and has been performing within specification.

Last Friday, during routine maintenance, the foreline (R1-L-FL) was replaced. The maintenance technician used available stock: a tube with 15 mm internal diameter and 1.5 metres in length, with two 90-degree bends. The original foreline was 25 mm internal diameter, 0.6 metres long, and straight (no bends).

On Monday morning, the first pump-down is performed. The viscous flow phase looks normal — 950 to 1 mbar takes about 95 seconds (previously 90 seconds).

But below 1 mbar, the pump-down slows dramatically. After 30 minutes of continuous pumping, R1-G-CH reads 0.18 mbar and is still dropping very slowly.

The operator isolates the system (R1-V-ISO closed) and performs a rate-of-rise test:

The pump is then tested independently by disconnecting the foreline and placing a gauge directly at the pump inlet. The pump reaches 0.008 mbar within 5 minutes — well within its rated specification of 0.01 mbar.

Entry Question 1: Pump-Down Phase Analysis

Compare the viscous flow phase (950 to 1 mbar) to the molecular flow phase (below 1 mbar) for this pump-down. Which phase is abnormal, and what does this tell you about where the problem lies in the system?

Your answer:

Entry Question 2: Rate-of-Rise Interpretation

The rate of rise for each interval in the isolation test has been calculated below:

Examine the rate-of-rise values. Is the rate constant or decreasing?

What gas load source does this pattern indicate — a leak or outgassing? How does this finding help narrow the diagnosis?

Your answer:

Entry Question 3: Pump Verification

The pump reaches 0.008 mbar when tested independently at its own inlet. The chamber stalls at 0.18 mbar through the new foreline.

What does the gap between these two pressures tell you? What component or section of the system is responsible for the performance loss?

Your answer:

Entry Question 4: Conductance and Geometry

The original foreline was 25 mm bore, 0.6 m long, straight. The replacement is 15 mm bore, 1.5 m long, with two 90-degree bends. Without performing any calculations, explain in descriptive terms why each of these three changes (diameter, length, bends) reduces the conductance of the foreline, and which change has the greatest impact in the molecular flow regime.

Your answer:

Entry Question 5: Escalation Note

Write a 3-sentence escalation note for the maintenance supervisor summarising: (1) what was observed, (2) what the evidence indicates, and (3) what additional information would help clarify the situation. Use specific component IDs and readings.

Your answer:

S2 Worked Example: Entry Ticket Model Answer

Entry Question 1 — Model Answer

The viscous flow phase (950 to 1 mbar) is essentially normal — 95 seconds vs the previous 90 seconds. The 5-second difference is minor and consistent with a slight increase in viscous flow resistance from the narrower foreline, but it does not represent a significant problem. The molecular flow phase (below 1 mbar) is severely abnormal — after 30 minutes, the system is at 0.18 mbar vs the expected 0.05 mbar at 8 minutes.

This tells us that the problem affects molecular flow much more than viscous flow. In viscous flow, conductance is relatively high and pressure-dependent, so even a restricted foreline passes gas adequately. In molecular flow, conductance depends on geometry alone — diameter, length, and bends — and the new foreline's geometry is creating a severe bottleneck that prevents the pump's capability from reaching the chamber.

Entry Question 2 — Model Answer

The rate decreases from 0.020 to 0.004 mbar/min over 10 minutes. This is a decreasing rate — the classic outgassing (surface desorption) pattern.

This rules out a real leak, which would produce a constant rate. The gas load is normal surface desorption, not an air ingress problem.

This finding is critical for the diagnosis: it eliminates both leaks and contamination as causes. The system's difficulty reaching base pressure is not due to excessive gas load entering the chamber — it is due to the pump's inability to remove gas fast enough through the restricted foreline.

Entry Question 3 — Model Answer

The pump reaches 0.008 mbar at its inlet. The chamber stalls at 0.18 mbar — approximately 22 times higher. This gap proves the pump is not the problem; it is performing well within its specification.

The entire performance loss is attributable to the gas path between the pump and the chamber — specifically, the new foreline (R1-L-FL).

In the molecular flow regime at 0.18 mbar, the conductance of the foreline determines how much of the pump's capability is "delivered" to the chamber. The new foreline's low conductance acts like a narrow bottleneck: the pump creates excellent vacuum at its own inlet, but that vacuum cannot propagate effectively through the restricted tube to the chamber. The effective pumping speed at the chamber is a small fraction of the pump's rated speed.

Entry Question 4 — Model Answer

Diameter reduction (25 mm to 15 mm): A narrower tube provides less cross-sectional area for gas to pass through. In molecular flow, molecules bounce randomly between walls — in a narrower tube, molecules are more likely to bounce backward toward the chamber rather than continuing toward the pump. Conductance in molecular flow scales approximately with the cube of the diameter, so this reduction alone cuts conductance dramatically.

Length increase (0.6 m to 1.5 m): A longer tube means molecules must undergo more wall collisions to traverse the passage. Each collision randomises the molecule's direction, so a significant fraction of molecules bounce back before reaching the pump. Conductance decreases inversely with length — doubling the length roughly halves the conductance.

Bends (two 90-degree elbows added): Each bend forces molecules to collide with the bend wall, randomising their direction. Some fraction bounce back the way they came. In molecular flow, bends are particularly costly because there is no collective flow to carry molecules around the corner — each molecule must individually navigate the geometry.

Greatest impact: The diameter reduction has the greatest impact because conductance scales with the cube of the diameter. The reduction from 25 mm to 15 mm alone reduces molecular flow conductance by roughly 80%. The length and bends compound the problem, but diameter is the dominant factor.

Entry Question 5 — Model Answer

"R1-A pump-down stalls at 0.18 mbar after 30 minutes — normal base is 0.05 mbar in 8 minutes. R1-P-RP verified to reach 0.008 mbar at its inlet (within spec); rate-of-rise test rules out leak (decreasing rate from 0.020 to 0.004 mbar/min).

The replacement foreline (R1-L-FL: 15 mm bore, 1.5 m, two bends) has severely reduced molecular flow conductance compared to the original (25 mm bore, 0.6 m, straight). Additional information needed: confirmation that the original foreline specification (25 mm bore, 0.6 m, straight) is available for comparison testing."

S3 Worked Example: Situation Report

Scenario Context (Facilitator-Provided During Synchronous Session)

R1-A has been pumping for 12 minutes. The viscous flow phase was normal. R1-G-CH currently reads 0.12 mbar and is dropping very slowly — approximately 0.005 mbar per minute.

The operator is uncertain whether this is normal behaviour in the molecular flow regime or an indication of a problem. The system was cleaned last week and has not been opened since.

Model Situation Report

System: R1-A State: ROUGHING (R1-V-ISO open, R1-V-VENT closed, R1-P-RP running) Time: 10:45

Observation: R1-G-CH reads 0.12 mbar at 12 minutes into the pump-down. Viscous flow phase (950 to 1 mbar) completed in ~90 seconds — normal.

The pressure is continuing to drop but at a very slow rate (~0.005 mbar/min).

R1-G-BX reads ~950 mbar. The pump sounds normal. No unusual exhaust odour from R1-FLT-EXH.

Interpretation: At 0.12 mbar, the system is in the molecular flow regime. The slow rate of pressure decrease is expected at this stage — the gas load is dominated by surface water desorption, and the molecular flow conductance of the foreline limits the effective pumping speed at the chamber.

The system logbook shows a normal base pressure of 0.05 mbar, typically reached at approximately 8-10 minutes. At 12 minutes with 0.12 mbar, the system is running slightly slow but within the expected range for a first pump-down after the system has been idle.

Comparison to expected: The logbook reference shows 0.065 mbar at 7 minutes. Today's 0.12 mbar at 12 minutes is slower, but the system was idle (though closed) for several days, which would allow some surface gas re-accumulation. If the pressure continues to decrease and reaches 0.05 mbar within the next 5-10 minutes, this is likely within normal variation.

Unknowns - Evidence Needed:

1. UNKNOWN: Will the system reach 0.05 mbar, or will it stall? Continue monitoring R1-G-CH for the next 10 minutes.
2. UNKNOWN: Is the pump-down rate still decreasing (approaching base) or has it levelled off (approaching a stall)? Record R1-G-CH at 2-minute intervals to characterise the trend.
3. UNKNOWN: If the system does not reach 0.05 mbar within 25 minutes total, a rate-of-rise test should be performed to check for a developing leak.

Escalation: "R1-A roughing in progress — 12 minutes in, R1-G-CH at 0.12 mbar and still dropping slowly. Viscous phase was normal. Currently monitoring the molecular flow phase to confirm the system reaches normal base pressure (0.05 mbar).

Slight delay consistent with idle-time surface gas accumulation. Will report if base pressure is not achieved within 25 minutes or if the pressure trend flattens."

S4 Worked Example: Evidence Brief

Scenario Context

Following a series of pump-down tests on R1-A, the following evidence has been collected. The system was recently returned to service after the foreline (R1-L-FL) was replaced with a smaller-bore tube (15 mm vs original 25 mm, 1.2 m vs 0.6 m, one bend vs straight). The system cannot reach its expected base pressure.

Evidence collected:

• Pump-down: 950 to 1 mbar in 95 seconds (reference: 90 seconds)
• Pump-down: 1 mbar to stall at 0.22 mbar after 30 minutes (reference: 1 to 0.05 mbar in 6.5 minutes)
• Rate-of-rise test (isolated at 0.22 mbar): decreasing rate from 0.018 to 0.003 mbar/min over 10 minutes
• Pump independent test: 0.008 mbar at pump inlet (within spec of 0.01 mbar)
• Chamber: cleaned last week, not opened since
• Maintenance record: foreline replaced with 15 mm bore, 1.2 m length, one 90-degree bend

Model Evidence Brief

System: R1-A State during test: ROUGHING (stalled at 0.22 mbar), then ISOLATED (rate-of-rise test) Investigation: Post-maintenance performance assessment

State Call: ROUGHING — R1-V-ISO open, R1-V-VENT closed, R1-P-RP running. Confirmed by valve positions.

Observed Evidence:

• Viscous flow phase essentially normal (5-second delay — not significant)
• Molecular flow phase severely degraded — stalls at 0.22 mbar vs expected 0.05 mbar
• Rate-of-rise: decreasing (0.018 to 0.003 mbar/min) — outgassing, not a leak
• Pump independent test: 0.008 mbar — pump meets specification
• Chamber clean and sealed — no contamination source identified
• Foreline geometry changed: bore reduced 40%, length doubled, bend added

Plausibility Check: The pump reaches 0.008 mbar independently. The chamber stalls at 0.22 mbar — a factor of 27.5 above the pump's capability.

With no leak, no contamination, and a functioning pump, the conductance of the gas path is the only remaining variable. The foreline was the only component changed during maintenance.

Hypotheses (ranked):

Discriminator Evidence: The definitive test is to restore the original foreline specification. If the base pressure returns to 0.05 mbar, the foreline conductance was the sole cause. This is a low-cost, reversible change that directly tests the leading hypothesis.

UNKNOWN - Evidence Needed:

1. Whether the original foreline is still available for reinstallation
2. Whether the maintenance stock includes 25 mm bore tubing of the correct length
3. Whether any other fittings or connections were disturbed during the foreline replacement

Escalation Note: "Post-maintenance investigation of R1-A performance degradation: base pressure stalled at 0.22 mbar (expected: 0.05 mbar). All evidence points to the replacement foreline (R1-L-FL: 15 mm bore, 1.2 m, one bend — original was 25 mm bore, 0.6 m, straight) as the single leading cause.

Pump verified independently at 0.008 mbar. Rate-of-rise rules out leak. Additional information needed: whether the original foreline is available for reinstallation to confirm the diagnosis."

S5 Worked Example: Sector Lens Output

Scenario Context

Using the conductance bottleneck scenario from S4, the student applies the thin-film coating sector lens. M03 begins the transition from the general industrial sector to thin-film coating applications.

Model Sector Lens Output

Base scenario: R1-A conductance bottleneck — replacement foreline (15 mm bore, 1.2 m, one bend) causing base pressure stall at 0.22 mbar vs expected 0.05 mbar.

Sector: Thin-Film Coating

Sector Lens Application:

In a thin-film coating environment (e.g., sputtering, evaporation, or chemical vapour deposition), this conductance bottleneck would have severe consequences:

• Process impact: Thin-film coating processes typically require base pressures between 0.001 and 0.01 mbar before process gas is introduced. A system stalling at 0.22 mbar is 20-200 times above the required base pressure. No coating process could begin under these conditions. Residual gas at 0.22 mbar would contaminate the deposited film, producing defective coatings with poor adhesion, incorrect composition, or unacceptable impurity levels.

• Cycle time impact: Even if the conductance issue were partially resolved, a restricted foreline extends the molecular flow pump-down phase — the phase that dominates cycle time in thin-film coating production. In a production environment running multiple coating cycles per shift, every additional minute in the pump-down phase reduces throughput. Over a production week, the cumulative time loss from a conductance-limited foreline can be significant.

• System design implication: This scenario illustrates why thin-film coating systems are designed with careful attention to molecular flow conductance. Short, wide forelines with minimal bends are standard practice. Full-bore isolation valves (where the valve bore matches the foreline diameter) are used to prevent the valve from becoming a bottleneck. In a production coating system, the foreline specification is not an afterthought — it is a critical design parameter that directly affects process capability and throughput.

• Maintenance discipline: In a thin-film coating facility, replacing a foreline with a smaller-bore tube would be treated as a non-conformance — a deviation from the engineered specification that compromises process performance. Maintenance procedures in coating environments typically specify exact tubing dimensions, materials, and routing to prevent exactly this type of problem. Spare parts stock should match original specifications.

Sector-Specific Escalation: "Conductance bottleneck identified on R1-A: replacement foreline (R1-L-FL) has reduced molecular flow conductance, stalling base pressure at 0.22 mbar. In a thin-film coating application, this would prevent the system from reaching the required base pressure range (0.001-0.01 mbar), making all coating processes impossible.

Film quality, adhesion, and composition would be unacceptable at this base pressure. Additional information needed: availability of original-specification foreline (25 mm bore, 0.6 m, straight) and whether maintenance stock records match the original engineering specifications."

S6 Reading List

Use these references to deepen your understanding of the concepts covered in Module 3. They are organised by topic and include section references for quick navigation.

How to Use This List:

• Start with Milne, Chapter 3 for a narrative introduction to flow regimes and the transition from viscous to molecular flow
• Read Varian, Chapters 4-5 for the clearest practical treatment of conductance and how foreline geometry affects system performance — this is the most directly relevant resource for Module 3
• Reference Pfeiffer, Sections 2.1-2.4 for detailed pump-down curve examples and the formal relationship between flow regime and conductance
• Browse the KJLC/ORNL deck, slides 130-200 for visual reinforcement of pump-down behaviour and effective pumping speed concepts

KJLC/ORNL Deck — Slide Guide for Module 3:

End of Assessment Content — Module 3