Module 3

Workbook

Flow Behaviour, Conductance & System Geometry
Facilitator: adjust scaffolding level before distributing

Module 3 Workbook: Flow Behaviour, Conductance & System Geometry

Estimated Completion Time: 45-60 minutes

Part A: Knowledge Check (20 marks)

Short-answer questions testing recall from Module 3 async learning content. 2 marks each. 10 questions.

A1. During a pump-down from atmospheric pressure (~950 mbar) to base pressure, the first phase is fast and the last phase is slow. In general terms, describe what distinguishes a fast pump-down phase from a slow pump-down phase and where in the pressure range the transition occurs.

(2 marks)

Clue: Think about how gas molecules move differently at high pressure versus low pressure.
Guide: At high pressure, molecules collide with each other frequently — this is viscous flow. At low pressure, molecules travel freely between wall collisions — this is molecular flow. The transition happens when the mean free path approaches the tube diameter.

A2. Describe the difference between viscous flow and molecular flow. What is the key factor that determines which type of flow is occurring inside a vacuum system?

(2 marks)

Clue: Think about what determines how easily gas can flow through a tube in the molecular flow regime.
Guide: Conductance in molecular flow depends on tube geometry: diameter (proportional to d^3), length (inversely proportional), and any restrictions like bends. Unlike viscous flow, pressure does not affect molecular flow conductance.

A3. In simple terms, explain what conductance means in a vacuum system. Why is conductance important when gas must travel from the chamber to the pump?

(2 marks)

Clue: Consider what happens at the boundary between viscous and molecular flow. What determines when the transition occurs?
Guide: The transition happens when the mean free path of gas molecules becomes comparable to the tube or chamber diameter. This is characterised by the Knudsen number (mean free path / tube diameter). Kn < 0.01 = viscous, Kn > 1 = molecular, in between = transitional.

A4. Explain, without using any calculations, how the flow regime influences the rate at which a pump can remove gas from a chamber. Why does a pump that works well at high pressure appear less effective at very low pressure?

(2 marks)

A5. A vacuum system has a long, narrow foreline connecting the pump to the chamber. Describe how this foreline geometry creates a bottleneck for gas flow, and state which flow regime is most severely affected by the restriction.

(2 marks)

A6. Describe, in plain language, how making a foreline longer and thinner reduces pumping speed at the chamber. Explain why a pump that reaches excellent pressure at its own inlet may deliver much poorer pressure at a chamber connected by a restrictive line.

(2 marks)

A7. A pump-down curve shows three distinct phases: a steep initial drop, a transitional middle section, and a slow asymptotic approach to base pressure. Identify which flow regime dominates in each phase, and state the primary gas source being removed in each.

(2 marks)

A8. A system reaches base pressure normally on its first pump-down of the day, but takes 20 minutes longer than the logbook reference to get there. The viscous flow phase was identical to the reference. Explain which flow regime was affected and identify the most likely factor responsible for the extended pump-down.

(2 marks)

A9. Two identical chambers are connected to identical pumps. Chamber A uses a short, wide foreline (25 mm bore, 0.5 m long, straight). Chamber B uses a long, narrow foreline (12 mm bore, 1.5 m long, with two 90-degree bends).

Both reach the same viscous-phase performance. Explain, without calculations, why Chamber B stalls at a much higher base pressure than Chamber A and describe which geometric change has the greatest impact in the molecular flow regime.

(2 marks)

A10. A colleague states: "The pump slows down at low pressures — that is why it takes longer to reach base pressure." Explain why this description is misleading. Describe, using the concepts of flow regime and conductance, what is actually happening during the slow final phase of a pump-down.

(2 marks)

Part B: Concept Diagnostic (20 marks)

Students examine a pump-down comparison and answer concept-diagnostic questions. 4 marks each. 5 questions.

Scenario Reference

Card: SC-M03-01 — Slow Pump-Down Diagnosis: Normal vs Abnormal Below 1 mbar Description: R1-A was left in VENTED state overnight in a moderately humid workshop environment.

This morning, the first pump-down is being compared to the logbook reference curve. The viscous flow phase is identical. Below 1 mbar, today's pump-down is noticeably slower.

Component State/Reading
R1-V-VENT CLOSED
R1-V-ISO OPEN
R1-G-BX ~950 mbar
R1-P-RP ON — running normally

Pump-Down Comparison Data

Time from start R1-G-CH — Today (mbar) R1-G-CH — Logbook Reference (mbar) Notes
0 min (start) 950 950 Identical start
1.5 min 1.0 1.0 Viscous flow phase identical
3 min 0.50 0.30 Today is slower below 1 mbar
5 min 0.25 0.12 Gap is widening
8 min 0.12 0.05 (base) Reference reaches base at 8 min
12 min 0.07 — Today still dropping
15 min 0.055 — Today approaching base
18 min 0.05 (base) — Today reaches base at 18 min

B1. Examine the pump-down comparison data. Classify the behaviour of each pump-down phase: is the viscous flow phase (950 to 1 mbar) normal or abnormal?

Is the molecular flow phase (below 1 mbar) normal or abnormal? State the evidence from the data that supports your classification. (4 marks)

Clue: Look at the scenario data. Which phase of the pump-down shows the abnormality?
Guide: Compare the viscous flow phase timing to the expected value, then compare the molecular flow phase. The phase that is most affected tells you whether the problem is in the bulk gas removal or in the low-pressure pumping path.
Step by step
Apply the diagnostic framework: if viscous flow is normal but molecular flow is affected, the problem lies in something that specifically restricts molecular flow — typically geometry (conductance), not pump capacity or leaks.

B2. Compare the two pump-down curves in the molecular flow region. Today's curve takes 16.5 minutes to go from 1 mbar to 0.05 mbar, while the reference took 6.5 minutes. Despite this difference, both curves reach the same base pressure of 0.05 mbar.

Describe what is the same about both pump-downs and what is different, and explain what the fact that the same base pressure is eventually reached tells you about the overall condition of the system. (4 marks)

B3. The system was left in VENTED state overnight in a humid environment. Explain, in terms of surface processes and flow regime, why this history causes the molecular flow phase to be extended. Describe what is happening on the chamber surfaces and why this affects the low-pressure pump-down more than the high-pressure phase. (4 marks)

B4. If, instead of reaching 0.05 mbar at 18 minutes, today's pump-down had stalled at 0.20 mbar after 30 minutes of continuous pumping, the assessment would change from "normal slow pump-down" to "abnormal." Identify what additional evidence you would look for to distinguish between a possible leak and a conductance bottleneck as the cause of the stall. For each, describe the specific pattern or test result that would confirm the diagnosis. (4 marks)

B5. This scenario involves the R1-A rig in a workshop environment. If this same pump-down behaviour occurred on a system being prepared for thin-film coating trials — where a base pressure of 0.005 mbar is required before process gas is introduced — explain how the extended molecular flow phase would affect the coating process. Describe the connection between the surface gas load observed here and the quality and cycle time requirements of a production coating environment. (4 marks)

Part C: Practical Reflection (10 marks)

Connects learning to real-world application. Open-ended. 5 marks each. 2 questions.

C1. Describe a situation where the concepts from this module — flow regimes, conductance, pump-down behaviour, or geometry effects on gas flow — would matter in a vacuum-related setting. This could be:

Explain what someone in that role would understand differently about system performance with the knowledge from this module. Be specific — reference at least one concept by name. (5 marks)

C2. What was the most surprising or counterintuitive idea you encountered in this module? Why did it challenge your expectations?

If you have workplace experience, describe how this changes your understanding of something you have observed on the job. If you are new to vacuum technology, describe how this concept changes the way you would think about why a vacuum system behaves differently at low pressures compared to high pressures. (5 marks)

Marking Summary

Part Available Achieved
A: Knowledge Check 20
B: Concept Diagnostic 20
C: Practical Reflection 10
Total 50

Assessor: ________________________________ Date: ________________________________ Comments: ________________________________________________________________________

Submit Your Workbook

When you have completed all sections, click below to submit your work to your facilitator.