Interpreting Pump-Down Curves
Estimated time: 20–25 minutes
Learning Outcome: Interpret pump-down curve shapes using flow regime and conductance concepts; distinguish normal pump-down from performance problems. Competency: M03-COMP-01, Indicators M03-IND-01.01, M03-IND-01.03
Orient
You can now explain why gas behaviour changes with pressure (Lesson 2) and why system geometry matters (Lesson 3). This lesson puts both together: reading the pump-down curve as a diagnostic tool.
A pump-down curve isn't just a line on a graph — it's a story about what's happening inside the system at every stage. Flow regime transitions, conductance effects, gas load contributions — all leave their signature on the curve.
Core Content: The Anatomy of a Pump-Down Curve
A pump-down curve plots chamber pressure (Y-axis, usually logarithmic) against time (X-axis). For R1-A:
Region 1: Steep decline (950 → ~10 mbar)
The pump is removing bulk atmospheric gas in viscous flow. Conductance is high, pump throughput is high, and the pressure drops rapidly. This region is dominated by pump speed — a bigger pump makes this phase faster.
Region 2: Bending transition (~10 → ~0.1 mbar)
The flow regime is transitioning. Conductance is dropping. Gas load from surfaces (water desorption) is becoming significant relative to the pump's throughput.
The curve bends — each decade of pressure takes longer than the last.
Region 3: Slow approach (below ~0.1 mbar)
Molecular flow. Conductance is at its minimum (geometry-limited). Gas load is dominated by outgassing and surface desorption.
The pump is still working, but between conductance losses and persistent gas sources, progress is very slow. This is where the system spends most of its time.
Region 4: Base pressure plateau
The system reaches equilibrium — gas load equals effective pumping speed. Pressure stabilises. The base pressure is determined by the balance between total gas load and effective pumping speed — more gas load or less effective pumping speed means a higher base pressure.
For reference, this is sometimes written as P_base = Q_total / S_eff, where S_eff includes the conductance losses. You don't need to calculate this — the balance concept is what matters.
Pump-Down in Motion — Video Reference
The four regions described above become much easier to recognise when you observe a pump-down in real time. The video below shows a gauge reading during a pump-down from atmospheric pressure to base pressure, with the flow-regime regions highlighted as the pressure drops through each transition.
As you watch the video, notice how the rate of pressure change is visible on the gauge — the digits spin rapidly during viscous flow, slow through transition, and barely move during molecular flow. This matches the curve shape you studied above and reinforces why Region 3 takes the longest.
Fast vs Slow Pump-Down: Diagnosis by Curve Shape
Normal curve: Follows the expected shape — steep decline, smooth transition, gradual approach to base pressure. All four regions are present and proportioned as expected for the system.
Slow initial decline (Region 1 too slow):
- Pump problem — low pumping speed, pump not warmed up, or pump oil low
- Or: large chamber volume (normal for bigger systems)
- Viscous flow should be pump-limited, so a slow Region 1 points at the pump
Extended transition (Region 2 too long):
- Gas load from contamination (M02 concept) — extra water or hydrocarbons slow the transition
- Or: conductance restriction — a partially closed valve or blocked foreline
- Check system history: was the chamber open for an extended period? Was handling clean?
Very slow approach or elevated base pressure (Region 3/4 worse than expected):
- Multiple possible causes:
- Gas load too high (contamination, leak — use rate-of-rise to diagnose, per M02)
- Conductance too low (foreline too narrow or too long for the pressure range)
- Pump ultimate pressure too high (pump degradation)
- Distinguish by checking: does a rate-of-rise test show a leak? Does the pump reach its rated base pressure on its own (disconnected from the chamber)?
Checkpoint — What You've Gained So Far You can now identify the four regions of a pump-down curve and diagnose whether a slow pump-down is caused by a pump problem, conductance limitation, or elevated gas load. The worked example below applies this to real R1-A data.
Worked Example
System: R1-A, identical conditions, two different days.
Monday pump-down:
| Time | R1-G-CH (mbar) |
|---|---|
| 0 min | 950 |
| 0.5 min | 100 |
| 2 min | 1 |
| 5 min | 0.1 |
| 8 min | 0.05 (base) |
Tuesday pump-down: (Chamber was left open over the weekend)
| Time | R1-G-CH (mbar) |
|---|---|
| 0 min | 950 |
| 0.5 min | 100 |
| 2 min | 1 |
| 12 min | 0.1 |
| 25 min | 0.07 (still dropping slowly) |
Interpretation:
- Region 1 (950 → 100 mbar) is identical: 30 seconds. The pump and viscous flow conductance are fine.
- Region 2 (100 → 1 mbar) is identical: about 1.5 minutes. Transition flow is unchanged.
- Region 3 diverges: Monday reaches 0.1 mbar at 5 minutes; Tuesday takes 12 minutes. Tuesday's base pressure is also higher.
- Diagnosis: The divergence in Region 3 points to a higher surface gas load on Tuesday (the weekend exposure allowed significant water re-adsorption). This is a gas load problem (M02), not a conductance or pump problem.
Annotated Pump-Down Curve — Visual Summary
The annotated curve below is the central diagnostic visual for this module. It shows a normal R1-A pump-down with all four regions labelled, plus an overlay of a second curve representing a system with elevated surface gas load. Study the point where the two curves diverge — that divergence is the diagnostic signature you will learn to interpret.
Notice that the two curves are identical through Regions 1 and 2 — the divergence appears only in Region 3 where molecular flow and surface gas load dominate. This confirms that Regions 1 and 2 are pump-limited and conductance-limited respectively, while Region 3 is where gas load differences become visible. When you analyse a pump-down curve, the region where the curve departs from expected behaviour tells you which factor is responsible.
Key Teaching Point
Misconception: All that matters is reaching target pressure. How the curve looks along the way is irrelevant.
Reality: The shape tells you what's limiting performance at each stage.
If the system barely reaches target pressure, the curve tells you where the problem is — pump, conductance, gas load, or some combination. Without reading the curve, you're guessing. With it, you're diagnosing.
What You Can Now Do
By the end of this section, you can:
- Identify the four regions of a pump-down curve and explain what's happening in each
- Diagnose whether a slow pump-down is caused by a pump problem, conductance limitation, or elevated gas load
- Compare two pump-down curves and identify where they diverge
- Explain why Region 3 is where most system problems become visible
- Use the pump-down curve as a diagnostic tool, not just a measurement
Next Steps
The next lesson applies everything to the broader picture — how these concepts apply beyond R1-A, including a first look at the thin-film coating sector where pump-down performance is critical.