| Time | R1-G-CH (mbar) | Rise | Rate (mbar/min) |
|---|---|---|---|
| 0 min | 0.08 | — | — |
| 1 min | 0.20 | 0.12 | 0.12 |
| 2 min | 0.29 | 0.09 | 0.09 |
| 5 min | 0.44 | 0.15 | 0.05 |
| 10 min | 0.56 | 0.12 | 0.024 |
Misreading the pressure rise pattern after isolation can lead to an unnecessary leak investigation — adding hours of diagnostic time when the system is behaving normally.
R1-A has completed a roughing pumpdown. At t = 0, R1-V-ISO is closed and R1-P-RP is switched off. R1-G-CH reads 0.08 mbar at isolation. Over the next 10 minutes, the gauge reading continues to rise. The technician records the data shown at left and asks: is this normal, or is something wrong?
Look at the Rate column — does the number go up, stay the same, or go down as time passes?
A gas load from the chamber walls is finite — it runs out. A gas load from outside the system (a leak) does not run out. Which type produces a decreasing rate? Which produces a constant rate?
1. The rate column reads: 0.12 → 0.09 → 0.05 → 0.024. It is falling each row.
2. A falling rate means the gas source is being depleted. That rules out a real leak (unlimited source).
3. This is outgassing — surface-adsorbed gas desorbing from R1-CH walls under vacuum.
4. Status note: quote starting pressure (0.08 mbar), ending pressure (0.56 mbar), direction of rate (decreasing). Conclude: normal outgassing, no escalation required.
| Phase | Expected | Today |
|---|---|---|
| 950 → 100 mbar | ~30 sec | ~30 sec ✓ |
| 100 → 1 mbar | ~2 min | ~2 min ✓ |
| 1 → 0.1 mbar | ~5 min | 20+ min, stalled ✗ |
Attributing a contaminated pumpdown to pump failure sends maintenance to the wrong component. The pump is fine — cleaning the chamber is what's needed. Misdiagnosis delays the fix by hours.
R1-A is in ROUGHING. R1-P-RP is running normally — no unusual noise, vibration, or exhaust odour. R1-V-ISO is open. Yesterday the chamber was opened three times and internal surfaces were handled without gloves during a component swap. Today's pumpdown proceeds normally above 1 mbar, but has been stalled at 0.35 mbar for 30 minutes. Normal base pressure for R1-A is 0.05 mbar.
The pumpdown is normal down to 1 mbar. At what pressure does it diverge? What kind of gas load only appears at low pressures — not at high ones?
Above 1 mbar, bulk gas dominates — surface contamination has no effect. Below 1 mbar, bulk gas is mostly gone and surface gas load becomes the dominant factor. A stall in the 1–0.1 mbar region points to the surface, not the pump.
1. First two pumpdown phases: normal → pump is fine.
2. Stall below 1 mbar → surface gas load region.
3. System history: ungloved handling → fingerprint oil (hydrocarbon). Multiple openings → water adsorption (humid air).
4. Both contamination types raise surface gas load above pump throughput at that pressure — creating a new equilibrium stall point.
5. Escalation note: 0.35 mbar stall; normal base 0.05 mbar; R1-P-RP normal; history of ungloved handling and repeated venting. Need: further pump-down cycling or chamber cleaning.
| Phase | Normal | Today |
|---|---|---|
| 950 → 1 mbar | ~2.5 min | ~2.5 min ✓ |
| 1 → 0.1 mbar | ~5 min | ~12 min ✗ |
Particle contamination in a vacuum chamber affects sample quality and pumpdown performance. Identifying the entry point (R1-FLT-VENT) determines the corrective action — a filter check, not a chamber rebuild.
R1-A was pumped to base pressure successfully this morning. The chamber was then vented using R1-V-VENT and brought to atmosphere for a brief inspection. After re-sealing, ROUGHING was restarted. R1-P-RP is running normally. However, visible particles are now present inside R1-CH that were not there before venting. The pumpdown from 1 to 0.1 mbar is taking approximately 12 minutes — normally it takes 5 minutes.
The chamber was clean before the vent and contaminated after. What happened between those two states? Only one event occurred.
R1-FLT-VENT is a sintered metal filter on the vent inlet. For particles to get past it: the filter could be degraded, the vent valve opened too quickly (turbulence bypasses the filter), the filter connection could be loose, or venting occurred via a different path that doesn't route through R1-FLT-VENT.
1. Timeline: clean → vent → contaminated. Only the vent event occurred. Particles entered during venting.
2. R1-FLT-VENT failure modes (need 2): filter degraded/saturated; vent valve opened too quickly; filter connection loose; venting via cracked flange bypassing the filter.
3. Extended pumpdown: particles add surface area inside R1-CH. More surface area = more adsorbed water and gas = higher surface gas load in the 1–0.1 mbar range. Pumpdown is slow but not stalled.
4. Escalation note: particles observed post-vent; R1-FLT-VENT likely compromised or bypassed; filter inspection and vent procedure confirmation needed before next use.