Diagnosing Vacuum Problems: A Step-by-Step Guide

When your vacuum system stops performing, the clock starts ticking. Downtime costs money. But before you panic or call for service, here's what you need to know: most vacuum problems follow predictable patterns. With the right diagnostic approach, you can often pinpoint the issue yourself - or at least give your service provider the information they need to fix it fast.

What's Wrong: The Three Main Categories

Vacuum problems fall into three buckets: pressure won't drop (pumpdown issues), base pressure won't hold (leak or contamination), or pump won't start (mechanical or electrical).

Each category tells you something different about what's failing.

Step 1: Read Your Pumpdown Curve

A pumpdown curve is your first and best diagnostic tool. It's the graph of pressure over time as your system goes from atmospheric pressure (760 Torr) down to high vacuum (10⁻⁶ Torr or better).

Plot your current pumpdown against a known-good baseline. If it matches, your system is fine. If it diverges, the point where it diverges tells you where the problem is:

• Diverges in the 1-0.1 Torr range? Roughing pump issue (backing pump, rotary vane pump, or its connection).

• Diverges in the 0.1-0.01 Torr range? Turbo pump or transition region problem.

• Diverges below 0.01 Torr? Cryopump degradation or leak downstream of the cryopump.

This single test eliminates 80% of guessing.

Step 2: Check for Leaks (Rate-of-Rise Test)

Isolate your pump and let the system sit for 10 minutes. Watch the pressure rise. A good system should have a rate of rise of less than 1 × 10⁻⁶ Torr/sec.

If it rises faster, you have a leak. Virtual leaks (water trapped in cold spots, absorbed gas outgassing slowly) are common culprits. Real leaks (helium via a connection or seal) are rarer but catastrophic.

Pro tip: Spray a soapy water solution around all connections while the system is under vacuum. Bubbles will form at leak points immediately.

Step 3: RGA (Residual Gas Analysis)

If your base pressure is high but your pumpdown looks normal, you likely have a contamination issue. An RGA scan tells you what gases are in your chamber - nitrogen, oxygen, water vapor, hydrocarbons, etc.

Common findings:

• High H₂O? Chamber moisture or outgassing from surfaces. Solution: bake out the chamber, increase pump speed.

• High N₂ or O₂? Air leak or virtual leak of atmospheric gas. Solution: find and seal the leak.

• High hydrocarbons? Oil backstreaming from the roughing pump or contaminated helium lines. Solution: install a backstreaming trap or change the compressor absorber.

Step 4: Inspect Physical Systems

Once diagnostics narrow the problem, do a visual inspection:

• Helium lines: Check for oil or moisture. Contaminated lines kill pump performance.

• Compressor absorber: If it's been >12 months since replacement, replace it. A spent absorber lets oil vapor into the helium circuit.

• Pump exterior: Check for frost (helium leak) or unusual vibration (bearing wear).

Step 5: Know When to Call for Help

If diagnostics show a cryopump problem (degraded pumpdown below 0.01 Torr, ice on the pump, or if WinOCC shows coldhead temperature drift), your pump likely needs field service or a rebuild. Don't wait - continued operation will cause more damage.

Why This Matters

Vacuum diagnostics aren't magic. They're just organized troubleshooting. Every data point eliminates possibilities. The faster you narrow it down, the faster you fix it - and the less uptime you lose.

We've been diagnosing these systems for 25+ years. If you collect this data and share it with us, we can give you an honest assessment over the phone. Often, we can tell you exactly what needs to happen before we ever see the system.

Ready to diagnose your system? Contact us with your pumpdown curve, RGA data, and symptoms. We'll help you figure out the next step.

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