Pumpdown Curves Explained: How to Read & Use Them for Diagnostics

Learn to read pumpdown curves and use them to diagnose vacuum system problems. Examples and information along with a complete guide with real examples from 25+ years of service.

3 min read

A pumpdown curve is a graph. The X-axis is time. The Y-axis is pressure. The line that goes down is your system reaching vacuum.

It looks simple. But buried in that curve is every secret your vacuum system is keeping.

What a Good Pumpdown Curve Looks Like

A healthy system has a characteristic shape:

  • 0-10 seconds: Steep drop from 760 Torr to ~50 Torr (roughing pump removing atmosphere fast)

  • 10 seconds-1 minute: Continuing drop from 50 Torr to ~0.1 Torr (still fast, but slowing)

  • 1-5 minutes: Shallow drop from 0.1 Torr to 10⁻⁵ Torr (turbo pump or turbomolecular pump ramping up)

  • 5-20 minutes: Very shallow drop from 10⁻⁵ Torr to base pressure (cryopump taking over, asymptotic curve)

The shape matters more than the absolute numbers. A curve that follows this pattern means your system is working correctly at each stage.

Reading a Curve: Pressure vs. Pumping Speed

Pressure changes because pumping speed is dropping as you go deeper into vacuum. This is normal. Different pump types dominate at different pressure ranges:

  • Roughing pump dominates at 760-1 Torr

  • Turbo pump dominates at 1-0.001 Torr

  • Cryopump dominates below 0.001 Torr

When you see a "flat spot" in the curve (pressure stalls for a while), it usually means you're at a transition between pump types. That's okay - it just means the next pump is starting to take over.

Diagnosing Problems from Curve Shape

Problem 1: Curve Diverges High (Around 100 Torr)

Your roughing pump isn't working well. Could be:

  • Backing pump failure

  • Rotary vane pump cavitation (oil level too low)

  • Bad valve or connection between roughing pump and main system

Problem 2: Curve Flattens Between 1-10 Torr and Won't Drop Further

Your turbo pump isn't starting or isn't reaching full speed. Could be:

  • Turbo motor failure

  • Magnetic bearing fault

  • Low inlet pressure (roughing pump not getting you low enough)

Problem 3: Curve Diverges Below 0.01 Torr

Your cryopump isn't working. Could be:

  • Coldhead not reaching temperature (cooling problem)

  • Charcoal array saturated (needs regeneration)

  • Helium circuit contamination (ratchetting, ice, moisture)

Problem 4: Curve Reaches Base Pressure But Pressure Creeps Back Up Over Hours

You have a leak or virtual leak. Could be:

  • Real leak in a fitting, seal, or weld

  • Virtual leak (outgassing from chamber walls, trapped water, absorbed gases)

  • Pump degradation (charcoal array saturation, absorbed gases in array)

Using Comparison Curves

The real power of pumpdown curves is comparison.

Get a baseline curve from your system when it's working well. Save it. Then every 6 months, run a new pumpdown curve and overlay it on your baseline.

  • Perfect match? Your system is fine.

  • Diverges at the same spot each time? That's your problem area - focus diagnostics there.

  • Progressively getting worse? Time for preventive maintenance.

Annotating Your Curve

When you run a pumpdown curve, note:

  • Time of day and date

  • Roughing pump type and operating condition

  • Turbo pump speed (if variable)

  • Cryopump model and coldhead temperature

  • Any audible sounds (ratchetting, vibration, etc.)

  • Recent maintenance or repairs

This context helps you (and your service provider) interpret the shape accurately.

Real-World Example: Contaminated Cryopump

We once saw a curve that looked fine down to 10⁻⁴ Torr, then flatlined completely. The customer thought the cryopump was broken.

Turns out: the charcoal array was saturated from back-to-back regeneration cycles (they'd been running the pump hard). One full regen cycle restored the curve to normal. No replacement needed. Cost: zero (besides the operator's time).

A curve told us that story. Without it, they would have shipped the pump back for rebuild.

How to Capture Your Curve

Most modern systems have data logging built in. On-Board pumps use WinOCC, which records pressure and temperature continuously. Older systems require a pressure gauge and a stopwatch.

Pro tip: Use a logarithmic scale on the Y-axis (pressure). It makes the curve shape much clearer.

Your Curve is Your Map

We've been reading pumpdown curves for 25+ years. If you have one - or if you're struggling to create one - send it to us. We can tell you what it means and what to do next.


Contact Us

Support and inquiries welcome anytime.

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sales@appliedcryogenics.com

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