Module 1

Gas Behaviour in Vacuum Systems

Vacuum Fundamentals & System Orientation

Gas Behaviour in Vacuum Systems

Estimated time: 15–20 minutes

Learning Outcome: Explain mean free path and flow regimes; describe gas load and its diagnostic use; apply systems thinking to vacuum problems.

Orient

You can read a pressure gauge and classify the reading. Now let's look at what's actually happening to the gas molecules — because their behaviour determines everything about how your system performs, how fast it pumps down, and what happens when something goes wrong.

Core Content: Mean Free Path and Flow Regimes

At atmospheric pressure at sea level (1013 mbar), molecules are packed tightly. The average distance a molecule travels before hitting another one — the mean free path — is about 0.07 micrometres (70 nanometres). Molecules collide constantly, forming a fluid that flows as a bulk stream, like water through a pipe.

This regime is called viscous flow or continuum flow (think "crowd flow" — molecules move together as a group, like people in a packed hallway). The gas acts like a continuous medium.

Now drop the pressure to 1 mbar

The mean free path grows to about 70 micrometres — a thousand times larger. Molecules are now so far apart they rarely hit each other.

Instead, they bounce off chamber walls in random directions. There's no bulk streaming — just individual molecules drifting based on probability.

This regime is called molecular flow or free molecular flow (think "individual wanderers" — each molecule moves independently, bouncing off walls at random). It's a fundamentally different physics.

The transition zone (Knudsen transition)

Between roughly 0.01 and 1 mbar, gas behaviour is in flux — sometimes more like a fluid, sometimes more like individual particles. This is Knudsen transition flow, and it's the hardest to predict because both mechanisms are happening.

Why this matters for the rig and your system:

In viscous flow, the roughing pump on R1-A pulls gas efficiently — you see steady, fast pressure drops.

Below 1 mbar, you enter molecular flow — the pressure drops more slowly because molecules must randomly drift into the pump inlet.

This is normal. It's not a problem. It's physics.

Technicians who don't understand this sometimes think the pump has failed when really the system has just entered a slower flow regime.

Mean Free Path Rule of Thumb

The exact relationship is complex, but the core idea is simple: as pressure drops, mean free path increases proportionally. Halve the pressure and the mean free path roughly doubles. This inverse relationship is all you need to predict when a system will transition from viscous to molecular flow — compare the mean free path at your operating pressure to the diameter of the tubes in the system.

For reference, this relationship can be expressed as a rule-of-thumb formula. You don't need to memorise or use this equation — the relationship described above is what matters.

Mean Free Path (micrometres) ≈ 66 ÷ Pressure (mbar)

At 1000 mbar: 66 ÷ 1000 ≈ 0.066 micrometres (viscous flow) At 1 mbar: 66 ÷ 1 = 66 micrometres (molecular flow) At 0.001 mbar: 66 ÷ 0.001 ≈ 66,000 micrometres = 66 mm (deep molecular flow)

The diagram below shows how mean free path changes with pressure — and where the flow regime transitions happen relative to typical tube diameters on the R1-A rig.

Mean free path vs pressure — flow regime transition when mean free path exceeds tube diameter
Mean free path vs pressure with flow regime transition bands

Notice where the diagonal line crosses the 25 mm reference (the KF25 tube diameter on R1-A). That crossover is where the rig's foreline transitions from viscous to molecular flow — and where you'd see the pumpdown rate slow down.

Checkpoint — What You've Gained So Far

You can now describe what mean free path is, explain why gas behaviour changes with pressure, and identify the transition point where a tube shifts from viscous to molecular flow. Next, you'll see how this connects to the gas entering the system — and why the pump isn't always the limiting factor.

Gas Load: The Inflow Side of the Equation

Gas load is the total amount of gas your pump must remove every second, measured in pressure × volume per unit time (often written as mbar·litre/second or Pa·m³/s).

Gas load comes from:

  1. Atmospheric leaks — Small holes or degraded seals letting air in
  2. Outgassing (literally: gas coming out of solid materials) — Gas molecules trapped in chamber walls, seals, and other materials slowly diffuse out as pressure drops. Think of it like moisture evaporating from a damp sponge — the lower the surrounding pressure, the more gas escapes from the surfaces
  3. Permeation (gas molecules slowly soaking through solid materials, the way water eventually seeps through a raincoat) — Gas molecules passing slowly through seals and gaskets, even when those seals are perfectly intact
  4. Backstreaming (exactly what it sounds like: contamination flowing backwards, from the pump back into the system) — Oil vapour or other contaminants migrating from the pump toward the chamber, opposite to the intended gas flow direction

The System Balance: Gas Load vs. Pump Speed

Think of it like this — a water analogy that works for vacuum, hydraulics, electrical circuits, or any flow-versus-capacity system:

A bathtub with the drain open:

If inflow and outflow are balanced, the water level stays constant. If someone turns on another faucet (new leak, sudden outgassing), the level rises — even though the drain hasn't changed. Close one faucet (seal the chamber), and the level drops as the drain empties the tub.

In vacuum:

If gas load is constant and pump speed is constant, pressure reaches a balance point — the level where inflow = outflow. If gas load suddenly increases (a new leak opens), pressure rises until the higher pressure helps drive more gas into the pump (in viscous flow) or provides more random molecules for the pump to capture (in molecular flow).

This balance is diagnostic. If the chamber pressure is rising when it should be steady, you know gas load is increasing. If pressure won't drop below a certain point despite good vacuum for hours, you know gas load is meeting pump speed at that level.

Rig Connection

When you watch R1-G-CH drop from 1000 to 100 mbar, the system is in viscous flow — R1-P-RP pulls gas efficiently. You see a steady, confident pressure drop. This is the rotary pump doing what it's designed to do.

Below 1 mbar, you're entering molecular flow — the pressure drops more slowly because molecules must randomly drift into the R1-P-RP inlet.

You might watch the pressure crawl from 1 mbar to 0.1 mbar over 20 seconds instead of the fast 20-second drop you saw from 1000 to 100 mbar.

This is normal. It's expected. It's not a failure — it's the gas behaving like individual molecules instead of a fluid.

Recognising this transition (slow pumpdown despite pump running smoothly) is your first real diagnostic skill. You can look at the gauges, understand what's happening physically, and know whether to escalate or wait.

Key Teaching Point

Misconception: The pumpdown is slow because something is wrong with the pump.

Reality: More likely you've entered molecular flow, where pumping is inherently slower. The pump is working perfectly. The gas is just behaving according to physics — individual molecules have a harder time finding the pump inlet by random chance than a fluid stream does.

Why this matters: Unnecessary alarm leads to unnecessary maintenance calls, system shutdowns, and wasted time.

Understanding flow regimes lets you distinguish real failures from expected behaviour. A slow pumpdown at 0.01 mbar is normal. A sudden pressure jump at 0.01 mbar might signal a real problem.

What You Can Now Do

By the end of this section, you can:

Next Steps

Next section takes you into schematic reading — the visual language of vacuum systems.

You'll learn to read the R1-A schematic and trace gas flow for any valve configuration. But if you need a break, take one. You've covered a lot of ground.