The vacuum jacket is the insulation layer that keeps liquid nitrogen cold.

How it works:

DEWAR CROSS-SECTION:
[Outer steel shell]
↓
[Vacuum space (2-5mm)]  ← This is the "vacuum jacket"
↓                         Insulates via absence of air
[Reflective coating]      (Nothing to conduct heat)
↓
[Inner aluminum container] ← Holds liquid nitrogen
↓
[Liquid nitrogen at -196°C]

Why vacuum works:

  • Heat transfer requires a medium (gas or liquid)
  • Vacuum has no medium
  • Therefore, minimal heat can conduct through
  • Keeps contents extremely cold

What “vacuum” means:

  • Nearly complete absence of air molecules
  • Pressure: <0.1 mm Hg (typical)
  • Air molecules too few to transfer heat
  • Sometimes <0.01 mm Hg (ultrahigh vacuum)

Vacuum degradation:

  • Over 10-15 years, vacuum can degrade
  • Tiny atmospheric leaks can occur
  • Results in increased evaporation rate
  • Solution: Recertification or replacement