Depends on dewar size and ambient temperature:

Standard Lab Temperature (20°C):

  • 20L Dewar: Every 4-6 weeks
  • 50L Dewar: Every 6-10 weeks
  • 100L+ Dewar: Every 10-16 weeks

Warm Environment (25-30°C):

  • 20L Dewar: Every 3-4 weeks
  • 50L Dewar: Every 5-7 weeks
  • 100L+ Dewar: Every 8-12 weeks

Why timing varies:

  • Room temperature affects evaporation rate
  • Dewar age affects insulation quality
  • Usage frequency (opening lid) increases loss
  • Ambient humidity impacts heat transfer

Best practice: Refill when dewar reaches 20% capacity, not when empty.