Several methods depending on dewar type:

Method 1: Visual Gauge (if equipped)

  • Graduated scale on side of dewar
  • Shows percentage capacity
  • Easiest method, no tools required
  • Accuracy: ±5%

Method 2: Dip Stick/Sounding Stick

  • Wooden stick or rod with markings
  • Insert slowly into dewar
  • Measure wet/dry lines
  • Accuracy: ±2-3 cm
  • Cost: £10-20

Method 3: Weight Method

  • Know empty dewar weight
  • Weigh current dewar
  • Calculate: (Current weight – Empty weight) / Density = Liters
  • Most accurate but requires industrial scale
  • Density of LN₂: 0.807 kg/liter

Method 4: Acoustic Method (Professional)

  • Uses sound reflection
  • Gives precise measurements
  • Used by service technicians
  • Cost: £50-150 per check

Best practice for IVF clinics: Combination of visual gauge + weight checks monthly