Liquid nitrogen at minus 196 degrees Celsius evaporates continuously, even in the most thermally efficient storage dewars. Heat always flows from warmer to cooler systems, and even through an excellent vacuum jacket, a small amount of heat enters the vessel every day, evaporating a corresponding amount of liquid nitrogen.
The evaporation rate of a dewar is expressed in litres per day and varies significantly between dewar types and quality levels. A high-specification medical-grade 35-litre dewar might have a static evaporation rate below 0.2 litres per day. A lower-quality unit might lose 0.4 litres or more per day.
The immediate concern is that if nitrogen levels drop too far, samples stored in the vapour space above the liquid may experience a temperature rise. A significant drop in nitrogen level can bring the storage temperature above minus 150 degrees Celsius, the threshold at which biological integrity may be compromised.
This is precisely why continuous monitoring with alarm systems is an HFEA requirement. The alarm should trigger well before nitrogen levels reach a point of concern, giving staff time to top up the vessel.
Practical considerations include topping up dewars on a scheduled basis rather than waiting for alarms, understanding the actual measured evaporation rate of each specific dewar, and planning nitrogen supply deliveries with sufficient frequency to maintain safe levels.
Cryolab supplies nitrogen monitoring systems and advises on top-up schedule planning for UK IVF clinics. Visit cryolab.co.uk“>Cryolab
