Liquid nitrogen evaporation rate in a cryogenic storage tank depends on the size and insulation quality of the vessel, how frequently the lid is opened and the ambient temperature of the storage room. A well insulated static storage dewar typically loses between 0.1 and 1 litre of liquid nitrogen per day under normal conditions. Larger tanks with a lower surface area to volume ratio tend to have better holding times relative to their capacity. Most quality cryogenic dewars used in UK IVF clinics are rated with a static holding time of several months before requiring a top up, though clinics operating active programmes with frequent access will see higher evaporation rates. Continuous liquid nitrogen level monitoring with automated top up systems or scheduled delivery contracts is standard practice to ensure the cold chain is never broken. Cryolab.co.uk supplies liquid nitrogen storage tanks and monitoring equipment for UK fertility clinics.