The liquid nitrogen Dewar tanks, which are storage containers, can store liquid nitrogen at -196°C and reduce its evaporation due to their vacuum structure and corrosion resistance made of aluminium alloy, which is not possible with other insulated containers. Therefore, although there are many cryogenic containers, the one that can be used for both liquid nitrogen and biological sample storage is the liquid nitrogen Dewar.
However, it is important to know that liquid nitrogen naturally evaporates and the amount of liquid nitrogen filled decreases over time, so how low can it go while keeping the biological sample active?
After years of observation by the KGSQ manufacturers, it has been found that the liquid nitrogen level is as low as it can go when it just covers the sample. This is generally the time to consider liquid nitrogen replenishment, as failure to do so is likely to result in damage to the sample, or even death.
There is, of course, another situation where only liquid nitrogen is stored in liquid nitrogen dewars and no biological samples are placed, in which case it can be replenished when the liquid nitrogen level is as low as 1/3 of the total.
Although, theoretically, whether storing samples or liquid nitrogen, it is possible to go lower when the sample is just covered, or as low as 1/3, due to practical considerations such as cells being damaged, or the need to pre-cool the tank again after the liquid nitrogen has been consumed to fill it, etc.
Therefore, as mentioned in the text, the liquid nitrogen should not be lower than 1/3 of the total content when it is about to fail to cover the biological sample.