Costlier than dry-ice and more dangerous to handle, liquid nitrogen is your only choice if you need to keep a sample, substance or intermediate product in the glass phase (i.e., colder than -150°C). Liquid nitrogen boils at -195.8°C and freezes at -210°C.
However, this popular coolant boils immediately in contact with warmer (or less cold) surfaces, thus limiting its efficiency. Furthermore, as this sub-zero boiling occurs, the chilled object acquires an insulating envelope of nitrogen bubbles: the Leidenfrost effect. To minimize this effect, you plunge objects into a slush of solid and liquid nitrogen instead of LN2 alone to achieve faster cooling.
At these ultra-low temperature levels, dedicated cryo compatible containers are necessary. They have a vacuum chamber and a relatively small payload, sometimes of only a few cubic centimeters.
LN2 or liquid nitrogen's ability to maintain extremely cold temperatures means that this refrigerant is useful for the:
Storage of cells for laboratory work.
Cryopreservation of blood, reproductive cells, other biological material and samples.
Preservation of tissue from surgical excisions for medical studies.
Cryoconservation of animal genetic resources. Apart from R&D and life science uses, cryogenically preserved specimens can revive breeds that are endangered or extinct.
Very low temperature reactions and the controlled evaporation typical of advanced processes.
Cooling baths and vacuum pump traps typical of chemistry and laboratory research.
Also known as cryo-cans, the holding time of vacuum flasks varies depending on their design and size. Their construction reduces heat transfer and vaporization Typically, their inner lining comprises borosilicate or stainless steel. For durability, the outer linings are usually carbon steel or stainless steel.
Research dewars typically have a capacity of between 0.5 and 6 liters. In contrast, the MVE Lab Series of dewars have internal volumes of between 4 and 50 liters.
Storage and Transport
Today's portable cryogenic containers feature spill-free designs. By following the recommended usage instructions and correct filling technique, the outer shell of these dewars remains dry. Consequently, they attract a non-hazardous classification, thus simplifying courier handling and reducing transport costs. Moreover, the method favors the maintenance of sample viability.
Crucially, the filling process must be slow. Depending on the type of container, two to four cycles may be needed as the absorbent layer soaks up the liquid nitrogen. However, for obvious safety reasons, you should avoid overfilling.
While older dewars might have maintained temperatures low enough for a few hours, today's modern equivalents can perform well for weeks. Over time, though, shippers tend to accumulate moisture which decreases their performance.
Inside a typical liquid nitrogen dewar container, the dry nitrogen vapor chills the contents to -150°C or colder. Internal capacity is usually sufficient for approximately twenty-two 10 milliliter vials or seventy-five smaller 2 milliliter vials.
The maximum holding time for most portable dewars is around 10-12 days. Handling and transportation are safer, thanks to the latest protective shipping container mechanisms that prevent tilting and keep the shipper upright.
The latest pressurized super-insulated vacuum vessels enable the storage of samples in liquid nitrogen for longer. Significantly, losses are as low as 2 percent per day. However, for larger volumes and long-term storage, liquid nitrogen freezers are more appropriate.
Cryogenic equipment manufacturers produce a choice of easy access wide neck freezers and high-efficiency versions designed for low LN2 consumption. The less voluminous models feature castors for occasional changes of position. Usually, these stainless steel units measure more than a meter in height.
Wiring and plumbing are usually fully enclosed, conferring safety and aesthetic advantages. Some of these cylindrical freezers have a standard single access step, while some taller variants have double-tier folding steps. On top, a robust metal handle operates the seal.
The latest product catalogs list cryogenic containers and freezers with vapor phase or liquid phase chilling options. Apart from accurate measurement, some advanced features include touchscreen controls, password security, user-defined alarms, and remote monitoring. In addition, their firmware permits communication with multiple devices and network connections. With ample data storage, the inbuilt memory can log thousands of events.