YBOTECH ultra-low temperature (ULT)” (typically −80 °C or −86 °C) freezers, as used for storing biological samples, also generally employ two stages of cooling, but in cascade. The lower temperature stage uses methane, or a similar gas, as a refrigerant, with its condenser kept at around −40 °C by a second stage which uses a more conventional refrigerant. Cryogenic chest freezers can achieve temperatures of down to −150 °C, and may include a liquid nitrogen backup.