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The instrument group ULT freezer is defined as freezers for -40 to -85°C. ULT is the abbreviation for ultra low temperature. There are upright and chest freezers. The inner volume is in general between 300 and 800 L.
In contrast to short term sample storage at +4 °C or -20 °C by using standard refrigerators or freezers, many molecular biology or life science laboratories need long term storage for biological samples like DNA, RNA, proteins, cell extracts, or reagents. To reduce the risk of sample damage, these types of samples need extremely low temperatures of -80 to -85 °C. Cells are stored in tanks of liquid nitrogen at -196 °C.
The samples in ULT freezers are typically stored in polymer tubes or microtubes. 64, 81 or 100 of these microtubes are located in one box made of cardboard or polymer. Standard ULT freezers can store ca. 350 to 450 of those standard boxes for microtubes.
When using an ULT freezer at full capacity, 40,000 or 50,000 microtubes are stored in one instrument. Clear sample labeling (type of sample, concentration, volume, owner, date, etc.) and exact documentation of the sample location within the freezer is mandatory to find the sample tubes.
The pull down time is defined as the necessary time to cool down the ULT freezer from ambient temperatures to the selected temperature of -80 to -85 °C. The time strongly depends on the type of insulation, the efficiency of the compressor system as well as the installed metal shelves within the freezer. Up-to-date instruments are able to cool down within 3 to 5 hours.
Due to the low temperature, ULT freezer consume high amounts of electric energy per day. Depending on the volume of the freezer, the opening frequency of the users as well as the number of samples, the energy consumption starts from ca. 11 KWh/ day and higher. To reduce the energy consumption, the insulation should be as efficient as possible. Additional inner doors reduce the loss of temperature when opening the main door. Icing within the ULT freezer should be reduced to a minimum.