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Performance of ULT Freezers
The performance of a ULT freezer is based on its ability to cool your valuable samples. One of the most obvious aspects to rate the performance of a freezer are pull-down times to -80 °C as well as the time which is needed by the freezer to recover after you opened the door. Actually, the pull-down performance has a direct influence on the recovery rate as the process is very similar: Cooling-down the inner chamber of the freezer.
Pull-down time to -80 °C
After installation as well as after de-frosting of the ULT freezer, the freezer needs to pull-down to -80 °C as soon as possible. In general, a good ULT freezer just needs 4 to 5 hours to pull-down from room temperature to -80 °C, saving precious time and protecting your samples. The short pull-down time is of high value for potential backup freezers: They are required to be ready in the shortest time possible. Therefore, many labs keep one backup freezer running 24/7, resulting in high power consumptions.
Based on fast pull-down times, backup freezers do not need to run all the time. They are ready within a short timeframe of within 4 hours. On top, the samples within the broken freezer unit are safe as the insulation slows down the sample warm-up.
Sample integrity after door opening
How often do you open the ULT freezer door per day? For how long? It is best practice to always access and exit the freezer as quickly as possible when storing new samples or attempting to locate a sample which is stored out of sight. However, this takes time.
The more time it takes, the more the temperature of both the cabinet and your frozen samples increases. The longer the door remains opened, the longer it takes for the freezer to regain the set temperature. Here again, a fast pull-down capacity is requested.
30 to 60 seconds is a realistic time frame to add a new sample or remove a stored one from the freezer.
The specified door open recovery time (DOR) is the time needed by the freezer to be back at -80 °C after the outer and one inner door was opened for a specific time (e.g. 60 sec). Be aware that some suppliers publish DOR for -75 °C as the adaption of the freezer to -80°C takes quite some time. Ask for real DOR back to -80 °C as your samples deserve -80 °C.