Laboratory freezers come with auto or manual defrost and depending on the stability needs of your samples one type of defrost might be a better fit for your lab than the other.
Auto defrost freezers contain evaporating coils that are located in the ceiling and have fans that circulate cold air when the freezer is closed. These fans give for an even air flow and provide consistent cooling all throughout the freezer. They shut off as soon as the door opens limiting the circulation of warm, outside air. The cold air that is circulated has a lower humidity level causing all ice or condensation that forms to evaporate and deposit on the coils. At regular intervals these deposits are evaporated by heating the evaporating coils. This heating melts and evaporates frost, defrosting the freezer. During the defrost cycle the temperature in the freezer fluctuates slightly due to the fact that the cooling coils are heated. This fluctuation can be minimized by keeping the freezer filled with your material. Furthermore, care should be taken to keep the door closed during the defrosting cycle as the air inside is not cooled at this time.
Manual defrost freezers cool by circulating refrigerant through coils in the walls. The internal temperature differential causes the cold air to circulate giving for a constant internal temperature although places closer to the wall and towards the bottom can be slightly colder than the rest of the freezer. Since there is no air circulation in these types of freezers they are preferred for open container storage as your material will not dry out. When condensation and ice build up on the walls they limit the air flow and the cooling power of the coils making it necessary to defrost the freezer.
Overall, auto defrost freezers require less maintenance and have more uniform internal temperatures. Their defrosting cycles are preprogrammed and happen automatically. Samples can be stored inside the freezer during defrosting but a small increase in temperature cannot be avoided. Manual defrost freezers are more energy efficient and have a more constant internal temperature. Defrosting them requires more time and effort as the freezer has to be completely defrosted and all samples have to be placed into a different freezer to keep them frozen.
So-Low Manual Defrost Freezer