This Frequently Asked Question concerns a "Liquid Nitrogen Supply Tank" that furnishes liquid nitrogen to a Cryopreservation Freezer. There are several issues that influence the usage of liquid nitrogen as follows:

1. The "static Evaporation Rate" of the Cryopreservation Freezer. The Static Evaporation Rate is usually provided by the manufacturer of the freezer. The manufacturer tests the Freezer with liquid nitrogen and determines an average daily loss rate. The lid of the tank is not opened during the test thus there is "Static Evaporation"
2. The "Transfer Loss" that occurs when liquid nitrogen of the supply tank is transferred to the freezer. A 4 Foot Flexible Hose is connected from the Supply Tank to the Freezer. There is also Plumbing on the external portions of the Supply Tank and the Freezer. The Flexible Hose and the Plumbing is normally at room temperature when liquid nitrogen is not transferred. When the "Transfer" occurs, the liquid nitrogen cools the Hose and Plumbing to liquid nitrogen temperature of -196 Degrees Celsius. The amount of liquid nitrogen used in this cooling is the "transfer Loss".
3. The "Static Evaporation Rate" of the supply Tank. The Static Evaporation Rate is tested by the manufacturer and is provided as a ""percentage of the total liquid nitrogen contents of the tank".
4. The "Cryopreservation Storage of Products" in the Freezer. "Vials" and "Bags" are the most typical containers used for Cryopresercvation Storage. Racks, Boxes, and Dividers are used to store vials. Frames and Canisters are used to store bags. Racks and Frames are made from Stainless Steel. Canisters are made from aluminum. Boxes are made from cardboard , aluminum, stainless steel, and plastic. Dividers are made from cardboard, Storing and retrieving of the products requires lifting of the racks and frames from the liquid nitrogen cooled chamber. There will be some warming of the racks, frames, boxes, canisters, etc depending on the amount of time that the products are out of the chamber. The storage and retrieval procedure will cause liquid nitrogen loss during this process.

   CBS is testing all of the above procedures to provide more information on the se of liquid nitrogen. We tested a V1500-ABIsothermal Freezer with 7 Aluminum Racks,Cardboard boxes and Dividers and a 5 Minute Retrieval of a Rack, Box and Vials.

   1.	V1500-AB Isothermal Freezer With 7 Aluminum Racks	6.0 Liters
   2.	Transfer Loss	3.5 Liters
   3.	Supply Tank - Daily Loss	3.0 Liters
   4.	5 Minute Retrieval of Racks, Box, and Vials	1.0 Liters
   5.	Total Daily Loss	13.5 Liters/Day

   If 180 Liter Supply Tank is Filler to 180 Liters, the usage rate (180 divided by 13.5Liter/day) equals 13 days of usage. however, there are many issues that can change the daily usage and the number of days provided by the Supply Tank.

   There could be many factors that shorten the time that a Supply Tank lasts. Here are some:

   1. Supply Tank filled with too much pressure (50 psi) or too low a pressure (10 psi). The optimum pressure for Supply Tanks used for Biological Storage is 22 psi.
   2. Frequently opening the freezer
   3. Transferring warm products into the freezer
   4. using long hoses for transferring liquid nitrogen.