Sample organization
The central component of the GRR is a liquid nitrogen freezer (model MVE 1830HE), located in the Florida Museum of Natural History of the University of Florida (Dickinson Hall). It is designed to mantain storage temperatures below -150C in vapor. The freezer can store close to 80,000 samples in 2 ml plastic vials. Below is a view inside the cryofreezer.
Currently, the freezer is equipped with 54 racks arranged in six sections following the layout shown below.
Each rack holds 13 boxes of 100 cells each. When the current configuration of storage racks reaches its sample capacity, we will add 30 more racks (designed to hold 25-cell boxes) to reach the largest capacity configuration possible for this freezer model. During regular operation, the racks are managed with leather gloves and operators wear a protective face mask to minimize risks associated with handling liquid nitrogen and materials at cryogenic temperatures.
Nitrogen Loading
The nitrogen level inside the cryofreezer is controlled by a small computer attached to the freezer. A pressure sensor and two temperature probes feed the computer the information it needs to regulate nitrogen flow into the tank, and regulate alarms alerting personnel of changes in temperature or liquid levels outside pre-set parameters. The computer is programmed to mantain a minimum of 5 inches of liquid nitrogen standing at the bottom of the freezer. When the level drops below this setting, the computer opens the valve connecting the tank to a liquid nitrogen reservoir. If liquid fails to flow from the reservoir, an alarm is triggered.
The high efficiency freezer model can hold up to 1,672 liters of liquid nitrogen. However, it is primarily designed for vapor storage so it only requires 296 liters of liquid nitrogen. This volume leaves all storage racks above the liquid nitrogen level. The rate of liquid nitrogen consumption varies depending on the frequency of access to the freezer contents. To reduce the level of nitrogen use, we designate one day a week to process loans and new accessions. At low levels of freezer access, we must replenish the reservoir dewar (180L, 22 psi) every eight to nine days to mantain the liquid level inside the freezer above a set minimum (5 inches). Below is an illustration of temperature changes at two levels inside the freezer as liquid nitrogen level drops.
Loading liquid nitrogen into the cryofreezer is an interesting process, since it is possible to evidence changes in pressure of the cryofreezer by the liberation of vapors of nitrogen like the ones captured in these pictures.
Notice the condensation in the hose while loading the cryoliquids.
Photos: Jeff Cage & Lorena Endara