Thermo Fisher Scientific CryoExtra CE8100 Series
Operation | 15
Operation
This section of the manual will detail the functions and
features of the Controller and demonstrate how to access
and adjust the various user settings and options.
All CryoExtra Freezers utilize LN
2
as the means of
refrigeration. Under atmospheric conditions, the temperature
of LN
2
is -196°C (-320°F, 77 K). Depending on the model, the
LN
2
resides either in or below the freezer storage space.
Through normal usage and time, the LN
2
will naturally boil off
reducing the amount of refrigerant in the freezer. It is
imperative that the LN
2
level be properly maintained in order
for the storage space temperature to be maintained. This,
along with monitoring and recording temperature, is the main
function of the Controller.
Figure 2. Illustration showing LN
2
as the
refrigerant.
Left: LN
2
below the storage space
naturally boils off and cools the freezer.
Right: LN
2
resides in the freezer storage
space.
Functions and Features
The Controller utilizes a variety of functions and features that
enables it to closely monitor and control the environment
inside a cryogenic freezer. This section will give an overview of
the following features and their functionality:
•
Liquid Nitrogen Level Measurement
•
Automatic Liquid Nitrogen Level Control
•
Liquid Nitrogen Usage
•
Temperature Measurement
•
User Defined Alarms
•
Passwords / Security
•
Communication Capabilities
•
Hot Gas Bypass
•
Battery Backup (Optional)
Liquid Nitrogen Level
Measurement
The LN
2
level in the tank is determined through the use of a
differential pressure sensor. This sensor operates on the
physical principle of hydrostatic head pressure. The pressure
generated by a column of fluid is proportional to the height or
depth, of the fluid column. In this application, the pressure
generated by the LN
2
at the bottom of the freezer will increase
as the LN
2
level increases. This differential pressure system
allows the Controller to measure the exact LN
2
level
accurately.
Figure 3. As the fluid level in the cylinder
increases, the imposed head pressure at
the bottom of the cylinder increases
proportionally. The pressure gauges
measure this increase in level.
This pressure signal generated by the LN
2
is transmitted from
the bottom of the inner vessel through the freezer’s annular
line to the differential pressure sensor inside the Controller.
The Controller then compares this pressure signal to its
atmospheric pressure readings and is then able to determine
the exact LN
2
level. The term “differential pressure” refers to
the fact that the level is determined from the difference
between the hydrostatic head pressure of the fluid column
and atmospheric pressure. The measured LN
2
level is
displayed in inches, millimeters or as percentage full.
