Taylor-Wharton 24K w/ AutoTend - Manual (Page 10)

Operating Parameters
When materials are immersed in liquid nitrogen, they will assume the temperature of
the liquid -196ºC (-320ºF). When material is stored in the vapor phase over the liquid,
the liquid nitrogen is still a very cold refrigerant, but the refrigerator’s interior tempera-
ture increases somewhat as product is stored higher over the liquid. This temperature
differential is not significant in many biological storage applications, and is affected by
the amount of product stored in the refrigerator, the type and size of inventory control
system, and the liquid level in the unit.
The liquid level in the refrigerator is determined by the position of the sensor probes
in the tube located at the front of the refrigerator. These probes are set at installation
to maintain a specific liquid level. The controller operates a fill cycle that adds liquid at
a low level, fills to a predetermined high level, then stops the fill (See Figure 5). The
cycle repeats when the liquid level drops to the low level sensor over time. Sensor
probe assignments may be changed on the controller keypad to define new high and
low levels, and these levels may be set independently to vary the liquid level differen-
tial between fills. Prior to the initial fill of the refrigerator, a determination should be
made whether vapor phase or liquid phase storage will be utilized.
The sensor probe contains seven thermistors that can be preprogrammed for any
liquid level application. The separate sensor in the sensor tube is the temperature
thermocouple. The thermocouple is normally positioned above the High Alarm sensor
to measure the warmest condition in the storage chamber. The factory sensor posi-
tions will maintain a liquid level between 3 in. (76 mm) to 6 in. (152 mm). The dimen-
sions used for the factory sensor installation are shown in Figure 5.
Liquid Phase Storage
Liquid phase storage is normally utilized when liquid nitrogen temperatures are
required to maintain stored product viability and the storage medium is adequate for
storage in liquid nitrogen.
In a typical liquid phase storage system, the liquid level sensors are positioned to
maintain the liquid level at or below the top level of the inventory control system.
During operation, the upper levels of the inventory control system will at times be-
come exposed as the liquid level fluctuates.
Care must be taken to ensure that the liquid level remains below the bottom of the
refrigerator lid. Exposure to liquid nitrogen may result in physical damage to the lid.
Additionally, operating the refrigerator with high liquid levels characteristic of liquid
phase storage may result in turbulence during fill cycles. Caution must be exercised if
the refrigerator lid is opened during a fill, and appropriate safety equipment should
always be worn.
Thermocouple Positioning
The thermocouple is a separate sensor designed in conjunction with the
controller temperature readout to monitor and control the temperature
within the refrigerator. The termocouple should be positioned about the
High Level Alarm sensor.
OPERATION