Taylor-Wharton CS Series - Manual (Page 13)

11
CS SERIES
Operational Theory
The CS SERIES Control System automatically maintains the Liquid Nitrogen (LN
2
)
level and monitors temperature in the Cryogenic freezer. Operational conditions
are monitored and any alarm is triggered if necessary. Operations data is stored in
memory on the control board.
#8
#7
High Level Alarm #6
#4
#1
#5 Stop Fill
#3 Start Fill
#2 Low Level Alarm
Figure 3.0 Thermister LN
2
Level
The CS CONTROL SYSTEM uses thermistors to measure the LN
2
level within
the vessel. A thermistor is a thermal resistor and its resistance changes as the
temperature changes. When a thermistor is submerged in LN
2
, its resistance will be
significantly greater than its resistance at room temperature. The control can detect
this resistance change and determine the level of the LN
2
within the freezer. The CS
CONTROL SYSTEM is designed to work with an 8 thermistor assembly. There are
four thermistors that can be selected that will maintain the LN
2
level. These selected
thermistors correspond to Low Alarm, Start Fill, Stop Fill and High Alarm. When
the LN
2
level drops below the Start Fill thermistor, the control opens a solenoid
valve allowing LN
2
to enter the vessel. This continues until the Stop Fill thermistor is
submerged in LN
2
at which point the solenoid valve is closed preventing the flow of
additional LN
2
into the vessel. The Low Alarm, Start Fill, Stop Fill and High Alarm
settings can all be changed by the user through the menu system.