Taylor-Wharton 24K w/ AutoTend User Manual
Page 35
connection to the controller board. At the completion of maintenance or repairs, install
the controller using the procedure outlined for your refrigerator model in the Remov-
ing/Installing the Controller section.
Controller Electrical Tests
If a controller is removed from the refrigerator for service, the liquid refrigerant level
must be maintained manually to protect stored product. The fill solenoid valve will be
inoperative with the controller removed. A flexible fill line terminated with a phase
separator may be used periodically through the open refrigerator lid to conduct
manual fill operations until automatic operation is restored.
Sensor Probes. The sensor probes used to detect liquid level by Taylor-Wharton
controllers are temperature-sensitive resistors called “thermistors.” Their resistance to
electrical current flow changes greatly with their temperature. The resistance of a
typical thermistor used in these controllers varies with its temperature as follows:
•
At Room Temperature ............................................... 2 ohms to 10 ohms
•
In Cold Nitrogen Gas ................................................. 8 K ohms to 18 K ohms
•
In Liquid Nitrogen ...................................................... 18 K ohms to 35K ohms
Controller Logic. The liquid level controllers read the values of the sensors as
indications of liquid level. The function of a sensor, and its value, are interpreted by
solid-state logic circuitry to set normal operating, fill, fill termination, and alarm
conditions. In addition, the refrigerator temperature is monitored and an alarm is
triggered if the temperature raises above a pre-determined point. The following values
will be interpreted by a controller as cause for a logic change – either an alarm, or in
some cases, a fill or fill termination action.
From cold gas to liquid Resistance greater than 22K ohms.
From liquid to cold gas Resistance less than 16K ohms.
Defective Sensor
Resistance greater than 50K ohms, or less that 5 ohms.