High Stage System Failure; Probe Failure Alarm - Thermo Scientific 902 Operating And Maintenance Manual

Section 3
Alarms
High Stage System

Probe Failure Alarm

3-2 Model 900 and 5900 Series
The "high stage system failure" condition is created when the high stage
compressor and fans run for 30 minutes and are not capable of cooling the
Failure
interstage heat exchanger to the proper temperature. Under this condition,
the high stage compressor and fans will turn off after 30 minutes and an
audible and visual alarm will occur. The audible alarm can be silenced and
will ring back every 15 minutes.
The microprocessor in Model 900 and 5900 Series freezers continually
scans all probes including the control probe, heat exchanger probe and
condenser probe to ensure that they are operating properly. Should an
error be detected, the "Probe Failure" alarm will occur as described above.
If an error is detected with the control probe, the high and low stage
compressors will run continuously. As a result, the cabinet temperature
will decrease until it reaches the lowest temperature that the refrigeration
system can maintain. If an error is detected with the heat exchanger probe,
the freezer will cycle properly at its temperature set point using a 5 minute
step start between the high and low stage compressors. If an error is
detected with the condenser probe, there is no impact on the performance
of the freezer; however, the hot condenser alarm may also occur. Contact
the Technical Services department or your local distributor.
Thermo Scientific