American Magnetics 135 & 136 Liquid Helium Level Instruments (CE-Marked) - Manual (Page 27)

Rev. 3 15
3 Operation
This section describes the front panel display and control functions for the
Model 135/136. In addition to the front panel operation described, the
remote communication ports provide methods for performing similar
operations and querying the current state of the instrument. Refer to the
Remote Interface Reference section beginning on page 21 for more details
regarding the communication functions.
3.1 Sensor-Burnout Protection Mode
Operating the sensor in a vacuum without contact with liquid helium can
cause thermal damage and/or destruction of the superconducting filament.
The Model 135/136 is equipped with circuitry that automatically prevents
this type of damage from occurring, if the instrument’s LENGTH setting is
properly set to the active length of the connected sensor. If excessive
sensor resistance is detected, the Model 135/136 display will read:
(indicating burnout protection is in effect) and the sensor current will be
turned off for a minimum of six seconds after which time the instrument
will attempt to resume normal operation. No relay or alarm states are
affected.
If an attached sensor is not actually installed in a cooled cryostat, then the
sensor burnout protection will activate as soon as the Model 135/136 is
powered-on. Please refer to the Service section for more information on
checking the function of the Model 135/136 and attached helium level
sensor before installation in a cryostat.
3.2 Normal Operational Mode
3.2.1 Turn on the power
Turn the POWER toggle switch to the on position. The green POWER LED
located above the POWER switch will be lit. The display will show
AA A A
for approximately one second upon power-up during the microprocessor
reset routine and then show the liquid helium level. Also during power-up,
all LEDs will briefly be energized and the alarm will briefly sound as a
self-test procedure.
606.0