American Magnetics 135 & 136 Liquid Helium Level Instruments (CE-Marked) - Manual (Page 60)

48 Rev. 3
Service Guide
No level reading
6.1.1 No level reading
1. Ensure that the sensor is immersed in liquid helium. If the sensor
is immersed and/or cooled, then proceed to step 2.
Note
When the sensor is not cooled, the instrument will enter into the
sensor burnout protection mode and turn off the sensor current.
Refer to paragraph 3.1 of the Operation section for a description of
this mode.
2. Check the sensor connections. With the sensor at room
temperature, check the following resistances between sensor leads:
RED to BLUE = approximately 5 ohms
BLUE to YELLOW = 13.75 ohms
x Active Length (inches)
YELLOW to BLACK = approximately 0.7 ohms
RED to BLACK = (RED to BLUE) + (BLUE to YELLOW)
If the indicated conditions do not exist, contact an Authorized AMI
Technical Support Representative.
Note
If sensor lead wires in excess of few feet are connected, the above
figures may not be applicable.
3. Ensure that the instrument is energized from a live power source of
proper voltage. The unit configured voltage is indicated on the rear
panel.
Warning
If the instrument has been found to have been connected to an
incorrect power source, return the instrument to AMI for evaluation
to determine the extent of the damage. Frequently, damage of this
kind is not visible and must be determined using test equipment.
Nevertheless, connecting the instrument to an incorrect power source
could damage the internal insulation and/or the ground
requirements, thereby, possibly presenting a severe life-threatening
electrical hazard.
4. Verify continuity of the line fuse, F2, located on the instrument
printed circuit board.