American Magnetics 286 Multi-Sensor Liquid Level Instrument (CE-Marked) - Manual (Page 103)

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Service
Troubleshooting Hints
d. Replace the fuse and securely fasten the instrument top cover.
Reconnect the power-cord.
3. Verify the input voltage selector switch (S1) on the instrument’s
printed circuit board is in the proper position for the available
input power. For 100 to 120 VAC input “115V” should be visible in
the switch window; for 200 to 240 VAC input “230V” should be
visible in the switch window. Checking the input voltage selector
requires removal of the top cover of the instrument. Observe the
same safety procedures as presented in step 2, above.
4. If the display appears blank but the instrument power-up appears
otherwise normal (all the LEDs energized), then the contrast of the
LCD display may need adjustment. The LCD display should always
be visible (especially when looking up at the unit), but may be only
faintly visible if set to a low contrast percentage. See paragraph
4.3.12.2 on page 55 for specific directions for adjusting the display
contrast; the display may have to be viewed from either a sharp
upward or a sharp downward angle to be legible until the contrast
is properly adjusted.
6.2.2 The Model 286 level reading is erratic or erroneous
1. If the erratic or erroneous reading is for inputs C or D, which
require an external oscillator, verify that the sensor is properly
connected to the oscillator cable and the extension cable (see the
system diagram on page 5).
2. Verify that cabling has no breaks or cuts.
3. If the erratic or erroneous reading is for inputs C or D, which
require an external oscillator, ensure the erroneous input’s
oscillator unit is not exposed to large temperature variations such
as those that occur near dewar vents. Extreme temperature
changes of the oscillator unit can cause readout errors.
4. Rapidly varying or sloshing liquids will sometimes make one think
the instrument is in error when it is actually operating properly.
5. Capacitance-based sensors used in cryogenic liquid systems are
sometimes exposed to humidified air when the cryogenic vessel is
emptied. This often happens when a cold trap runs out of liquid. As
the sensor warms, the electronics can show large errors (readings
greater than 20% are not uncommon). This is due to the fact that
air contains moisture that will condense between the cold sensing
tubes. This small film of moisture can cause a shorted or partially
shorted condition. The electronics may recognize this as a higher
level reading and display some positive level. As the sensor warms
over some period of time, the moisture can evaporate and the