American Magnetics 185 & 186 Liquid Level Instruments (CE-Marked) - Manual (Page 73)

Rev. 3 63
Service Guide
Erratic or erroneous level reading
selector requires removal of the top cover of the instrument.
Observe the same safety procedures as presented in step 2.
7.1.2 Erratic or erroneous level reading
1. Verify that the sensor is properly connected to the oscillator cable
and the extension cable (see the system diagram on page 7).
2. Verify the cabling has no breaks or cuts.
3. If the Model 185/186 suddenly reads 100% without a corresponding
level, there is a possibility of moisture in the connector at the top of
the sensor. Disconnect the BNC connection and remove any
moisture. Moisture or contaminants in any of the BNC coaxial
connectors can short out the sensor and cause a false ‘full’ level
indication or other erroneous readings. A pack of non-conductive
electrical connection lubricant (ECL or “Dielectric Tune-up
Grease”) has been included with the liquid level sensor packaging
to reduce the possibility of this occurring. Apply a small amount of
ECL to any of the BNC connectors that may be exposed to moisture.
Mate the doped connectors then remove any excess ECL from the
outside of the connector. Added protection can be achieved by
covering the doped connections with a short section of heat-shrink
tubing.
Note: MSDS sheets for the ECL are available upon request.
4. Ensure the oscillator unit is not exposed to large temperature
gradients such as those that occur near dewar vents. Extreme
temperature changes of the oscillator unit can cause readout errors.
5. Rapidly varying or sloshing liquids will sometimes make one think
the instrument is in error when it is actually operating properly.
6. 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 which 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
sensor will again approach the correct reading of 0%. This condition
can also be corrected immediately if liquid nitrogen is added to the
cold trap freezing the residual moisture. This is a physical
phenomenon and does not indicate any problem with your AMI
level equipment.