56
Troubleshooting
Erratic or erroneous level reading
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 5).
2. Verify the cabling has no breaks or cuts.
3. If the Model 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) 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.
7. Verify the sensor is free of contaminants and not subject to any physical
distortion. Disconnect the BNC connector at the top of the sensor and measure
the sensor resistance by placing an ohmmeter across the center pin and the
outer barrel of the connector. The resistance of the sensor should typically be
>10 M
Ω
.
Summary of Contents for 186
Page 3: ...2 Introduction ...
Page 13: ...12 Installation Configuring power ...
Page 23: ...22 Calibration Approximate Calibration ...
Page 29: ...28 Operation Sensor contamination ...
Page 37: ...36 RS 232 Communication Data Logger Option Error Codes ...
Page 47: ...46 IEEE 488 Communication Option Serial Poll Status Byte ...
Page 55: ...54 Virtual Instrument Operation Running multiple GPIB devices ...
Page 65: ...64 Index ...