American Magnetics 188CPS Point-Sensing Instrument - Manual (Page 25)

24
Troubleshooting
Erratic or erroneous reading
3. Verify the input voltage selector switch on the instrument’s printed wiring
board is in the proper position for the available power receptacle at the
customer’s facility. Checking the input voltage selector requires removal of
the top cover of the instrument. Observe the same safety procedures as
presented in step 2.
Erratic or erroneous point-sensor 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. Ensure the 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. The Model
188CPS instrument implements a hysteresis band in order to minimize the
effects of minor sloshing.
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 display errors. This is due to the fact that air contains moisture
which will condense between the cold sensing plates. This small film of
moisture can cause a shorted or partially shorted condition. The electronics
may recognize this as a higher reading and display an immersed condition. 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.
6. Verify the sensor is free of contaminants and not subject to any physical
distortion. Disconnect the BNC connector for a specific plate 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 plate sensor should
typically be >10 M
Ω.
If the cause of the problem cannot be located, contact an AMI Customer Service
Representative at (865) 482-1056 for assistance. Do not send the unit back to AMI
without prior return authorization.