5-5
Troubleshooting and Maintenance
Unexpected Process Variable
(PV) Readings
Use a Model 275 HART Communicator to display the current process
variables. Before connecting a HART-based communicator in an
explosive atmosphere, make sure the instruments in the loop are
installed in accordance with intrinsically safe or non-incendive field
wiring practices.
The following performance limitations may inhibit efficient or safe
operation. Critical applications should have appropriate diagnostic and
backup systems in place.
Pressure devices contain an internal fill fluid. It is used to transmit the
process pressure through the isolating diaphragms to the pressure
sensing element. In rare cases, oil loss paths in oil-filled pressure
devices can be created. Possible causes include:
• Physical damage to the isolator diaphragms.
• Process fluid freezing.
• Isolator corrosion due to an incompatible process fluid.
A device with oil fill fluid loss may continue to perform normally for a
period of time. Sustained oil loss will eventually cause one or more of
the operating parameters to exceed published specifications while a
small drift in operating point output continues. Symptoms of advanced
oil loss and other unrelated problems include:
• Sustained drift rate in true zero and span or operating point
output or both.
• Sluggish response to increasing or decreasing pressure or both.
• Limited output rate or very nonlinear output or both.
• Change in output process noise.
• Noticeable drift in operating point output.
• Abrupt increase in drift rate of true zero or span or both.
• Unstable output.
• Output saturated high or low.
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