29 Field Instrumentation GEN Rev)
• If the control logic is remotely located and temperature transmitters (milli-volt to milli- amp transducers) are used, a power supply failure will of course cause faulty readings.
Failure Modes An open circuit in the thermocouple detector means that there
is no path for current flow, thus it will cause a low (off-scale) temperature reading. A short circuit in the thermocouple detector will also cause a low temperature reading because it creates a leakage current path to the ground and a smaller measured voltage.
30 Field Instrumentation GEN Rev)
2. PRESSURE Learning Objectives Upon Completion of this topic one should be able to understand 1. Pressure
and its different units 2. Definitions Of pressure related terms and head pressure calculation
3. Types of pressure
instruments and their detail 4. Types of pressure sensors and transducers
5. Pressure
transmitter installation 6. State the effect of the following failures or abnormalities a) Over-pressuring a differential pressure cell or bourdon tube b) Diaphragm failure in a differential pressure cell c) Blocked or leaking sensing lines and d) Loss of loop electrical power.
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