An RTD suddenly shows -200°C on DCS. What is the most likely cause?
A. High process temperature
B. Open circuit
C. Grounding
D. Calibration error
View Answer
Answer: B
Explanation:
A sudden indication of -200°C (or another extreme low value) on a DCS is most commonly caused by an open RTD circuit.
When an RTD element or its wiring becomes open, the transmitter detects an abnormally high resistance (effectively infinite resistance). Depending on the transmitter configuration, it may drive the output to a predefined fault value, which the DCS interprets as an extremely low temperature such as -200°C.
Common Causes of an Open Circuit
Broken RTD element
Broken cable or lead wire
Loose terminal connection
Open junction box connection
Failed transmitter input connection
Typical Symptoms
- Sudden temperature drop to an extreme value
- Sensor fault alarm
- Bad PV indication
- Diagnostic warning from transmitter
Why the Other Options Are Incorrect
High process temperature
- Would cause the RTD resistance to increase and the indicated temperature to rise, not fall to -200°C.
Grounding
- May introduce noise or unstable readings but usually does not produce a fixed extreme low value.
Calibration error
- Causes measurement offset, not an abrupt jump to -200°C.
Because an open RTD circuit is the most common cause of an extreme low-temperature indication on a DCS, Option B (Open circuit) is the correct answer.