Instrumentation, Sensors and Control Systems
What the exam tests
This domain covers sensors, transmitters, loops, PLC I/O, calibration, shielding, and fail-safe behaviour. Expect signal scaling, loop checks, and questions on why a reading is wrong even when the sensor appears powered.
Core rules
- A 4-20 mA loop uses 4 mA as the live zero and 20 mA as full scale. A reading of 0 mA usually means a broken loop or lost supply, not a true zero process value.
- Scale linearly: process value = lower range + (mA − 4) / 16 × span. For voltage signals, use the specified 0-10 V or 2-10 V range.
- Two-wire transmitters receive loop power and return current on the same pair. Check loop resistance and supply voltage against the transmitter datasheet.
- Shielded signal cable is normally earthed at one end to avoid earth loops; follow the site standard. Keep signal and power cables separated.
- PLC input and output modules have voltage, current, and isolation limits. A failed output may be a shorted load, not a failed module.
Worked example
A pressure transmitter is ranged 0-16 bar and outputs 4-20 mA. The measured loop current is 12.8 mA. Fraction of span = (12.8 − 4) / 16 = 0.55. Pressure = 0 + 0.55 × 16 = 8.8 bar. If the local gauge shows 9.2 bar, check calibration and reference, not just the wiring.
Traps
Treating 4 mA as zero pressure; assuming a 24 V supply is present at the transmitter without measuring under load; earthing a shield at both ends and creating noise; and replacing a PLC module before checking the field load.