Instrumentation, Control Systems, and Safety Devices
What the exam tests
This domain covers sensors, transmitters, signal ranges, controllers, interlocks, flame safeguards, relief devices, and trip testing. You should be able to interpret a 4-20 mA signal and explain fail-safe behavior.
Core rules
- A 4-20 mA transmitter uses a live zero: 4 mA is the lower-range value and 20 mA is the upper-range value. A 0 mA reading usually means a fault, not a true zero process value.
- Thermocouples generate voltage; RTDs change resistance. Both need the correct extension wire or lead compensation.
- PID control combines proportional, integral, and derivative action. Integral removes offset; derivative reacts to rate of change and can amplify noise.
- Safety devices must fail safe and be tested. Bypassing a low-water cutoff, flame scanner, or high-pressure trip defeats the protective function.
Worked example
A pressure transmitter has range 0-200 psig and output 4-20 mA. At 12 mA, fraction of span = (12 - 4) / (20 - 4) = 8/16 = 0.50. Pressure = 0.50 x 200 = 100 psig. The reasoning: subtract the live zero first, then scale the remaining 16 mA span to the engineering range.
Traps
Do not treat 4 mA as a failed signal; do not calibrate only the upper end; do not leave a trip in bypass after maintenance; and do not confuse an alarm with a trip that actually stops fuel or power.