Measurements, Instrumentation and Sensors
What the exam tests
Measurements, Instrumentation and Sensors tests indicating instruments, bridges, instrument transformers, transducers, and error analysis. Questions often ask for extension ranges, bridge balance conditions, and sensor selection.
Core rules
- PMMC responds to average value and is used for DC; moving-iron responds to rms and works on AC/DC; electrodynamometer responds to true rms and can measure power.
- Ammeter shunt: Rsh = Rm/(m−1), where m = I/Im. Voltmeter multiplier: Rmult = V/Im − Rm.
- Wheatstone bridge balances when R1/R2 = R3/R4; Maxwell bridge measures inductance, Schering bridge capacitance and loss angle.
- CT secondary must not be open-circuited because dangerous voltage appears; VT secondary must not be short-circuited.
- Strain gauge gauge factor = (ΔR/R)/(ΔL/L); RTD has positive temperature coefficient; thermocouple measures temperature difference.
Worked example
A PMMC meter has 100 μA full-scale current and 100 Ω coil resistance. To read 10 V full scale, add a series multiplier. At full scale, total resistance = 10 V/100 μA = 100 kΩ. Multiplier = 100,000 − 100 = 99,900 Ω. The meter deflects full scale at 10 V, and power loss in the multiplier is negligible compared with the load.
Traps
Do not use PMMC for AC rms without a rectifier and scaling correction. Do not open a CT secondary even with the primary de-energized if burden is connected. Bridge sensitivity is maximum when all arms are comparable, not when one arm is extreme. Instrument transformer ratio and phase errors affect power measurement.