Sensors and Measurement Fundamentals
Domain 1 tests the measurement vocabulary and sensing hardware that every other instrumentation topic depends on: what accuracy, precision, resolution and drift actually mean, how a physical quantity becomes an electrical signal, and how that signal is conditioned, converted and used in a closed loop. Questions are mostly definitions plus short numerical reasoning.
Measurement vocabulary
- Accuracy - closeness of an indication to the true value; precision - repeatability of the same reading; resolution - smallest change the system can detect or display.
- Sensitivity - output change per unit input; linearity - deviation from a straight-line transfer function; hysteresis - a different output for the same input depending on the direction of change; dead band - an input range that produces no output change.
- Drift - slow change of zero or span with time and temperature; stability is the absence of it. Systematic (bias) errors shift every reading; random errors scatter them.
Sensing elements
- Resistive: potentiometer (wiper wear, noise), strain gauge in a bridge (force, pressure), platinum RTD (positive temperature coefficient), thermistor (large, non-linear, usually negative coefficient).
- Capacitive: pressure capsules and fuel-quantity probes. Inductive: LVDT for linear displacement, RVDT for rotary position. Synchro and resolver for angular transmission between instruments.
- Piezoelectric elements for vibration and pressure pulses; thermocouples for high temperature; optical and MEMS devices in modern AHRS/ADAHRS.
Conditioning and conversion
A Wheatstone bridge turns a tiny resistance change into a voltage; the signal is then amplified, filtered and digitised. ADC resolution = full-scale range ÷ 2ⁿ. A servo loop drives a motor until feedback equals the command, so the error signal is zero at rest - the principle behind autopilot servos, synchro repeaters and servo altimeters.
Worked example
A 12-bit ADC spans a 0–1000 hPa sensor. Steps = 2¹² = 4096, so resolution = 1000 ÷ 4096 ≈ 0.24 hPa. If the sensor's stated accuracy is ±2 hPa, the extra resolution is not extra accuracy - quoting 0.01 hPa would be false precision.
Traps that cost marks
- Choosing precision when the question says "closeness to the true value" (that is accuracy).
- Confusing gauge, absolute and differential pressure: gauge = absolute − ambient.
- Assuming better resolution means a better sensor; accuracy and resolution are independent.