Measurement, Instrumentation and Control
What this domain tests
Measurement, Instrumentation and Control tests whether you can choose an instrument, read it correctly, quantify error, and describe a control loop.
Core rules
- Accuracy is closeness to the true value; precision is repeatability. A precise instrument can be consistently wrong.
- Errors: systematic (zero, calibration, parallax bias) can be corrected; random errors are reduced by averaging.
- Calibration compares an instrument with a traceable standard; sensitivity is output change per unit input; hysteresis is the difference between increasing and decreasing readings; linearity describes deviation from a straight-line characteristic.
- Vernier caliper: least count = main-scale division − vernier division, commonly 0.02 mm. Micrometer: pitch 0.5 mm with a 50-division thimble gives 0.01 mm.
- Control: open loop ignores output; closed loop uses feedback and compares measured value with setpoint. Proportional control leaves offset; integral action removes offset; derivative action damps response.
- Sensors: thermocouple (voltage), RTD (resistance rise), thermistor (large nonlinear resistance change), strain gauge (resistance change with strain).
Worked example
A vernier caliper has a main scale in 1 mm divisions and 50 vernier divisions. Least count = 1 − 49/50 = 0.02 mm. The main scale reads 12 mm and the 7th vernier line coincides. Reading = 12 + 7 × 0.02 = 12.14 mm. If the instrument had a zero error of +0.03 mm, the corrected value would be 12.11 mm.
Traps
Subtracting a zero error with the wrong sign; reading the main scale after the vernier zero instead of before; quoting a micrometer to 0.001 mm; confusing accuracy with precision; expecting P-only control to remove steady-state error; ignoring sensor nonlinearity.