Measurements & Instruments (Electrical)

Basic Concepts:

• Accuracy: closeness to true value.
• Precision: repeatability.
• Resolution: smallest measurable.
• Sensitivity: output/input ratio.

Errors:

• Systematic: consistent, can be corrected.
• Random: unpredictable, reduced by averaging.
• Gross: mistakes; must be eliminated.

Voltmeter:

• High impedance (parallel with circuit).
• Range extended by series resistance (multiplier).

Ammeter:

• Low impedance (in series with circuit).
• Range extended by parallel resistance (shunt).

Wattmeter:

• Measures real power (P = VI cos φ).
• Current coil (low Z) in series; potential coil (high Z) across load.

Energy meter:

• Measures kWh.
• Electromechanical (induction) or electronic.
• Smart meter: bi-directional, communication.

CRO (Cathode Ray Oscilloscope):

• Shows waveforms.
• Modern: DSO (digital storage oscilloscope).
• Used for: frequency, amplitude, phase measurement.

Bridge methods:

• Wheatstone bridge: unknown resistance.
• Kelvin double bridge: low resistance.
• Maxwell bridge: inductance.
• Schering bridge: capacitance.

Transducers:

• Thermocouple: temperature.
• RTD: temperature (resistance-based).
• LVDT: linear displacement.
• Strain gauge: strain.
• Hall sensor: magnetic field.

Multimeter:

• Measures V, I, R, capacitance, frequency.
• Digital (DMM) or analog.

Calibration:

• Comparing instrument with standard.
• Periodic.

RRB JE focus: instrument connections (V parallel, A series), wattmeter use, bridge methods.