(a) Analogue vs Digital Measuring Instruments and Their Working Principles

Analogue Instruments

Definition:

• An analogue measuring instrument displays the measured value as a continuous movement of a pointer over a graduated scale.

Working Principle:

• The instrument converts the measurand (input quantity) into a proportional mechanical deflection.
  • In a typical analogue electrical meter:
    • The current flowing through a coil generates a magnetic torque.
    • This torque causes the pointer to move across the scale.
    • A spring provides a balancing torque.
    • The steady deflection of the pointer is proportional to the input signal (e.g., current or voltage).

  Digital Instruments

  Definition:

  • A digital measuring instrument displays the measured value as numerical digits on an electronic display (such as an LCD).

  Working Principle:

  • The instrument works by sampling the input signal, converting it into digital form, and processing it electronically to produce a precise reading.
    • In a typical digital meter:
      • The input signal passes through protection and signal conditioning circuits.
      • An Analogue-to-Digital Converter (ADC) converts the input voltage into a stream of digital bits.
      • A microcontroller or processor computes the final value.
      • The processed measurement is displayed on the LCD screen.

    (b) Examples of Analogue and Digital Instruments Used Onboard

    1. Analogue Instrument: Bourdon Tube Pressure Gauge

    Working Principle:

    • The Bourdon gauge measures fluid pressure using the elastic deformation of a C-shaped metal tube.
      • When internal pressure increases, the curved tube tends to straighten.
      • This motion is transmitted through a link and sector gear mechanism to a pointer, which moves proportionally across a calibrated dial.
    • Applications Onboard:
      • Commonly used in lube oil, fuel oil, and cooling water lines.
      • Advantages:
        • Rugged construction and no power requirement.
        • Provides an instant visual indication and helps monitor trends easily.

      2. Digital Instrument: Digital Multimeter

      Working Principle:

      • A digital multimeter measures voltage, current, and resistance electronically.
        • The input passes through protection and range selection networks.
        • The signal is digitised by an ADC.
        • The internal microprocessor computes the corresponding electrical value.
        • The result is shown numerically on the LCD display.
        • For AC measurements, a true-RMS converter or sampling algorithm ensures accurate readings even for non-sinusoidal waveforms.
      • Applications Onboard:
        • Checking 24V DC control circuits.
        • Verifying generator phase voltages.
        • Measuring sensor loop currents such as 4–20 mA signals in control systems.