Introduction to Measuring Instruments

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Questions and Answers

What happens to the pointer in an underdamped instrument?

  • It takes a long time to deflect.
  • It immediately remains stationary at zero.
  • It oscillates before coming to rest. (correct)
  • It reaches the final position without oscillations.

What is a consequence of using double springs in measuring instruments?

  • They eliminate temperature dependency.
  • They increase the friction at the pivot.
  • They provide a uniform scale. (correct)
  • They simplify the calibration process.

What is a main disadvantage of using springs for control in measuring instruments?

  • They require constant adjustment of torque.
  • They can develop an inelastic yield. (correct)
  • They provide too much damping.
  • They cause instability in the reading.

In gravity control instruments, what is the purpose of the adjustable balance weight?

<p>To make the center of gravity aligned with the spindle. (A)</p> Signup and view all the answers

Which factor affects the stiffness of springs used in measuring instruments?

<p>The temperature of the environment. (D)</p> Signup and view all the answers

Which classification of instruments does not require calibration with respect to standard instruments?

<p>Absolute instruments (D)</p> Signup and view all the answers

What type of instrument is specifically designed to give a continuous record of the variation of the quantity being measured?

<p>Recording instruments (B)</p> Signup and view all the answers

Which of the following instruments is considered a secondary instrument?

<p>Ammeter (B)</p> Signup and view all the answers

Integrating instruments measure quantities based on what factor?

<p>Time over which the quantity is supplied (D)</p> Signup and view all the answers

Which instrument category includes devices that can measure both A.C. and D.C. quantities?

<p>Moving iron instruments (B)</p> Signup and view all the answers

What primary function distinguishes indicating instruments from other types?

<p>They directly show the instantaneous value of the measured quantity. (C)</p> Signup and view all the answers

Which type of instrument typically does not provide relevant data without calibration?

<p>Secondary instruments (D)</p> Signup and view all the answers

What is the primary function of deflecting torque in an indicating instrument?

<p>To cause the moving system to move from its position of rest (A)</p> Signup and view all the answers

Which type of torque acts in the opposite direction to deflecting torque?

<p>Controlling torque (D)</p> Signup and view all the answers

What occurs when damping torque is absent in an indicating instrument?

<p>The pointer oscillates back and forth (D)</p> Signup and view all the answers

Which of the following is NOT a function of controlling torque?

<p>Initiating movement of the pointer (A)</p> Signup and view all the answers

What happens to the damping torque when the moving system of an indicating instrument is at rest?

<p>It becomes zero (A)</p> Signup and view all the answers

Which effect of electric current would NOT typically be used to produce deflecting torque?

<p>Chemically induced reaction (A)</p> Signup and view all the answers

What classification of damping occurs when the system returns to its static position without oscillating?

<p>Critically damped (B)</p> Signup and view all the answers

How do deflecting and controlling torques behave at steady-state conditions in an indicating instrument?

<p>Deflecting torque equals controlling torque (B)</p> Signup and view all the answers

Which torque is responsible for quickly bringing the moving system to rest?

<p>Damping torque (C)</p> Signup and view all the answers

Which statement regarding digital instruments is incorrect?

<p>Digital instruments have lower resolution than analog instruments (D)</p> Signup and view all the answers

Flashcards

Underdamped Instrument

An instrument where the pointer oscillates but eventually comes to rest.

Overdamped Instrument

An instrument where the pointer takes a long time to reach its final position without oscillating.

Critical Damping

The state where the pointer reaches its final position quickly without oscillations.

Spring Control Mechanism

Using one or two hairsprings to provide restoring torque for the pointer.

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Double Springs Advantage

Using two springs wound oppositely to ensure torque is proportional to angle of deflection.

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Measurement

The act of quantitative comparison between a standard and an unknown quantity.

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Absolute Instruments

Devices that provide magnitude without needing comparison to standards.

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Secondary Instruments

Instruments requiring calibration against absolute instruments for accurate readings.

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Indicating Instruments

Instruments that show the instantaneous value of a quantity.

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Recording Instruments

Instruments that provide a continuous record of a measured variable over time.

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Integrating Instruments

Instruments that measure the total quantity over a specified period.

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AC and DC Instruments

Instruments classified by their ability to measure alternating current (AC) or direct current (DC).

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Analog Information

Continuous and stepless function of time used in analog instruments.

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Digital Information

Discrete pulses or steps representing data in digital instruments.

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Deflecting Torque

The force that moves the instrument's moving system from rest.

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Controlling Torque

Limits movement to ensure consistent deflection for a given input measurement.

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Damping Torque

Quickly brings the moving system to a rest position to avoid oscillation.

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Steady State

Condition when deflecting torque equals controlling torque in an instrument.

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Underdamped

A state where the system overshoots its final position and oscillates slowly.

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Overdamped

A state where the system returns to rest slowly without oscillating.

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Study Notes

Introduction to Measuring Instruments

  • Measurement is comparing an unknown quantity to a predefined standard
  • An instrument is a device that determines the value of a quantity

Classification of Instruments

  • Absolute Instruments:
    • Determine a quantity's magnitude based on instrument constants and deflection
    • Do not require comparison with a standard instrument
    • Example: tangent galvanometer for measuring electric current
  • Secondary Instruments:
    • Need calibration against absolute instruments
    • Determine quantity value based on instrument deflection
    • Calibration is essential; without it, deflection readings are meaningless
    • Examples: ammeters, voltmeters, wattmeters

Classification Based on Operation

  • Indicating Instruments:

    • Show the instantaneous value of the measured quantity
    • Examples: ordinary voltmeters, ammeters, and wattmeters

  • Recording Instruments:

    • Provide a continuous record of a quantity's variation over time
    • Used in power plants and process industries
    • Examples: ECG (Electrocardiogram)

  • Integrating Instruments:

    • Measure a quantity over a period of time
    • Examples: ampere-hour meter, energy meter

Electrical Instrument Classification

  • A.C. Instruments: measure alternating current quantities

  • D.C. Instruments: measure direct current quantities

  • A.C./D.C. Instruments: measure both alternating and direct current quantities

    • Examples include moving iron instruments and dynamometer instruments

  • Analog Instruments:

    • Display information continuously
    • Easy to understand, calibrate, and maintain

  • Digital Instruments:

    • Display information using discrete pulses
    • High resolution and readability

Operation of Indicating Instruments

  • Deflecting Torque: Causes the moving system to move from rest
    • Generated by various effects of electric current (magnetic, electromagnetic induction, heating, chemical, electrostatic)
  • Controlling Torque: Limits the movement of the moving system; ensures constant deflection magnitude for a given input
    • Acts in the opposite direction to deflecting torque
    • Brings the pointer back to zero when the instrument is disconnected
  • Damping Torque: Brings the moving system to rest quickly
    • Necessary to prevent oscillations
    • Active only when the moving system is in motion

Damping Torque

  • Critical Damping:

    • Prevents oscillations and the pointer quickly reaches the final position.

  • Under-damped:

    • Oscillations occur before settling to the final position.

  • Over-damped:

    • Takes a long time to settle to the final position without oscillations.

Spring Control

  • Most common control method
  • Uses hairsprings (often phosphor bronze)
  • Wind in opposite directions to produce a proportional controlling torque
  • Balance weight helps center the moving system

Gravity Control

  • Uses a small control weight attached to the moving system
  • Adjusted to make the zero position vertical
  • Control weight exerts a force proportional to sinθ

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