Characteristics of Measuring Instrument MCQ || Measuring Instrument Characteristics Questions and Answers

MCQ, Measurement and Instrumentation / March 12, 2022

11. The gauge factor is defined as:

  1. $\frac{{\Delta L/L}}{{\Delta R/R}}$
  2. $\frac{{\Delta R/R}}{{\Delta L/L}}$
  3. $\frac{{\Delta R/R}}{{\Delta D/D}}$
  4. $\frac{{\Delta R/R}}{{\Delta P/P}}$

Answer:2. $\frac{{\Delta R/R}}{{\Delta L/L}}$

Explanation: 

The gauge factor is defined as the ratio of per unit change in resistance to per unit change in length. It is a measure of the sensitivity of the gauge.

Gauge factor, ${G_f} = \frac{{{\rm{\Delta }}R/R}}{{{\rm{\Delta }}L/L}}$

$\frac{{{\rm{\Delta }}R}}{R} = {G_f}\frac{{{\rm{\Delta }}L}}{L} = {G_f}\varepsilon $

Where

ε = strain = $\frac{{\Delta L}}{{L}}$

The gauge factor can be written as:

= Resistance change due to change of length + Resistance change due to change in the area + Resistance change due to the piezoresistive effect

${G_f} = \frac{{{\rm{\Delta }}R/R}}{{{\rm{\Delta }}L/L}} = 1 + 2v + \frac{{{\rm{\Delta }}\rho /\rho }}{\varepsilon }$

If the change in the value of resistivity of a material when strained is neglected, the gauge factor is:

${G_f} = 1 + 2v$

The above equation is valid only when the Piezoresistive effect that changes in resistivity due to strain is almost neglected.

For wire-wound strain gauges, the Piezoresistive effect is almost negligible.

 

12. The unit of sensitivity of an instrument is:

  1. Ampere/sec
  2. Volt/ohm
  3. Volt-amp
  4. Ohm/volt

Answer:4. Ohm/volt

Explanation: 

  • Sensitivity is defined as the ratio of the changes in the output of an instrument to a change in the value of the quantity being measured.
  • It denotes the smallest change in the measured variable to which the instrument responds.

The Unit of instrument sensitivity is expressed in Ohm/Volt.

 

13. The sensitivity of an instrument is

  1. Smallest increment in the output that can be detected with certainty
  2. Largest input change to which the instrument fails to respond
  3. Ratio of the change in the magnitude of the output to the corresponding change in the magnitude of the input
  4. Closeness of the output values for repeated applications of constant input

Answer:3. Ratio of the change in the magnitude of the output to the corresponding change in the magnitude of the input

Explanation: 

Sensitivity is defined as the ratio of the changes in the output of an instrument to a change in the value of the quantity being measured. It denotes the smallest change in the measured variable to which the instrument responds.

 

14. Meter A has a range of 0 – 100 V and a multiplier resistance of 28 kΩ and internal resistance of 2 kΩ. Its sensitivity is:

  1. 0.6 kΩ/kV
  2. 0.3 kΩ/V
  3. 0.5 kΩ/V
  4. 3 kΩ/V

Answer:2. 0.3 kΩ/V

Explanation: 

Total internal resistance of a voltmeter is given by

Rm = Vfsd/Ifsd

The sensitivity (S) of a voltmeter is the reciprocal of full-scale deflection current (Ifsd)

Rm = full scale range of voltmeter × sensitivity

Rv = Vfsd × S

Calculation:

Given that, full scale range of voltmeter (Vfsd) = 100 V

Internal resistance (Rm) = 2 kΩ

Multiplier resistance = 28 kΩ

Total resistance = 30 kΩ

Sensitivity (S) = 30/100 kΩ/V = 0.3 kΩ/V

 

15. Find the total resistance of a voltmeter if the range of voltmeter is 50 V and sensitivity is 20 kΩ/V.

  1. 10 kΩ
  2. 1 MΩ
  3. 0.4 kΩ
  4. 2.5 kΩ

Answer:3. Inter-symbol interference

Explanation: 

Total internal resistance of a voltmeter is given by

Rm = Vfsd/Ifsd

Sensitivity (S) of a voltmeter is the reciprocal of full-scale deflection current (Ifsd)

Rm = full scale range of voltmeter × sensitivity

Rv = Vfsd × S

Calculation:

Given that, full scale range of voltmeter (Vfsd) = 50 V

Sensitivity (S) = 20 kΩ/V

Internal resistance (Rm) = 50 × 20 = 1000 kΩ = 1 MΩ

 

16. In an electrical measuring instrument, the controlling torque is also known as:

  1. Damping torque
  2. Operating torque
  3. Restoring torque
  4. Deflecting torque

Answer:3. Restoring torque

Explanation: 

  • Restoring torque is used to control the pointer to a definite value which is proportional to the quantity being measured
  • In absence of controlling torque, the pointer will swing beyond its final steady-state position and the deflection will be indefinite
  • After removal of the moving mechanism, the pointer has to come back to its initial position, but in absence of controlling torque, the pointer won’t come back to its initial position

 

17. A digital voltmeter has a read-out range from 0 to 9999 counts. If the full-scale reading is 9.999 V, the resolution is:

  1. 1 V
  2. 0.01 V
  3. 1 milli V
  4. 1 micro V

Answer:3. 1 milli V

Explanation: 

The resolution (R) in an N bit DVM is given by:

R = Range of voltmeter/10N

Where N is the number of full digits.

In a DVM, a full digit counts 0 to 9 and a half digit counts from 0 to 1.

Calculation:

The full-scale reading = 9.999 V

It is a 4-digit voltmeter i.e. N = 4.

Range of voltmeter = 10 V

Resolution for the given DVM is

= 10/104 = 1 mV

 

18. Accuracy is defined as

  1. The measure of the consistency of the readings
  2. Closeness with which an instrument reading approaches the true value
  3. The smallest measurable input change
  4. The ratio of the input to output

Answer:2. Closeness with which an instrument reading approaches the true value

Explanation: 

The accuracy of measurement is the closeness of agreement between a quantity value obtained by measurement and the true value of the measurand (quantity intended to be measured).

 

19. An instrument has sensitivity of 1000 ohm/volt. On the 100 volt scale, this instrument will have an internal resistance of:

  1. 10 ohms
  2. 10,000 ohms
  3. 100,000 ohms
  4. 1000 ohms

Answer:3. 100,000 ohms

Explanation: 

Sensitivity(S) is the reciprocal of full-scale deflection current (IFSD).

Sensitivity S = 1/Ifsd

Full scale voltage (VFSD) = IFSD Rm

Where Rm is the internal resistance of the meter.

Calculation:

Sensitivity (S) = 1000 ohm/volt

Full scale voltage (VFSD) = 100 V

IFSD = 1/S = 1/1000 = 1 mA

Rm = Vfsd/Ifsd

Rm = 100/(1 × 10−3)

Rm = 100 kΩ

 

20. Full-scale deflection current of a D’Arsonval movement is 10 mA and internal resistance is 100 Ω. If it is converted to a multirange voltmeter, the value of multiplier required for voltage of 0-50 V is:

  1. 5.9 kΩ
  2. 4.1 kΩ
  3. 5.1 kΩ
  4. 4.9 kΩ

Answer:4. 4.9 kΩ

Explanation: 

To increase the range of a voltmeter, we need to the series resistance and it is given by

Rse = Rm(V/Vm − 1)

Where V is the required voltmeter range

Vm is the voltmeter range

Rm­ is the meter internal resistance

Calculation:

Given that,

Meter full scale current reading (Im) = 10 mA

Internal resistance (Rm) = 100 Ω

Voltmeter range (Vm­) = I m­ Rm = 10 × 10-3 × 100 = 1.0 V

Required voltmeter range (V) = 50 V

Rse = 100(50/1 − 1)

Rse = 4.9 kΩ

 

21. A LVDT produces an RMS output voltage of 2.6 V for the displacement of 0.4 μm. Calculate the sensitivity of LVDT.

  1. 6.5 V/μm
  2. 4.5 V/μm
  3. 8.5 V/μm
  4. 12.5 V/μm

Answer:1. 6.5 V/μm

Explanation: 

Sensitivity(S) = output voltage / displacement measured

Given, RMS output voltage = 2.6 V

Displacement = 0.4 μm

S = 2.6/0.4

Sensitivity = 6.5 V/μm

 

22. ______ cannot be expressed as a numerical value.

  1. Accuracy
  2. Sensitivity
  3. Precison
  4. All of the above

Answer:1. Accuracy

Explanation: 

The accuracy of measurement is the closeness of agreement between a 3 quantity value obtained by measurement and the true value of the measurand (quantity intended to be measured). Accuracy cannot be expressed as a numerical value.

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