100 Most Important MCQ Of Measurement and Instrumentation with explanation

Ques.51. In series type ohmmeter, full-scale deflection current indicator is marked

  1. Zero
  2. Infinite
  3. 100 ohm
  4. 1 Megaohm

Answer.1. Zero

Explanation:-

Series Type Ohmmeter

Instead of measuring both voltage and current, many instruments keep one of the two quantities constant. Thus the measurement of other quantity is nothing but proportional to the value of the resistance. If the current is kept constant, a voltmeter reading across the resistance is directly proportional to the value of the resistance. This is the principle of the Ohmmeter. Similarly, if the voltage is kept constant, an ammeter in series will have deflection proportions to the conductance but the meter can be calibrated in terms of the resistance.

Series ohmmeter

A series type ohmmeter consists of a deflecting meter in series with a resistance R1 and a battery which is connected to the terminals A-B, across wh the unknown resistance is connected. 

R1 = Current Limiting Resistance

R2 = Zero adjust Resistance

Rm = Meter Resistance

RX = Unknown Resistance

V = Battery Voltage

The current flowing through the meter depends on the magnitude of the unknown resistance. Thus the meter deflection is proportional to the value of He resistance. Hence Be meter scale should be calibrated accurately.

When the unknown resistor Rx = 0 (terminals A and B shorted), maximum current flows in the circuit. Under this condition, the shunt resistor R2, is adjusted until the movement indicates full-scale current (Ifsd). The full-scale current position of the pointer is marked ‘0Ω” on the scale.

Similarly, when RX = 0 (terminals A and B open), the current in the circuit drops to zero and the movement indicates zero current, which is then marked “∞” on the scale.

By connecting different known resistances across the terminals A-B, intermediate markings can be done.

As current is inversely proportional to the resistance, the scale is marked from ∞ to 0, rather than 0 to ∞. This is shown in Fig. 

Series ohmmeter scale

 

Ques.52.  For the measure of voltage and current in the ratio frequency range, a suitable instrument is

  1. Moving iron type
  2. Moving coil type
  3. Electrothermic type
  4. Electrostatic type

Answer.3. Electrothermic type

Explanation:-

Electrothermic type Instrument

The principle of operation of those instruments is based on the properties of certain circuit elements which gets heated when current flows through them. The average effect is dependent upon the mean square value of current and therefore, these instruments are used to indicate the true r.m.s. values. Consequently, the reading obtained is virtually free from frequency and waveform errors and also errors due to stray magnetic fields. The accuracy is equally good on a.c. as well as d.c. circuits.

The main significance of such an instrument is that they can be used to measure current at very high frequencies without many errors. So obviously these instruments are used for current measurement at the frequency well above the range of moving Iron and electrodynamometer type Instrument. These instruments can be used for Precision voltage measurement at moderate frequencies.

The electrothermic instruments are classified into the following three categories:

(i) Hot-wire instruments

(ii) Thermocouple instruments

(iii) Bolometers

Hot-wire instruments

  • Thermoelectric element used in hot wire type instruments is made up of platinum-iridium.
  • The sensitivity of electrothermic instruments is higher than electrodynamometer type instruments.

Advantages and Disadvantages—

(i) Not affected by frequency hence can be used at the higher frequency (more than 50 MHz), these indicate RMS value irrespective of the waveform.

(ii) Not affected by the magnetic field.

(iii) Can be used AC & DC measurements.

(iv) Measurement in an electrothermic instrument is independent of the waveform.

(v) The instruments have disadvantages like instability due to stretching of wire, sluggish response, high power consumption and inability to withstand overloads and mechanical shocks.

Thermocouple Type instruments

  • These instruments are generally used upto 500 V and at the frequency more than 50 MHz. At frequencies more than 50 MHz the skin effect is dominant and can cause an error. But this can be minimized by using Tubular Conductor. This is preferable for current I > 3A.
  • The thermoelectric instruments give the same calibration for AC & DC, therefore, these instruments can be calibrated by DC and can be used for AC measurement that is why these instruments are also called transfer instruments.
  • At low frequencies voltmeter act as a precision instrument.

Bolometer

Such an instrument is based on the fact that when current passes through the circuit element, it gets heated. Due to this dimension changes and resistance of the circuit elements changes.

 

Ques.53.  When a 10 µF capacitor is connected across the terminals of an ohmmeter, the instrument initially shows low resistance and then slowly the value of resistance rises to a very high value. What conclusion can be drawn about the condition of the capacitor?

  1. The capacitor is faulty
  2. The capacitor is leaky
  3. The capacitor has some initial charge
  4. The capacitor is straight

Answer.4. The capacitor is straight

Explanation:-

Testing of Capacitor

Testing capacitors difficult too really test a capacitor, two conditions must be assessed. One is the condition of the dielectric, and the other is the condition of the plate area. The first test can be made with an ohmmeter.

If an ohmmeter is connected across the terminals of a capacitor, the meter should show a deflection upscale and then return to infinity ohms. The deflection up scale indicates current flow to the capacitor when it is being charged by the ohmmeter battery. If the leads of the ohmmeter are reversed, the meter should deflect twice as far upscale and then return to infinity occurs. If a digital ohmmeter is employed, it should indicate either infinity or a very high resistance in the megohm range. That indicates that the capacitor is working fine

Capacitor Testing

The ohmmeter test for a capacitor is actually a very poor test. This test will not indicate a broken plate that would lower the value of capacitance. The ohmmeter test can indicate a show circuit, but it cannot test the condition of the dielectric. To test the dielectric rated voltage must be applied to the capacitor.

 

Ques.54. Which of the following meters will require the smallest shunt resistance?

  1. 0-10 mA
  2. 0-100 mA
  3. 0-1 mA
  4. 0-10 A

Answer.4. 0-10A

Explanation:-

According to the Ohm’s law

I = V/R

So more the value of current the value of resistance will be less therefore  0-10 A current will require low shunt resistance.

 

Ques.55. A permanent magnet moving coil type ammeter and a moving iron type ammeter is connected in series with the output of a half wave rectifier. If the moving iron type instrument reads 5-ampere magnet moving coil type instrument is likely to be

  1. Zero
  2. 5 A
  3. 3.18 A
  4. 2.5 A

Answer.4. 3.18A

Explanation:-

For half-wave rectifier

IDC = IPeak

IRMS = IPeak/2

IDC = 2 × IRMS ⁄ π

Where

IDC = DC output current

IPeak = Peak value of the current

IRMS = output current of root mean square value = 5 A

∴ PMMC Reading would be

IDC = (2 × 5) ⁄ π = 3.18 A

 

Ques.56. In shunt type ohmmeter, full-scale deflection current indicator is marked

  1. Zero
  2. Infinite
  3. 100 mΩ
  4. 100 MΩ

Answer.2. Infinite

Explanation:-

The shunt-type ohmmeter is used for measuring low values of resistance. It operates on the same basic principle as an ammeter shunt. When using a shunt-type ohmmeter, place the unknown value of resistance in parallel with the meter movement. This causes part of the circuit current to bypass the meter.

Shunt type ohmmeter

The shunt type ohmmeter consists of a battery in series with an adjustable resistance R1 and a meter movement. 

The switch is provided to disconnect the battery when the instrument is not in use. The unknown resistance is connected in parallel with the meter hence the name given to the instrument is shunt type ohmmeter. 

When the terminals A-B are shorted then the meter gets bypassed by the short circuit. Hence entire current flows through the short circuit and meter current is zero. This pointer position is marked as zero and the corresponding Rx = 0 as terminals A-B are shorted.

When the terminals A-B are opened i.e. Rx = ∞ then the entire current flows through the meter hence pointer deflects to the maximum. The resistance R1 is then adjusted such that current through the meter is its full-scale deflection current. This position of the pointer is marked as .

Thus the scale is marked form 0 to ∞ and not reversely from ∞ to 0 as in the case of the series ohmmeter.

 

Ques.57. Thermocouples are generally used for accurate temperature measurement upto

  1. 100°C
  2. 250°C
  3. 500°C
  4. 1600°C

Answer.4. 1600°C

Explanation:-

A thermocouple is a device which converts thermal energy to electric energy. The amount of electric energy produced can be used to measure temperature. The thermocouple is generally used for accurate temperature measurement upto −250°C to 2000°C.

 

Ques.58. The commonly used material for the thermocouples is

  1. Chromel copel
  2. Chromel-alumel
  3. Platinum-rhodium
  4. Any of the above

Answer.4. Any of the above

Explanation:-

The material used for the thermocouple is

Chromel-Copel = −200°C to +800°C

Iron-Copel = 0°C to +800°C

Copper-Copel = −200°C to +300°C

Chromel-Alumel = −200°C to +1300°C

Platinium-Rohdium = 0°C to 1400°C

Tungsten-Rhenium = 0°C to 2320°C

Copper-Constantan = −200°C to 900°C

 

Ques.59. Bolometers are used for the measurement of

  1. Optical inputs
  2. Thermal radiations
  3. Electrical signals
  4. Temperature inputs

Answer.4. Any of the above

Explanation:-

A bolometer is a very sensitive electrical resistance thermometer that is used to detect and measure weak thermal radiation. Consequently, it is especially well suited as an IR detector. The bolometer used in older instruments consisted of a thin metal conductor, such as the platinum wire. Incident IR radiation heats up this conductor, which causes its electrical resistance to change. The degree of change of the conductor’s resistance is a measure of the amount of radiation that has fallen on the detector. In the case of platinum, the resistance change is 0.4% per °C. The change in temperature depends on the intensity of incident radiation and on the thermal capacity of the detector. It is important to use a small detector and to focus the radiation on it. The rate at which the detector heats up or cools down determines how fast the detector responds to a change in radiation intensity as experienced when an absorption band is recorded. This constitutes the response time of the detector. For these older types of bolometers, the response time is long, on the order of seconds. Consequently, a complete mid-IR scan using a dispersive instrument and halometer could take 20 min.

The modern bolometer is micro-machined from silicon. This type of halometer is only a few micrometers in diameter and is usually placed in one arm of a Wheatstone bridge for measurements. The modern micro-bolometer has a fast response time and is particularly useful for detecting far-IR radiation (600-20 cm−1).

 

Ques.60. A barometer is an element which

  1. Senses optical input and delivers thermal output
  2. Senses thermal input
  3. Senses electrical input and delivers electrical output
  4. Senses optical input and delivers electrical output

Answer.1. Senses optical input and delivers thermal output

Explanation:-

Bolometers are radiation power detectors. They measure the power of the radiation through absorption and a thus resulting change of a measurable quantity. The most commonly used halometers are resistive bolometers. They use a change in resistance induced by absorption of radiation and measure the resulting current/voltage change. This change is proportional to the absorbed power.

The resultant temperature rise is then detected by measuring Me change in bolometer resistance by means of an auxiliary bridge circuit. 

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