100 Most Important MCQ Of Measurement and Instrumentation with explanation

100 Most Important MCQ Of Measurement and Instrumentation| Objective Type Question Measurement and Instrumentation with Explanation

Ques.1.  A measuring system consists of

  1. Sensors
  2. Variable conversion elements
  3. Signal processing elements
  4. All of these

Answer.4. All of these

Explanation:-

A measuring system exists to provide information about the physical value of some variable being measured. In simple cases, the system can consist of only a single unit that gives an output reading or signal according to the magnitude of the unknown variable applied to it. However, in more complex measurement situations, a measuring system consists of several separate elements as shown in Figure. 

The measurement sensor includes

  1. Primary Sensors
  2. Variable conversion elements
  3. Signal processing
  4. Signal recording

Primary Sensors:- The first element in any measuring system is the primary sensor: this gives an output that is a function of the measurand (the input applied to it). Some examples of primary sensors are a liquid-in-glass thermometer, a thermocouple, and a strain gauge. In the case of the mercury-in-glass thermometer, the output reading is given in terms of the level of the mercury, and so this particular primary sensor is also a complete measurement system in itself. However, in general, the primary sensor is only part of a measurement system. 

Variable conversion elements:- They are needed where the output variable of a primary transducer is in an inconvenient form and has to be converted to a more convenient form. For instance, the displacement-measuring strain gauge has an output in the form of a varying resistance. The resistance change cannot be easily measured and so it is converted to a change in voltage by a bridge circuit, which is a typical example of a variable conversion element. In some cases, the primary sensor and variable conversion element are combined, and the combination is known as a transducer.

Signal processing elements:- Signal processing elements exist to improve the quality of the output of a measurement system in some way. It is important that the measurement system is designed properly such that the output from measurement sensors is of an appropriate amplitude and is as free from noise contamination as possible. The electronic amplifier is a very common type of signal processing element. This is used to amplify low-amplitude outputs from the primary transducer or variable conversion element, thus improving the sensitivity and resolution of the measurement.

For example, amplification is needed for thermocouples, which have a typical output of only a few millivolts. Other types of signal processing element are those that filter out induced noise and remove mean levels, etc. In some devices, signal processing is incorporated into a transducer, which is then known as a transmitter.

 

Ques.2.  An instrument in which the value of ethnical quantity to be measured can be determined from the deflection of the instrument when it has been precalibrated by comparison with an absolute instrument

  1. Absolute instrument
  2. Secondary instrument
  3. Recording instrument
  4. Integrating instrument

Answer.2. Secondary Instrument

Explanation:-

In a broad sense, analog instruments may be classified into two ways:

  1. Absolute instruments
  2. Secondary instruments

Absolute instruments:- Absolute instruments give the value of the electrical quantity to be measured in terms of the constants of the instruments and to its deflection, no comparison with another instrument being required. For example, the tangent galvanometer gives the value of the current to be measured in terms of the tangent of the angle of deflection produced by the current, the radius and the number of turns of galvanometer coil, and the horizontal component of the earth’s magnetic fierce. No calibration of the instrument is thus necessary.

Secondary instruments:- Secondary instruments are so constructed that the value of current, voltage or other quantity to be measured can be determined from the malefaction of the instruments, only if the latter has been calibrated by comparison with either an absolute instrument or one which has already been calibrated. The deflection obtained is meaningless until such a calibration has been made.

This class of instruments is in most general use, the absolute instrument being seldom used except in standard laboratories and similar institutions.

The secondary instruments may be classified as

  1. Indicating instruments
  2. Recording instruments
  3. Integrating instruments

Indicating instruments are instruments which indicate the magnitude of a quantity being measured. They generally make use of a dial and a pointer for this purpose.

Recording instruments give a continuous record of the quantity being measured over a specified period. The variation of the quantity being measured is recorded by a pen Attache to the moving system of the instrument; the moving system is operated by the quantity being measured on a sheet of paper that moves perpendicular to the movement of the pen.

Integrating instruments record totalized events over a specified period of time. The summation, which they give, is the product of time and an electrical quantity. Ampere-hour and watt-hour (energy) meters are examples of this category.

 

Ques.3. A pointer of an instrument once deflected returns to zero position, when the current is removed due to

  1. Action of gravity
  2. Mass of the pointer
  3. Controlling Torque
  4. Damping Torques

Answer.3. Controlling torque

Explanation:-

Basically, there are three types of Force or torque act upon the Measuring Instrument

  1. Deflecting Torque
  2. Controlling Torque
  3. Damping Torque

Deflecting Torque: In order to move the pointer from its zero position on the scale deflecting torque is required. The deflecting torque works on the moving system which the pointer is attached. Obviously, the magnitude of deflecting torque produced is proportional to the magnitude of the quantity being measured, say, the current I flowing through the instrument. A deflecting torque is required to overcome the inertia, damping effect and controlling effect of the moving system.

This deflecting torque can be produced by any of this effects of current (or of voltage) such as

  1. Magnetic effect
  2. Electrostatic effect
  3. Electromagnetic effect
  4. Thermal effect
  5. Chemical Effect

Controlling Torque: With the help of the deflecting torque, the pointer deflection will take place on the calibrated scale but to stop the pointer at the definite position, controlling torque (Tc.) comes into action. As the deflection of the pointer increases, the controlling torque also increases and stops the pointer at the measured value.

Controlling torque is also known as restoring torque i.e., it brings back the pointer to its zero position when deflecting torque is withdrawn. The pointer attains a steady position when controlling torque becomes numerically equal to deflecting torque i.e., Tc = Td

Spring control and gravity control are used for controlling torque.

Damping Torque: At the final deflected position, when the deflecting and controlling torques are equal, the pointer starts oscillating owing to its inertia and therefore, cannot immediately settle at its final deflected position. If no extra force is provided to damp these oscillations, the moving system will take considerable time before coming to settle to the final deflected position. This is especially undesirable if the number of readings to be taken is quite large.

 

Ques.4. In hot wire instrument, the sensing wire is made of

  1. Copper
  2. Silver
  3. Platinum-iridium
  4. Copper-Nickel

Answer.3. Platinium-iridium

Explanation:-

These instruments utilize the principle of expansion of wire, heat due to the current being measured, passing through the wire.

It consists of a hot wire of platinum-iridium (because it can withstand high temperatures of about 300°C without being oxidized), and about 0.2 mm in diameter.

 

Ques.5. An ammeter

  1. Is inserted in series in a circuit and current to be measured flows through it
  2. Is inserted in series in a circuit and part of the current to be measured flows through it
  3. Is connected in parallel in a circuit and current to be measured flows through it
  4. Is connected in parallel in a circuit and only part of the current to the measured flows through it.

Answer.1. Is connected in parallel in a circuit and current to be measured flows through it

Explanation:-

An ammeter is connected in series with the load

To measure current, you want most of the current to pass through it. Hence ammeter is connected in series with the low resistance

An ammeter measures the value of current flowing in the circuit, so current should flow inside ammeter to give proper result. it acts like a closed switch so it indicates the current. And it has very low resistance to ensure the correct measurement of current in the circuit. If it is connected in parallel across any load then all current in the circuit will choose the lower resistive path (i.e ammeter) to cause its circuit to be damaged. Hence it is used in series.

 

Ques.6. An ammeter is convertible to a voltmeter by

  1. Changing the scale
  2. Putting a large resistance in parallel with the actual measuring part of the instrument
  3. Putting a large resistance in series with the actual measuring part of the instrument
  4. Simply installing the instrument in parallel with the circuit

Answer.3. Putting a large resistance in series with the actual measuring part of the instrument

Explanation:-

An ammeter is used to. measure current in amperes. It is a moving coil galvanometer in which a low resistance wire is connected in parallel.   An ammeter is converted into voltmeter by putting a large resistance in series with the actual measuring part of the instrument.

Let

Vr- Voltage range you required to measure (KNOWN VALUE)

If– Current required to full-scale deflection of your Ammeter (KNOWN VALUE)

Ra- Internal Resistance of Ammeter (Ideally Ra=0)

Rs- Resistance Should be Connected in Series with your existing Ammeter=?

Vr=(Ra Rs× (If)Vr=(Ra+Rs) × (If)

Vr=(Rs×If)Vr=(Rs×If) (We know that Ra=0 )

Rs=Vr/If

 

Ques.7. Which of the following material will be preferred as a shunt for extending the range of measurement of a voltmeter

  1. Copper
  2. Steel
  3. Aluminum
  4. Manganin

Answer.4. Manganin

Explanation:-

The range is the maximum value of the measured that can be measured by a given meter. The range of voltmeters, ammeters, etc., is limited by the current carrying capacity of the control springs, high measuring parameters, etc. Thus, for the measurement of large current and voltage values, it is necessary to employ a device which reduces the current and voltage of the instrument by a known proportion. 

Shunts are small resistances connected in parallel to increase the range of an ammeter. For multi-range ammeters, the moving coil remaining same; separate shunts are used to increase the range of measurement of a single instrument.

To extend the current-measuring capabilities of the meter movement it is necessary to connect a resistor in parallel with the coil. This causes the current that is to be measured to divide, a small part flowing through the coil and the remainder passing through the shunt resistor. The wire from which the coil is made is nearly always copper; unfortunately, the resistance of a copper coil varies appreciably with temperature.  A shunt resistor is then used whose resistance is practically constant over the operating temperature range; this guarantees a constant ratio between the coil resistance and the shunt resistance. Manganin is the usual alloy used for shunts because this material has a negligible temperature coefficient at the temperatures involved. There is also a negligible thermoelectric effect when manganin is used with copper.

 

Ques.8. What should be the size of the slide wire of the potentiometer to make it to achieve high accuracy?

  1. As long as possible
  2. As short as possible
  3. 1 meter
  4. Neither too thin nor too thick

Answer.1. As long as possible

Explanation:-

A potentiometer is an instrument designed to measure an unknown voltage by comparing it with a known voltage. This known voltage is supplied by a standard cell or through a reference source. The potentiometer makes use of null or balanced condition for the purpose of measurement.

The potential difference across a length of the potentiometer wire is directly proportional to its Length (or) when a steady current is passed through a uniform wire, potential drop per unit length or potential gradient is constant.

E ∝ L ⇒ E = φL

Where φ is the potential gradient

So In the case of longer wire, the fall of potential per unit length is small. In other words, the potential gradient is small. Lesser the potential gradient, more accurate is the potentiometer. 

 

Ques.9. A resistance of 75 Ohms is connected in shunt of a galvanometer, having an internal resistance of 25 Ohms, to convert it into an ammeter. What is the value of current (in A) flowing through the galvanometer, if the total current in the circuit is 5 A?

  1. 2
  2. 2.5
  3. 3.65
  4. 3.75

Answer.4. 3.75

Explanation:-

Extension of Ammeter Range

The current range of a DC moving coil ammeter is extended by connecting a shunt resistance Rs (low resistance) across the coil, the circuit as shown in Figure

I = Total current = 5A
Im = full-scale deflection current of ammeter = ?
Ish = shunt current
Rm = resistance of the ammeter = 25Ω
Rsh = shunt resistance = 75Ω

The full-scale deflection current Im is given as

\begin{array}{l}{I_m} = \dfrac{{I \times {R_{sh}}}}{{{R_m} + {R_{sh}}}}\\\\{I_m} = \dfrac{{5 \times 75}}{{(25 + 75)}}\\\\{I_m} = 3.75A\end{array}

 

Ques.10. Which of the following is not an integrating instrument?

  1. Ampere-hour meter
  2. Watt-hour meter
  3. Voltmeter
  4. All of the above

Answer.3. Voltmeter

Explanation:-

Integrating Instruments:

The instruments which add up the electrical quantity, i.e electrical energy and measure the total energy (in kilowatt hours) or the total Amper hours (in ampere-hours) supplied to a circuit in a given period are called integrating instruments.

In such instruments, there are sets of dials or gears which register the total quantity of electricity or the total amount of electrical energy supplied to a circuit in a given period. Household energy meter is an integrating instrument. Ampere-hour meter another example of integrating instruments.

Recording Instrument: The recording instruments are those instruments which give a continuous record of the variations of the electrical quantity to be measured.

Example: The example of such an instrument is Recording voltmeters which are used in power stations to record the generated voltage.

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