100 Most Important MCQ Of Measurement and Instrumentation with explanation

Answer.3. Attraction type moving instrument

Explanation:-

Moving iron instrument are of two types

1. Attraction Type
2. Repulsion Type

Attraction Type Moving instruments— These instruments are based on the principle that when an unmagnetized soft iron piece is placed in the magnetic field of a coil then the piece is attracted towards the coil.

Repulsion type Moving instruments— These instruments are based on the principle of repulsion between the two iron pieces similarly magnetized. Since deflection θ is proportional to the square of current through the coil, therefore scale of such instruments in non-uniform being crowded in the beginning.

Advantages of moving Iron Instrument

The various advantages of moving iron instruments are,

1) The instruments can be used for both a.c. and d.c. measurements.

2) The torque to weight ratio is high, errors due to the friction are very less.

3) A single type of moving element can cover the wide range hence this instrument; are cheaper than other types of instruments.

4) There is no current-carrying part; in the moving system hence these meters are extremely rugged and reliable.

5) These are capable of giving good accuracy. Modern moving iron instruments have a d.c. an error of 2% or less.

6) These can withstand large loads and are not damaged even under severe overload conditions;

7) The range of instruments can be extended.

Disadvantages of moving Iron Instrument

The various disadvantages of moving iron instruments are,

1) The scale of the moving iron instruments is not uniform and is cramped at the lower end. Hence accurate readings are not possible at this end.

2) There are serious errors due to hysteresis, frequency changes, and stray magnetic fields.

3) The increase in temperature increases the resistance of the coil, decreases the stiffness of the springs, decreases the permeability and hence affect the reading severely.

4) Due to the non-linearity of the B-H curve, the deflecting torque is not exactly proportional to the square of the current.

5) There is a difference between a.c and d.c calibrations on account of the effect of the inductance of the meter. Hence these meters must always be calibrated at the frequency at which Hey are to be used. The usual commercial moving iron instrument may be used within its specified accuracy from 25 to 125 Hz frequency range.

6) Power consumption is on the higher side.

Answer.4. Photoconductive cell

Explanation:-

PHOTOCONDUCTIVE CELLS

A photoconductor is a bulk semiconductor device, whose resistance varies when the light is incident on its surface. All semiconductors can be considered as photoconductors. This is because any semiconductor will produce electron-hole pairs (EHP) when it is irradiated by electromagnetic radiation of the optical frequencies. These include frequencies in the infrared, visible and ultraviolet radiations. 

The photoconductive cell is a  two terminal semiconductor device whose resistance varies inversely with the intensity of light that falls upon its photosensitive material shows a typical photoconductive cell and the schematic symbol. These devices are also called photoresistive cells or photoresistors since the change in conductivity appears as a change in resistance.

The photoconductive materials most frequently used include cadmium sulfide (CdS) and cadmium selenide (CdSc). The response time of CdS units is about 100 ms and of CdSe cells is 10 ms. 

Answer.4. All of the given option are correct

Explanation:-

LINEAR VARIABLE DIFFERENTIAL TRANSFORMER (LVDT)

This is the most widely used inductive transducer for translating linear motion into an electrical signal. As we know that displacement is a vector quantity representing a change in position of a body or a point with respect to a reference. It can be linear or angular (rotational) motion. With the help of the displacement transducer, many other quantities, such as force, stress, pressure, velocity, and acceleration can be found. 

The main electrical displacement transducers work on the principle of

Variable resistance: transducer is a strain gauge.

Variable inductance: transducer is a linear variable differential transformer

Variable capacitance: transducer is a parallel plate capacitor with a variable gap

Synchros and resolvers: used to measure angular displacement

Advantages of LVDT:

1. Linearity: The output voltage of LVDT is almost linear for displacement up to 5mm.
2. High Output: The LVDT gives reasonably high output and hence requires less amplification.
3. High Sensitivity: The LVDT has a high sensitivity of about 300 mV/mm. i.e., 1mm displacement of the care produces an output voltage of 300 mV.
4. Ruggedness: The LVDT is mechanically rugged and can withstand mechanical shock and vibrations.
5. Less friction: Since there is no sliding contact, the friction is very less.
6. Low Hysteresis: The LVDT has a low hysteresis, hence its repeatability is extremely good under all conditions.
7. Low Power Consumption: Most LVDTs consume Less than the watt of power. The LVDT transducers are small, simple and light in weight. They are stable and easy to align and maintain.

Disadvantages of LVDT

1. Comparatively, large displacement is necessary for appreciable differential output.
2. They are sensitive to stray magnetic fields. However, this interference can be reduced by shielding.
3. The temperature effects transducers.
4. The dynamic response is limited.

Applications of LVDT

1. The LVDT can be used in all applications where displacement ranging from fractions of a few mm to a few cms have to be measured.

2. Acting as a secondary transducer, LVDT can be used as a device to measure force, weight & pressure etc.

Answer.1. Piezoelectric

Explanation:-

Piezoelectric Effect is the ability of certain materials to generate an electric charge in response to applied mechanical stress. One of the unique characteristics of the piezoelectric effect is that it is reversible, meaning that materials exhibiting the direct piezoelectric effect (the generation of electricity when stress is applied) also exhibit the converse piezoelectric effect (the generation of stress when an electric field is applied).

The most used piezoelectric materials are PZT, a ceramic containing lead titanate and lead zirconate and PZLT, a ceramic of lead-lanthanum-zirconate-titanate.

Natural piezoelectric materials such as quartz (SiO₂) and Rochelle salt (NaKC₄H₄O_6–4H₂O) are also used for piezoelectric transducers.

Answer.3. Thermoelectric pyrometer

Explanation:-

A pyrometer is a device for measuring very high temperatures and uses the principle that all substances emit radiant energy when hot, the rate of emission depending on their temperature. The measurement of thermal radiation is, therefore, a convenient method of determining the temperature of hot sources and is particularly useful in industrial processes. There is two main type of pyrometer, namely the total radiation pyrometer and the optical pyrometer.

Pyrometers are very convenient instruments since they can be used as a safe and comfortable distance from the hot source. Thus applications of pyrometer are found in measuring the temperature of molten metals, the interiors of furnaces or the interiors of volcanoes. Total radiation pyrometers can also b used in conjunction with devices that record an control temperature continuously.

Total radiation pyrometers are used to measure temperature in the range 700°C to 2000°C.

 Optical pyrometers may be used to measure temperatures up to, and even in excess of, 3000°C.

Measurement of temperature using the thermometer

1. Liquid thermometer:- 
   1. Low temperature: as low as −30°C (freezing point of mercury is −39°C) can be measured using mercury and as low as -100°C can be measured by using alcohol (Remains liquid upto-130°C) in glass thermometers.
   2. High temperature: A mercury thermometer (boiling point of mercury is 357°C) can be used to measure temperature upto 300°C. The range can be increased to 600°C by filling the space above mercury with some inert gas. A gallium boiling point 1700°C thermometer made of quartz can be conveniently used for measurement of temperature upto 1000°C.
2. Gas thermometers
   1. Low temperature: For measurements of low temperatures upto −250°C constant volume hydrogen thermometers are used and for measure merits below -250°C upto -270°C hydrogen is replaced by Helium. The Helium. at the pressure below its vapor pressure can be used to measure temperature down to 1 K.
   2.  High temperature: For high-temperature measurement upto 500°C the standard hydrogen gas thermometer with platinum-iridium bulb can be used, and for the temperature upto 1100°C porcelain bulb is used. Using nitrogen in place of hydrogen in Rhodium bulb, the temperature as high as 1600°C can be measured.
3. Resistance thermometers
   1. Low-temperature range: The platinum resistance thermometer is suitable for measurement upto -190°C. A lead thermometer may be used to measure temperature as low as -250°C and below these temperature constant and manganin thermometers are used. Phosphor bronze can be used to measure still lower temperatures with great accuracy.
   2. High-temperature range: Platinum resistance thermometers can be used to measure temperature upto 700°C with the coil enclosed in the glass bulb and lead made of copper and upto 1300°C with the coil enclosed in porcelain with platinum leads.
4. Thermoelectric-thermometer
   1. Low-temperature measurement: Thermocouples of Iron-constantan and copper constantan are used to measure temperature as low as 18 K and gold-platinum and silver platinum thermocouples are used to measure below 18 K with high accuracy.
   2. High-temperature measurement:- Iron constantan or copper constants are used to measure temperate upto 300°C and thermocouples of nickel-iron are used to measure temperature opts 600°C. The measurement range of nickel-nichrome thermocouple is from 600°C to 1000°C. Between 1000°C and 1600°C Platonism Rhodium thermocouple is used.

Answer.4. Same as a photovoltaic cell

Explanation:-

A solar cell or photovoltaic cell is a device that converts light energy into electrical energy. Sometimes the term solar cell is reserved for devices intended specifically to capture energy from sunlight, while the term photovoltaic cell is used when the light source is unspecified.

Answer.2. Very low pressure

Explanation:-

Piezometers are used to monitor pore water pressure and are sometimes referred to as pore pressure cells.

Piezometer tube

The piezometer tube is a vertical glass tube inserted in the wall of a pipe or a vessel containing the liquid whose pressure is to be measured. The tube is kept vertical and its other end is open to atmosphere. The height of the liquid in the tube above any point in the fluid indicates the pressure of the liquid at that point. In the figure, the pressure at the point above atmospheric pressure is:

PA = wh_A 

where

h_A  is the height of the liquid column in the piezometer tube.

Since the other end of the tube is open to atmosphere, the pressure obtained using the piezometer tube is gauge pressure. This type of manometer can only be used for measuring small and moderate pressures because as the pressure to be measured increases, the corresponding length of the piezometer tube required for measurement also increases. When the length of the tube becomes longer, it cannot be handled conveniently.

The piezometer, when used for the measurement of pressure in a pipe in which fluid is flowing, must be kept such that the axis of the tube is normal to the direction of flow. Also, to reduce the surface tension and capillarity effects. the diameter of the tube must be at least 12 mm.

Answer.1. Strain gauge

Explanation:-

 A load cell is a transducer. It is used to create an electrical signal whose magnitude is directly proportional to the force being measured. Various types of load cells include hydraulic load cells, pneumatic load cells, and strain gauge load cells.

Strain Gauge Load Cell

Strain gauge based load cells have a sensing junction/element that when loaded responds to the load by producing an electrical output. The circuitry to convert the response to a quantifiable electrical signal is a configuration known as the Wheatstone bridge.

Strain-gauge-based load cells are the most commonly used type. Strain gauges are made up of a long, thin strip of conductive material arranged in a zigzag manner. Load cells typically contain multiple strain gauges aligned and wired in a Wheatstone bridge circuit. When stress is applied to a strain gauge, the resistance of the strain gauge changes and unbalances the Wheatstone bridge, resulting in a signal output (voltage) that is proportional to the stress.

The rated capacities of strain-gauge load cells range from 1 N to more than 50 MN, and they can be used with high-resolution digital indicators as force transfer standards. The shape of the elastic element used in load cells depends on a number of factors, including the range of force to be measured, dimensional limits, final performance, and production costs.

Answer.2. Temperature

Explanation:-

• Load cells can measure tension, compression, or shear.
• Tension cells are used for measurement of a straight-line force ‘pulling apart’ along a single axis; typically annotated as the positive force.
• Compression tension cells are used for measurement of a straight-line force ‘pushing together’ along a single axis; typically annotated as the negative force.
• Shear is induced by tension or compression along offset axes. Most load cells actually measure the displacement of a structural element to determine force.
• The force is associated with a deflection as a result of calibration.
• There are many form factors or packages to choose from—S-beam, pancake, donut or washer, plate or platform, bolt, link, miniature, cantilever, canister, load pin, rod end, and tank weighing.
• Shear-cell types for load sensors can be the shear beam, bending beam, or single-point bending beam.
• The most common sensor technologies are piezoelectric and strain gauge.
• For piezoelectric devices, a piezoelectric material is compressed and generates a charge that is conditioned by a charge amplifier.
• For strain gauge devices, strain gauges (strain-sensitive variable resistors) are bonded to parts of the structure that deform when making the measurement.
• These strain gauges are typically used as elements in a Wheatstone bridge circuit, which is used to make the measurement.
• Strain gauges typically require an excitation voltage and provide output sensitivity proportional to that excitation.

Answer.1. 105 mV or 95 mV

Explanation:-

The limiting error at full scale is

δE = ± 5% of 100

δE = ±5mv

The actual value of limiting error can be expressed as

Ea = Es ± δE

Where

Ea = Actual value

Es = Specified or rated value

δE = Limiting error

Ea = 100 ± 5

Ea = 105 to 95 mV