Basic Characteristics of an Instrument MCQ Quiz - Objective Question with Answer for Basic Characteristics of an Instrument - Download Free PDF

Explanation

A lux meter is used to measure the illumination level (in lux) of a lighting system.

During an energy audit, it is essential to assess whether the lighting system is providing adequate illumination for the energy consumed. This helps in optimizing lighting efficiency and identifying potential energy savings.

Applications of other equipments:

• Ohm meter: Measures electrical resistance (ohms).
• Frequency meter: Measures the frequency of AC power supply (Hz).
• Energy meter: Measures total electrical energy consumption (kWh).

Solution:

For a moving coil galvanometer, the current sensitivity is given by:

θ = (BNA) / k

Where:

• B = magnetic field
• N = number of turns
• A = area of the coil
• k = constant

The current sensitivity is proportional to N (number of turns), so if the number of turns is doubled, the current sensitivity is also doubled.

For voltage sensitivity, the formula is:

V = (BNA) / (kR)

Since the number of turns (N) is doubled, R (resistance) is also doubled. However, as a result, the voltage sensitivity remains unchanged because the factor of doubling the number of turns cancels out with the doubling of the resistance.

Hence, the voltage sensitivity will remain unchanged (Option 4).

Primary Instruments and Their Functions:

Primary instruments, also known as measuring instruments, are devices used to measure physical quantities. These instruments are the first in the chain of measurement and are responsible for detecting the quantity to be measured. Below is an explanation of why the magnetic needle is considered a primary instrument:

• Magnetic Needle: This is a primary instrument used for detecting the direction of the magnetic field. It is a simple device that aligns itself with the Earth's magnetic field, thereby indicating the direction of the magnetic north and south poles. It is fundamental in various applications, including navigation (as a compass) and in certain types of analog measuring instruments.

Explanation of Other Options:

• Ammeter: This is a secondary instrument used to measure electric current in a circuit. It converts the detected current into a readable format but relies on primary instruments to function correctly.
• Voltmeter: Similar to an ammeter, a voltmeter is a secondary instrument used to measure electric potential difference between two points in a circuit. It also depends on primary instruments for accurate functioning.
• Kilowatt Hour Meter: This is an energy meter used to measure the amount of electrical energy consumed over time. It is a secondary instrument used in electrical engineering and relies on primary measurements to compute energy consumption.

Conclusion:

From the above explanation, it is evident that a magnetic needle is a primary instrument because it directly detects and indicates the direction of the magnetic field without requiring additional measurements or conversions. This fundamental role distinguishes it from secondary instruments like ammeters, voltmeters, and kilowatt-hour meters, which process and convert primary measurements into a readable format.

Integrating Instrument:

• An integrating instrument is a type of electrical measuring device that calculates the total quantity of electrical energy used over a specific period. These instruments sum up the electrical quantity continuously over time.
• One of the most common examples of an integrating instrument is the energy meter, which measures the total electrical energy consumed by a load in kilowatt-hours (kWh).
• The primary function of integrating instruments is to monitor and record the cumulative value of an electrical quantity such as electric charge, electric energy, or ampere-hours.
• These instruments are essential in various applications, including billing purposes in residential, commercial, and industrial settings.
• Integrating instruments work on the principle of continuous integration of the electrical quantity over time, thus providing an accurate measure of the total consumption or generation.

Examples of Integrating Instruments:

• Energy Meter: Measures the total electrical energy consumed by a load in kilowatt-hours (kWh).
• Watt-Hour Meter: Records the total amount of electrical energy (in watt-hours) used over a period.
• Ampere-Hour Meter: Measures the total charge flow in ampere-hours, commonly used in battery monitoring.

Integrating instruments are crucial for tracking energy usage, enabling accurate billing, and ensuring efficient energy management. They provide valuable data for both consumers and utility providers, helping to optimize energy consumption and reduce costs.

Effect of Electric Current on Electric Measuring Instruments:

Electric measuring instruments are designed to measure various electrical quantities such as current, voltage, resistance, etc. The working principle of these instruments is based on one or more effects of electric current. The three primary effects of electric current that are utilized in electric measuring instruments are:

• Heating Effect: This effect is based on the principle that when an electric current flows through a conductor, it produces heat. This effect is used in devices such as electric fuses and some types of ammeters and voltmeters.
• Lighting Effect: This effect occurs when electric current passes through a medium, causing it to emit light. This effect is utilized in devices like light bulbs and some types of indicators but is not typically used in measuring instruments.
• Magnetic Effect: When an electric current flows through a conductor, it generates a magnetic field around it. This effect is the basis for most electric measuring instruments, such as galvanometers, ammeters, and voltmeters. These instruments use the interaction between magnetic fields and electric currents to measure electrical quantities.

Given that electric measuring instruments can work on more than one effect of electric current, the correct answer is:

All of the Above:

• Electric measuring instruments can utilize the heating effect, magnetic effect, and even other effects depending on the design and application of the instrument.

Therefore, the correct option is option 4) All of the above.

concept:

Sensitivity(S) of instrument is defined as change in output with respect to change in input

S = \(\frac{dV_0}{dV_i}=\frac{Δ V_0}{Δ V_i}\)

Where

V₀ = Output voltage of instrument

V_i = Input voltage of instrument

Application:

Given:

ΔV₀ = 0.18 V

ΔV_i =0.2 V

S = \(\frac{0.18}{0.2}\) = 0.9

Important Points

•  For any Instrument, Even for a small change in input there should be a large change in output i.e sensitivity should be higher
•  For any disturbances in the system the output should not be affected much i.e sensitivity should be lower.

Concept: 

Precision: A measurement of consistency or repeatability of measurement i.e., successive reading does not differ.

The average of the measured value is given by,

\(\overline {{X_n}} = \sum \frac{{{X_n}}}{n}\)

Where X_n is the recorded readings, n is no. of recorded readings.

Precision for particular reading is given by,

Precision \( = 1 - \left| {\frac{{Respective\;Reading - \overline {{X_n}} }}{{\overline {{X_n}} }}} \right|\)

Calculation:

The average measurement value

\(\overline {{X_n}} = \frac{{98 + 101 + 99 + 100 + 102}}{5}\)

\(= \frac{{500}}{5} = 100\)

Given third measurement reading  = 99

The 3rd reading precision is given as

\(= 1 - \left| {\frac{{99 - 100}}{{100}}} \right|\)

\( = 1 - \frac{1}{{100}} = 0.99\)

The linear variable differential transformer (LVDT) is a type of electrical transformer used for measuring linear displacement (position).

Sensitivity is defined as the ratio between the output signal and the measured property.

Here the output of LVDT is a voltage signal and the measured quantity is displacement.

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

Sensitivity: 

• It 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.

\(Sensitivity\left( S \right) = \frac{{change{\rm{ }}in{\rm{ }}output}}{{unit{\rm{ }}change{\rm{ }}in{\rm{ }}input}} = \frac{{{\rm{\Delta }}output}}{{{\rm{\Delta input}}}}\)

• The sensitivity of an instrument is determined by dividing the sum of the resistance of the meter (Rm) and the series resistance (Rs), by the full-scale reading in volts.

Mathematically, sensitivity is expressed as:

\(Sensitivity=\frac{R_m+R_s}{E}\)

Expressing the above expression in units, we get:

\(Sensitivity=\frac{Ohms}{Volt}\)

Unit of instrument sensitivity is expressed in Ohm/Volt

Important Points

Sensitivity is also expressed as:

\(Sensitivity=\frac{1}{ampere}\)

So, sensitivity is said to be equal to the reciprocal of the full-scale deflection current.

Concept:

Sensitivity or Figure of merit:

Sensitivity is the measure that indicates the relation between change in output readings for change in input quantity for an instrument.

Sensitivity (s) = (change in output)/(change in input) = tanθ 

• For an instrument, we always prefer s = 1, so that the change in output is directly proportional to the change in input.
• If s<1 or s>1, the instrument indicates the non-linear reading.
• So, sensitivity plays a crucial role in designing the type of scale at the time of designing.
• In order to reduce the loading effect or error, high sensitive instruments are preferred.

Calculation:

Let's assume output as resistance and input as temperature, so that for change in temperature there will be some change in resistance.

From the above table, we can observe that

For every 50°C temperature (input) change,  there is a 5 Ω change in resistance(output).

Then, the sensitivity is given by

S = (5 / 50) = .1 Ω/°C

Precision: It is the measure of reproducibility i.e., given a fixed value of a quantity, precision is a measure of the degree of agreement within a group of measurements.

• Precision of an instrument does not guarantees accuracy
• An instrument with more significant figures has more precision
• Deflection factor is reciprocal of sensitivity

Additional Information

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

Accuracy: It is the degree of closeness with which the reading approaches the true value of the quantity to be measured.

Explanation:

Accuracy:

Accuracy is how close a measured value is to the actual (true) value.

Accuracy has two definitions:

1) More commonly, it is a description of systematic errors, a measure of statistical bias; low accuracy causes a difference between a result and a "true" value. ISO calls this trueness.

2) Alternatively, ISO defines accuracy as describing a combination of both types of observational error above (random and systematic), so high accuracy requires both high precision and high trueness.

Precision:

1) Precision is how close the measured values are to each other.

2) Precision is a description of random errors, a measure of statistical variability.

 In the first, a more common definition of "accuracy" above, the two concepts are independent of each other, so a particular set of data can be said to be either accurate, or precise, or both, or neither.

The below diagram shows the difference between Accurate and Precise systems:

Concept:

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

 \(R= \frac{1}{{{{10}^N}}} \times range\;of\;voltage\)

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

 \(= \frac{1}{{{{10}^4}}} \times 10 = 1 \ mV\)

Accuracy:

• It is the; closeness to the true value; closeness with which an instrument reading approaches the true or accepted value of the variable (quantity) being measured.
• It is considered to be an indicator of the total error in the measurement without looking into the sources of errors.

Precision:

• The most repeatable value (or) the reproducible value out of the set of records is known as precision.
• It is the measure of consistency. It gives the closeness of individual measured value to the average of all measured values.

For accuracy of the instrument, both conformity and precision are necessary.

Concept:

Total internal resistance of a voltmeter is given by

\({R_m} = \frac{{{V_{fsd}}}}{{{I_{fsd}}}}\)

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

R_m = full scale range of voltmeter × sensitivity

R_v = V_fsd × S

Calculation:

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

Sensitivity (S) = 20 kΩ/V