Electrical & Electronic Measurements- 1 - GATE EE Engineering SSC JE Free

Precision type electrodynamometer is used in standardization process of potentiometer.

It is a transfer instrument. A transfer instrument is one which is calibrated with a d.c. source and used without any modifications for a.c. measurements. It has same accuracy for both D.C. and A.C. measurements.

Hence an electrodynamometer type of instruments are used as both standard and transfer instruments.

Instrument transformers are of two types.

1. Potential transformer

2. Current transformer

These are used to extend the range of induction type of instruments.

Reproducibility: It is the degree of closeness with which given value may be repeatedly measured. It may be specified in terms of units for a given period of time.

Perfect reproducibility means that the instrument has no drift.

No drift means that with a given input the measured values do not vary with time.

Given that,

Measured value = 25.34 W

Absolute error = -0.11 W

Absolute error = Measured value – true value

⇒ -0.11 = 25.34 – true value

⇒ 25.34 + 0.11 = 25.45 W

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

Gauge factor,

Where ε = strain = ΔL/L

The gauge factor can be written as

= Resistance change due to change of length + Resistance change due to change in area + Resistance change due to piezo resistive effect

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

The above equation is valid only when Piezo resistive effect that is change in resistivity due to strain is almost neglected.

For wire wound strain gauges, Piezo resistive effect is almost negligible.

Hysteresis error in PMMC Instrument: It can be reduced by providing Al frame in the moving system since AL is having a thin hysteresis loop. So that the difference between magnetic fields is less. We can’t eliminate hysteresis error but we can reduce.

Hysteresis error in MI Instrument: Hysteresis error is more in MI instruments. Due to hysteresis effect, the value flux density for the same current, while ascending and descending are different. While descending, the flux density is high and while ascending it is lesser. So we can reduce this error by using small iron parts which can demagnetize quickly.

Hysteresis error in EMMC Instrument: Hysteresis error is absent in EMMC instruments, since there is no iron related material in the moving system.

Order of Hysteresis error in descending order

MI > PMMC > EMMC

The accuracy of an instrument is crucial in obtaining reliable and precise measurements. To ensure accuracy, certain factors need to be considered. Among these factors, conformity and precision play significant roles. Let's explore these concepts further.

• Conformity refers to the extent to which an instrument adheres to a recognized standard or specification.


• An instrument that conforms to the established standards is more likely to provide accurate measurements.


• Conformity ensures that the instrument is calibrated correctly and operates within acceptable limits.


• It involves verifying and validating the instrument against known standards or reference materials.

• Precision refers to the degree of consistency or reproducibility in obtaining repeated measurements using the same instrument.


• A precise instrument will produce measurements that are close to each other, even if they are not close to the true value.


• Precision is determined by the instrument's ability to minimize random errors and variations in measurements.


• It can be assessed by calculating statistical parameters such as standard deviation or coefficient of variation.

• Conformity and precision are both essential for ensuring accuracy in measurements.


• Conformity ensures that the instrument is properly calibrated and aligned with recognized standards, reducing systematic errors.


• Precision ensures that the instrument produces consistent and reproducible measurements, minimizing random errors.


• While conformity addresses systematic errors, precision addresses random errors, collectively improving the overall accuracy of measurements.

In conclusion, both conformity and precision are necessary for achieving accuracy in instrument measurements. Conformity ensures that the instrument is calibrated and aligned with recognized standards, while precision ensures that measurements are consistent and reproducible. By considering both factors, the accuracy of the instrument can be maximized, leading to reliable and trustworthy measurements.

The accuracy of null type instruments is higher than that of deflection type. This is because the opposing effect is calibrated with the help of standards which have high degree of accuracy. Accuracy of deflection type instruments is dependent upon their calibration which depends upon the instrument constants which are normally not known to a high degree of accuracy.

In the null type of instruments, the measured quantity is balanced out. This means the detector has to cover a small range around the balance (null) point and therefore can be made highly sensitive. Also the detector need not be calibrated since it has only to detect the presence and direction of unbalance and not the magnitude of unbalance. A deflection type of instrument must be larger in size, more rugged, and thus less sensitive if it is to measure large magnitude of unknown quantity.

Deflection type of instruments have faster response than null type instruments.

The controlling torque is to control the pointer to a definite value which is proportional to 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 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. The following mechanism can be used for producing controlling torque.

The deflecting torque is used for deflection, the controlling torque acts opposite to the deflecting torque. So before coming to rest the pointer always oscillates due to inertia, so to bring the pointer rest within a short time we will use damping torque without effecting controlling torque (or) inertia.

Now IFSD 1=0.1mA ,IFSD2=0.05mA

So,voltmeter 2 reads the full scales current allowed is 0.05mA and and voltmeter one reads half of its full scale value so the total reading is 150V.

Given that,

I_m = 1 mA

R_m = 100 Ω

V_m = I_mR_m = 0.1 V

We know that, power factor is given by

Given that,

W₁ = +ve value

W₂ = -ve value

Here power factor angle varies from 60 degrees to 90 degrees.

Hence power factor varies from zero to 0.5 lagging

Electrodynamometer instrument has two fixed coils and one moving coil. For using this instrument as a wattmeter to measure the power, the fixed coils acts as a current coil and must be connected in series with a load. The moving coil acts as a pressure coil and must be connected across the supply terminals.

In the question given that, the current and potential coils of a dynamometer type wattmeter were accidentally interchanged while connecting. Hence total voltage applies across current coil which has very less resistance which leads to damage the current coil.

Moving coil permanent magnet instrument can be used as ammeter and voltmeter but not as wattmeter.

Electrodynamometer instrument has two fixed coils and one moving coil. For using this instrument as a wattmeter to measure the power, the fixed coils act as a current coil and must be connected in series with a load. The moving coil acts as a pressure coil and must be connected across the supply terminals.

Electrostatic watt-meters are used for measurement of small amount of power, practically when the voltage is high and power factor is low. This type of wattmeter is also used for measurement of dielectric loss of cables on alternating voltage and for calibration of watt-meters and energy meters.

According to Blondel’s theorem the no. of watt-meters to be required to measure the total power in n-phase system is either N (or) (N – 1). When separate neutral wire is available in the system the no. of watt-meters to be required is N. When the neutral wire is not available in the system, then the no. of watt-meters to be required is (N – 1). One line is acts as a common line for return path.

Hence minimum number of watt-meters required = 5

Given that,

Voltage (V) = 230 V

Current (I) = 5 A

Time (t) = 5 hours

Number of revolutions = 1940

Meter constant = 400

True Kwh = 200 × 10 = 2 kWh

Given that, disc rotates at 4.2 revolutions per minute.

Recorded revolutions = 4.2 × 60 × 10 = 2520

Meter constant = 1200 revolution/kWh

Measured value of kWh = 2520/1200 = 2.1 kW

Excess value of record = measured value – true value = 2.1 – 2 = 0.1 kW

Thermocouple has constant temperature at one end.

With a negative temperature coefficient thermistor, when the temperature increases the resistance will decreases.

A strain gauge is a sensor whose resistance varies with applied force. It converts force, pressure, tension, weight, etc., into a change in electrical resistance which can then be measured.

Linear variable differential transformer is a common type of electromechanical transducer that can convert the rectilinear motion of an object to which it is coupled mechanically into a corresponding electrical signal.

There must be synchronization between the sweep and the signal being measured. Synchronization is done to produce stationary pattern.

SYNC control in a CRO is used to lock the display of signal.

Transducers which require an external power source for their operation is called as a passive transducer. They produce an output signal in the form of some variation in resistance, capacitance or any other electrical parameter, which then has to be converted to an equivalent current or voltage signal. LVDT is an example of passive transducer. LVDT is used as an inductive transducer that convert motion into the electrical signal.