Measuring and quantity like Length, Mass, Time, Speed, Velocity, Pressure, Temperature, Current, Voltage, Power etc. is nothing but the comparison of the quantity against some standards. In other words, any individual who enters the Electrical Engineering laboratory to conduct basic experiments will be using the Ammeter, Voltmeter & Wattmeter. It is always helpful to understand the basic principles & operation of some of these measuring instruments before actually using them.
Classification of Electrical Instruments:
- Indicating Secondary Instrument
- Recording Secondary Instruments
- Integrating Secondary Instruments
1. Absolute Instrument:
- The absolute instrument is one that gives the value of the quantity to be measured in terms of the constants of the instruments and their deflection only.
- It is not required to compare this reading with that of any standard instrument, nor does it need any calibration.
- Examples - Absolute electrometer, Rayleigh current balance, Tangent galvanometer.
2. Secondary instrument:
- The secondary instrument is one in which the value of Current, Voltage, or any other quantity to be measured can be determined from its deflection only.
- In other words, secondary instruments need calibration.
- The secondary instruments are normally used at large, whereas absolute instruments are rarely used except in standards laboratories & instruments.
- The secondary instruments are classified as:
- Indicating Secondary Instruments
- Recording Secondary Instruments
- Integrating Secondary Instruments
1. Indicating Instruments:
- Indicating Instruments are those that indicate the instantaneous value of the electrical quantity being measured.
- The indicated instruments indicate the R.M.S value of the electrical quantity.
- The indication is by means of the deflection of the pointer on a calibrated scale.
- Examples: Ammeter, voltmeter, Wattmeter, etc.
2. Recording Instruments:
- Recording instruments are those that give a continuous record of variations of the electrical quantity over a selected period of time.
- The moving system of the instrument carries an inked pen which rests tightly on a sheet of graph paper.
- Examples: Cardiogram, Chromatogram, X-Y plotter.
3. Integrating Instruments:
- The Integrating instruments are those that measure, register & integrate the electrical quantity (in Amp-hr) being measured over a pre-defined period of time.
- The integrated (added) value is displayed with the help of pointers or by means of an odometer array.
- Examples: Odometer, Energy meter etc.
Examples of Indicating, Recording, Integrating Instruments:
Indicating Instruments:
Recording Instruments:
- Chart Recorder: Records variations in electrical quantities (e.g., temperature chart recorder, pressure chart recorder).
- Oscillograph: Records waveforms of electrical signals (e.g., cathode-ray oscilloscope).
Integrating Instruments:
- Energy Meter: Measures and accumulates energy consumption over time (e.g., electromechanical energy meter, electronic energy meter).
- Watt-hour Meter: Measures and integrates energy consumption in watt-hours.
|
Basis
of Difference |
Indicating
Instruments |
Recording
Instruments |
Integrating
Instruments |
|
Definition |
Indicating
instruments indicate the value of an electrical quantity being
measured. |
Recording
instruments record the value of the quantity being measured on a
graph paper or digital recording system. |
Integrating
instruments measure and accumulates the total quantity over a
specific period. |
|
Measuring
Mechanism |
Examples:
Voltmeter, Ammeter, Wattmeter. |
Examples:
Chart recorder, Oscillograph. |
Examples:
Energy meter, Watt-hour meter. |
|
Need
of Observer |
Requires
an observer to monitor changes in the measured quantity. |
Records
readings automatically, so no need for an observer. |
Typically, it does not require an observer. |
|
Power
Consumption |
Consumes
less power compared to recording instruments. |
Consumes
comparatively more power due to continuous recording. |
Varies
based on the specific integrating instrument. |
|
Zero
Adjustment |
Requires
zero adjustment to ensure accurate readings. |
Does
not require zero adjustment. |
May
or may not require zero adjustment, depending on the design. |
|
Parallax
Error |
May
have parallax error due to the observer’s perspective. |
Not
applicable (no observer). |
Not
applicable (no observer). |
Frequently Asked Questions (FAQs) on Electrical Measuring Instruments
Q1. What is electrical measurement?
Electrical measurement is the process of determining the magnitude of an electrical or physical quantity such as current, voltage, power, energy, temperature, pressure, length, mass, or time by comparing it with a standard unit.
Q2. Why are electrical measuring instruments important?
Electrical measuring instruments are essential for accurately measuring, monitoring, and analyzing electrical quantities in laboratories, industries, and power systems to ensure safe and efficient operation.
Q3. How are electrical measuring instruments classified?
Electrical measuring instruments are broadly classified into two categories:
- Absolute Instruments
- Secondary Instruments
Q4. What is an absolute instrument?
An absolute instrument is one that gives the value of the quantity to be measured in terms of the instrument constants and deflection only. It does not require calibration or comparison with any standard instrument.
Q5. Give examples of absolute instruments.
Examples of absolute instruments include Tangent Galvanometer, Rayleigh Current Balance, and Absolute Electrometer.
Q6. What is a secondary instrument?
A secondary instrument is one in which the value of the electrical quantity such as current or voltage is obtained from the deflection of the pointer on a calibrated scale. These instruments require calibration.
Q7. Why are secondary instruments commonly used?
Secondary instruments are simple, accurate, and convenient to use. Therefore, they are widely used in practical applications, whereas absolute instruments are mostly used in standard laboratories.
Q8. How are secondary instruments further classified?
Secondary instruments are classified into the following three types:
- Indicating Instruments
- Recording Instruments
- Integrating Instruments
Q9. What are indicating instruments?
Indicating instruments are those instruments that indicate the instantaneous RMS value of the electrical quantity being measured by the deflection of a pointer over a calibrated scale.
Q10. Give examples of indicating instruments.
Common examples of indicating instruments are Ammeter, Voltmeter, and Wattmeter.
Q11. What are recording instruments?
Recording instruments continuously record the variation of an electrical quantity over a specific period of time on a graph paper or digital recording system.
Q12. Give examples of recording instruments.
Examples of recording instruments include Chart Recorder, Oscillograph, and X-Y Plotter.
Q13. What are integrating instruments?
Integrating instruments measure and accumulate the total value of an electrical quantity over a predefined period of time.
Q14. Give examples of integrating instruments.
Examples of integrating instruments are Energy Meter and Watt-hour Meter.
Q15. Which type of instrument is an energy meter?
An energy meter is an integrating secondary instrument because it measures and accumulates electrical energy consumption over time.
Q16. Do indicating instruments suffer from parallax error?
Yes, indicating instruments may suffer from parallax error due to the observer’s viewing angle while reading the pointer position.
Q17. Is an observer required for recording and integrating instruments?
No, recording and integrating instruments work automatically and do not require continuous observation.
Q18. Are electrical measuring instruments important for competitive exams?
Yes, questions related to classification, working principles, and examples of electrical measuring instruments are frequently asked in competitive exams such as SSC JE, RRB JE, and other electrical engineering examinations.