Basic concepts of measurement & Classification of Electrical Instrument

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:

1. Absolute Instruments

2. Secondary instruments

1. Indicating Secondary Instrument
2. Recording Secondary Instruments
3. Integrating Secondary Instruments

1. Absolute Instrument:

1. The absolute instrument is those which gives the value of the quantity to be measured in terms of the constants of the instruments and their deflection only.
2. It is not required to compare this reading with that of any standard instrument, neither it needs any calibration.
3. Examples - Absolute electrometer, Rayleigh current balance, Tangent galvanometer.

2. Secondary instrument:

• The secondary instrument is those 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:

1. Indicating Secondary Instruments
2. Recording Secondary Instruments
3. Integrating Secondary Instruments

1. Indicating Instruments:

• Indicating Instruments are those which indicate the instantaneous value of the electrical 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 which 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 graph chart.
• 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:

• Voltmeter: Measures voltage (e.g., digital voltmeter, analog voltmeter).
• Ammeter: Measures current (e.g., analog ammeter).
• Wattmeter: Measures power (active power) in watts.

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 accumulate 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 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 observer’s perspective.	Not applicable (no observer).	Not applicable (no observer).