Intruduction ot Electical Measuring Instruments

Introduction to Electrical Measuring Instruments

The instruments, which are used to measure electrical quantity such as current, voltage, power, energy, frequency, power factor etc. are called Electrical Measuring Instruments. These instruments are generally named after the electrical quantity to be measured. Thus the instruments which measure current voltage, power, energy and frequency are known as ammeter voltmeter, wattmeter, energy meter and frequency meter respectively.

Different Types of Measurements Instruments According to Principle of Working

Different Types of Measuring Instruments According to Principle of Working

The principle of working of all measuring instruments depends upon the various effects of electric current or voltage. The instruments generally classified according to effects utilized in their operation is given below in table

Definition of Indicating, Intergrating and Recording Instruments

Indicating, Intergrating and Recording Instruments

Measuring instruments (i.e. secondary instruments) may be classified according to their function as:

1.       Indicating instruments

2.       Integrating instruments

3.       Recording instruments

1.       Indicating instruments: “Indicating instruments are those which indicate the magnitude of an electrical quantity at the time when it is being measured.”

Their indications are given by a pointer moving over calibrated dial.

Example: Ordinary voltmeters, ammeters and wattmeter’s, etc.

Classification of Measuring Instruments

Classification of Measuring Instruments

Measuring instruments may be broadly classified as follows

1.       Absolute instruments

2.       Secondary instruments

(a)     Indicating instruments

(b)     Integrating instruments 

(c)     Recording  instruments

Absolute and secondary instruments

 Absolute instruments: “absolute instruments are those which gives the value of the quantity to be measured, in terms of the constants of the instruments and their deflection only”.

Such instruments do not require any previous calibration or comparison.

Example: Tangent galvanometer (is a very common example) and Rayleigh’s current balance instruments.

Uses: These instruments used as standardizing instruments in laboratories.

Secondary instruments: “secondary instruments are those in which the value of electrical quantity to be measured can be determined from the deflection of the instruments”.

Such instruments are need to be pre-calibrated by comparison with an absolute instrument. Without calibration, the deflection of such instruments is meaningless.

Example: An ammeter, voltmeter, a glass thermometer, and a pressure gauge instruments.

Uses: These instruments are generally used in practical work.

Introduction of Measuring Instruments

Introduction to measuring instruments

Measurements play an important role in achieving goals and objectives of Engineering. Measurements are basically a means of communication and are used by scientists for understanding natural phenomenon, by the society for transacting business and by the engineers for practical ends. The advancement of science and technology is dependent upon a parallel progress in measurement techniques. The reason for this are obvious. The importance of measurement is simply and eloquently expressed in the following statement of the famous physicist Lord Kelvin.

“I often say that when you can measure what you are speaking about and can express it in numbers, you know something about it; when you cannot express in it numbers your knowledge is of a meagre and unsatisfactory kind.”

Measurement is essentially a process in which the magnitude of a quantity is determined in comparison with another similar quantity.

Measurements involve the use of instruments as a physical means of determining quantities or variables. The instrument serves as an extension of human faculties and enables the man to determine the value of unknown quantity or variable which his unaided human faculties cannot measure, a measuring instrument exists to provide information about the physical value of some variable being measured. In simple cases, an instrument consists of a single unit which gives an output reading or signal according to the unknown variable applied to it. In more complex measurement situations, a measuring instrument may consists of transducing elements which convert the measurand to an analogous form. The analogous signal is then processed by some intermediate means and then fed to the end devices to present the results of the measurement for the purpose of display and/or control. Because of the modular nature of the elements within it, it is common to refer the measuring instruments as a measurement system.

The first instrument used by mankind were mechanical in nature and the principles on which these instruments worked are even in vogue today. The earliest scientific instruments used the same three essential elements as our modern instruments do. These elements as

(a)     A detector,

(b)     An intermediate transfer device, and

(c)     An indicator or recorder or a storage device.

Modern science and technology is associated with sophisticated methods of measurements and measuring instruments.

The history of development of instruments gives three phases of instruments, viz.:

1.       Mechanical instruments

2.       Electrical instruments, and

3.       Electronic instruments