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Measurements characteristics

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الكلية كلية الهندسة/المسيب     القسم هندسة الطاقة     المرحلة 4
أستاذ المادة حامد حسين هدوان الخفاجي       22/10/2017 03:03:11
Measurements are the basic means of acquiring knowledge about the parameters and variables involved in the operation of a physical system. Measurement generally involves using an instrument as a physical means of determining a quantity or variable. An instrument or a measuring instrument is, therefore, defined as a device for determining the value or magnitude of a quantity or variable. The electrical measuring instrument, as its name implies, is based on electrical principles for its measurement function.
True or Expected Value: The true or expected value of a quantity to be measured may be defined as the average of an infinite number of measured values when the average deviation due to the various contributing factors tends to zero. It also refers to a value of the quantity under consideration that would be obtained by a method (known as exemplar method) agreed upon by experts. In other words, it is the most probable value that calculations indicate and one should expect to measure.
Note that the value of the unknown obtained by making use of primary standards and measuring instruments considered its true value.
CLASSIFICATION OF MEASURING INSTRUMENTS
First: Measuring instruments can be classified into two groups:
(a) Absolute (or primary) instruments.
(b) Secondary instruments.
- Absolute Instruments
• These instruments give the value of the electrical quantity in terms of absolute quantities (or some constants) of the instruments and their deflections.
• In this type of instruments no calibration or comparison with other instruments is necessary.
• They are generally not used in laboratories and are seldom used in practice by electricians and engineers. They are mostly used as means of standard measurements and are maintained by national laboratories and similar institutions.
• Examples of absolute instruments are:
* Tangent galvanometer
* Raleigh current balance
* Absolute electrometer.
- Secondary Instruments
• They are direct reading instruments. The quantity to be measured by these instruments can be determined from the deflection of the instruments.
• They are often calibrated by comparing them either with some absolute instruments or with those, which have already been calibrated.
• The deflections obtained with secondary instruments will be meaningless until it is not calibrated.
• These instruments are used in general for all laboratory purposes.
• Some of the very widely used secondary instruments are ammeters, voltmeter, wattmeter, energy meter (watt-hour meter), ampere-hour meters etc.


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