Principles and methods of measurement. Classification of methods of measurement. Electrical measuring instruments and their classification
Today, the techniques of measurement are of immense importance in most facets of human civilization. Present-day applications of measuring instruments can be classified into three major areas. The first of these is their use in regulating trade, and includes instruments which measure physical quantities such as length, volume and mass in terms of standard units.
The second area for the application of measuring instruments is in monitoring functions. These provide information which enables human beings to take some prescribed action accordingly. Whilst there are thus many uses of instrumentation in our normal domestic lives, the majority of monitoring functions exist to provide the information necessary to allow a human being to control some industrial operation or process. In a chemical process for instance, the progress of chemical reactions is indicated by the measurement of temperatures and pressures at various points, and such measurements allow the operator to take correct decisions regarding the electrical supply to heaters, cooling water flows, valve positions, etc. One other important use of monitoring instruments is in calibrating the instruments used in the automatic process control systems.
Use as part of automatic control systems forms the third area for the application of measurement systems. The characteristics of measuring instruments in such feedback control systems are of fundamental importance to the quality of control achieved. The accuracy and resolution with which an output variable of a process is controlled can never be better than the accuracy and resolution of the measuring instruments used. This is a very important principle, but one which is often inadequately discussed in many texts on automatic control systems. Such texts explore the theoretical aspects of control system design in considerable depth, but fail to give sufficient emphasis to the fact that all gain and phase margin performance calculations, etc., are entirely dependent on the quality of the process measurements obtained.
Here are "six guiding principles" from United Kingdom's National Physical Laboratory's website, concerning Principles of Measurement.
1.Make the right measurements
2.Use the right measurement tools
3.Use the right measurement procedures
4.Use the right people to do the measurement (this refers to training)
5.Review your measurement regularly
6.Demonstrate how consistent the measurement is
Measurement of any quantity involves two parameters: the magnitude of the value and unit of measurement. For instance, if we have to measure the temperature we can say it is 10 degree C. Here the value “10” is the magnitude and “C” which stands for “Celsius” is the unit of measurement. Similarly, we can say the height of wall is 5 meters, where “5” is the magnitude and “meters” is the unit of measurement.
There are two methods of measurement: 1) direct comparison with the standard, and 2) indirect comparison with the standard. Both the methods are discussed below:
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