What is Measurement and Error?

In this tutorial, we are going to learn about what is Measurement and Error?

Measurement consists of an act or result of comparison between the unknown quantity and a predefined standard. The result in expressed in numerical values with a unit which identifies the characteristic or property measured for example elongation in measure in unit of length i.e. meters or its fraction. Temperature in measured in degrees Centigrade or Fahrenheit or Kelvin. For the result of measurement to be meaningful, there are two basic requirements viz.

  • The standard used for companion purpose must be accurately define and should be commonly accepted and
  • The apparatus used for comparison and the method adopted must be provable.

There are two major functions of all branches of engineering- 

  • Designing of equipment and processes
  • Proper operation and maintenance of equipment and processes.

Both these functions require measurements. Measurements give feedback information and play an important role is achieving goals and objective of engineering, measurements are essential both for design of equipment and proper operation and maintenance of equipment and processer.

There are basically two methods of measurement

  • Direct
  • Indirect

In direct methods the quantity being measured by comparing it with a standard. The measured quantity is expressed as a number and a unit. Physical quantities like length; Mass and time can be measured directly. Due to human factors, it is not easily possible to make very accurate measurements by direct methods. In engineering application, Measurement systems are used and these require indirect methods of measurements. As already discussed, a measurement system or instrumentation system consists of different modules viz. sensor module, processing module and indicating or recording module. Depending on the type of quantity used for measurement the instrument can be mechanical, electrical or electronic.

Mechanical Instruments-

These types of instruments are very reliable for static and stable conditions of measurement. They are unable to respond rapidly to measurement of dynamic and transient conditions. This is so because mechanical instruments have moving parts that are rigid, heavy and bulky and have a large mass. Mass presents inertia problems. As such, these instruments cannot faithfully follow rapid changes involved in dynamic measurements. Mechanical instruments for example, may be suited to measurements say 50 Hz voltage. Further, mechanical instruments are potential sources if noise which is an unwanted quantity and disturbs the accuracy and sensitivity of measurement,

Electrical Instruments-

Electrical methods of indicating the output of detector are more rapid than mechanical methods. An electrical system normally depends upon a mechanical meter movement as indicating device. This mechanical movement has some inertia and therefore, these metersmovements have some inertia and therefore, these meters have limited frequency response. Majority of industrial under have response time of a fraction of a second to tens of seconds. Even these are too slow for present day requirements of fast measurement.

Electronic Instruments-

Present day electronic instruments and their associated circuitry make it possible to step upthe response time and detect dynamic changes in certain parameters. Monitoring time of some ms and many times us is possible with electronic instruments. Electronic instrument generally use semiconductor devices where the movement involved in that of electrons withvery small inertia, enabling very small inertia, enabling very small time intervals for measurement. For example, a cathode ray oscilloscope (CRO) is capable of following  dynamic and transient changes of the order of a few nano seconds (10-9sec).Electronic instruments are more popular now –a- days due to their following characteristics:

  • Electronically controlled power supplies provide stable voltages for studies in the field of chemical reactions and nuclear instrumentation.
  • Improvements in design and manufacturing process of semiconductor materials make electronic instruments more reliable.
  • Very weak signals can be amplified by using preamplifiers and amplifiers and detected, making it possible to increase the sensitivity of electronic instruments.   This is particularly important in bio instrumentations where signals in the form bio instrumentations where signals in the form of bioelectric potentials of the order of lower than 1mv are encountered.
  • Electronic instruments offer the advantage of obtaining indication at a remote location.
  • Power amplification is possible in electronic instruments, thereby making it possible to present data to devices like stylus type recorders, galvanometers. CRO’s and magnetic tape recorders where higher power is needed for their operation.
  • Transducers of various types can be use to convert non – electrical quantities into electrical quantities in case of electronic instrumentation systems.
  • Electromagnetically produced signals such as radio, video and microwave can be detected very conveniently.
  • Electronic instruments are light, compact, have high degree of reliability and low power consumption.
  • Electronic instruments and associated apparatus make communication, use in made of air borne transmitters and receivers to interpret the signals.
  • Building up of analog and digital computers which are essential in today’ developments in science are made possible with electronic instruments only. To summarize, electronic instruments have :
  • Higher sensitivity
  • A faster response time
  • A greater flexibility
  • Lower weight
  • Lower power consumption and
  • A higher degree of reliability as compared to their mechanical or purely electrical counterparts.

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