In this topic, you study the definition of measurement in science.
By measurement of a quantity. we mean “an act of comparison” between the quantity to be measured and a predefined standard, e.g., when we say a “5 meter piece”, we mean that the piece is 5 times of 1 meter standard “kept in the laboratory”. Measurements help in the proper and economical design of equipment and processes. Various “measuring instruments” are used for measurement of various quantities. We always measure quantities “through comparison”.
TERMS RELATED TO MEASUREMENTS
Important terms related to measurements are defined below.
Instrument: Is a device to measure value or magnitude of a quantity or a variable.
True Value: The true value of a quantity under measurement may be defined as the “average of an infinite no. of measured values when the deviation tends to zero”. It is not possible to measure exact true value of the quantity.
Error: The difference between the “measured value” and “true value” of a quantity is called the “Error” which can be represented by the following expression:
Accuracy: Accuracy means “conformity to truth”. The accuracy is the “closeness Wilh which an instrument reading approaches the true value of the quantity being measured”. It is expressed as a percentage of true value.
Precision: By precision, we mean that if a quantity is measured no. of times by an instrument it always gives the same reading: i.e., there is no drift and the measured values do not vary with time. In other words precision is the measure of the degree to which successive measurements differ from one another. An instrument may possess a high degree of “precision”, however the readings may not be “accurate”.
Sensitivity: Is the “ratio of output or response of the instrument to a change of input or a measured variable”
OR
The sensitivity of an instrument is the “ratio of the magnitude of the output signal to the magnitude of the input signal’.
Efficiency: The efficiency of an instrument is the “ratio of the measured quantity at full scale to the input power taken by the instrument”.
Resolution or Discrimination: This is the smallest input of the quantity under measurement, which can be detected by an instrument. OR, “this is the smallest change in the measured value to which the instrument will respond.
Torque Weight (T/W) Ratio: This is the ratio of deflecting torque produced on the moving system and weight of the moving system. For accuracy this ratio
Fidelity: If an instrument produces output which is an exact “replica” of the input, it is said to possess 100% fidelity e.g., if a sine wave is fed to a CRO. the output on its screen should be an exact sine wave and there should be no distortion.
Response Speed & Response Time: By response speed, we mean the rapidity, with which an instrument responds to the changes in the input under measurements. By response time, we mean the time taken by an instrument to respond to the changes in the input.