We may explain the meaning and importance of uncertainty in measurement in several ways. It is widely recognized that the value of a measured quantity is determined within a certain range. The range depends upon instruments, quality of measurements taken and the confidence level at which the final result is to be stated. Leaving aside the formal definition, half of this range may be called uncertainty of measurement. The uncertainty in a measurement result will dependupon all the three aforesaid elements. Therefore, quantifying a measurable quantity through any measurement process is meaningful only if the value of the quantity measured is given with a proper unit of measurement and is accompanied by an overall uncertainty in measurement.
The quality of a measurement may also be characterized by the semi-range in which the measured value is expected to lie. Incidentally, the word measurement should be understood to mean both a process and the output of that process. The measurements are carried out at different levels. The measurements in industry have assumed greater significance in view of the fact that measurements provide the very
basis of all control actions.
Importance of accurate measurement in science may also be illustrated by the following examples.
Discovery of Inert Gases
The density of nitrogen gas was measured, taking samples of nitrogen from air and the chemical reaction in which pure nitrogen was produced. The density of nitrogen sample taken from atmosphere after removing oxygen and CO2 was found to be
more than that of nitrogen through a chemical reaction. The persistence and significant difference of the two values of density of nitrogen made us reach the conclusion on the existence of inert gases such as Helium, and Argon present in atmosphere.
Correction in Composition of Air
More recently, the composition of air has been revised as a result of precise measurements of air density. The density of air used to be calculated by using the CIPM formula [1, 2] expressing density of moist air in terms of pressure, temperature, humidity and involving the composition of air and its molar mass. The density of moist air is calculated by CIPM formula by measurements of pressure, temperature and humidity with reasonable small uncertainty. The density of air has also been measured by gravimetric (artefacts) method [3]. The values obtained by the two methods, though agreed very well within any one of the methods, did not agree with each other. The relative discrepancy was 6:4 10:5 [4]. Density obtained
by gravimetric method was found to be more than that obtained by CIPM formula. Independent analysis of air samples through spectroscopic means [5] suggested the change in molar fraction of Argon. The CIPM in 2008 [6] subsequently changed the molar fraction of Argon from 0.0917 to 0.09332. This is an example of the benefits of high precision measurements.
Meaning of Quantity Being Exact
he value of velocity of light in vacuum is taken as exact by the international agreement. However, it does not mean that there was no uncertainty in its measurement but by assigning a specific value to the velocity of light in vacuum, we have assigned a new value to the metre. Similar is the case of the value of permeability of free space or magnetic constant, which is taken as
. This value comes from the definition of the unit of electric current – the ampere, through a specific theoretical formula. The force F acting per unit length on the two current-carrying parallel wires is given as
International Agreement with Uncertainty
Having understood, in 1970s, the benefits of precise measurements along with uncertainty, all metrology laboratories recognized the fact that each measurement result is to be associated with an uncertainty declaration. As a result of which each laboratory started giving the result along with an uncertainty. But there was no uniformity in either achieving or expressing the uncertainty.
With the initiation of globalization, more and more national metrology laboratories started sharing their results of measurements. For better and easy understanding, the results and in assigning some mean value to the results of various laboratories, it was necessary that all laboratories express the measurement results in a uniform way.
Initiation by BIPM
In 1978, the International Committee on Weights and Measures (CIPM) – world’s highest authority in metrology, requested the International Bureau of Weights and Measures (BIPM), in conjunction with a few national metrology laboratories and other international bodies interested in metrology to look into this problem.
The BIPM prepared a detailed questionnaire covering the issues involved and distributed that to 32 national metrology laboratories and to five international organizations known to have interest in the subject. Almost all agreed that it was necessary to arrive at an internationally accepted procedure for expressing measurement uncertainty and also a uniform method of combining uncertainty
components into a single total uncertainty; however, a consensus was not apparent on the method of combining several components. Eleven national laboratories send their experts to the meeting convened by the BIPM. This working group developed a recommendation on statement of uncertainty in 1980, which CIPM adopted
in 1981 and reaffirmed in 1986.
Source : measurement uncertainty
