GENERAL MEASUREMENT CONCEPTS
We know that the primary objective of measurement in industrial inspection is to determine the quality of the component manufactured. Different quality requirements, such as permissible tolerance limits, form, surface finish, size, and flatness, have to be considered to check the conformity of the component to the quality specifications. In order to realize this, quantitative information of a physical object or process has to be acquired by comparison with a reference. The three basic elements of measurements (schematically shown in Fig. 1.3),
which are of significance, are the following:
1. Measurand, a physical quantity such as length, weight, and angle to be measured.
2. Comparator, to compare the measurand (physical quantity) with a known standard (reference) for evaluation.
3. Reference, the physical quantity or property to which quantitative comparisons are to be made, which is internationally accepted.
All these three elements would be considered to explain the direct measurement using a calibrated fixed reference. In order to determine the length (a physical quantity called measurand) of the component, measurement is carried out by comparing it with a steel scale (a known standard).
Calibration of Measuring Instruments
It is essential that the equipment/instrument used to measure a given physical quantity is validated. The process of validation of the measurements to ascertain whether the given physical quantity conforms to the original/national standard of measurement is known as traceability of the standard. One of the principal objectives of metrology and measurements is to analyse the uncertainty of individual measurements, the efforts made to validate each measurement with a given equipment/instrument, and the data obtained from it. It is essentialthat traceability (which is often performed by a calibration laboratory having conformity with a proven quality system with such standards) should disseminate to the consumers. Calibration is a means of achieving traceability. One of the essential aspects of metrology is that the results of measurements obtained should be meaningful.
To accomplish this, calibration of any measuring system/instrument is very essential. Calibration is the procedure used to establish a relationship between the values of the quantities indicated by the measuring instrument and the corresponding values realized by standards under specified conditions. It refers to the process of establishing the characteristic relationship between the values of the physical quantity applied to the instrument and the corresponding positions of the index, or creating a chart of quantities being measured versus readings of the instrument. If the instrument has an arbitrary scale, the indication has to be multiplied by a factor to obtain the nominal value of the quantity measured, which is referred to as scale factor. If the values of the variable involved remain constant (not time dependent) while calibrating a given instrument, this type of calibration is known as static calibration, whereas if the value is time dependent or time-based information is required, it is called dynamic calibration. The relationship between an input of known dynamic behaviour and the measurement system output is determined by dynamic calibration.
The main objective of all calibration activities is to ensure that the measuring instrument will function to realize its accuracy objectives. General calibration requirements of the measuring systems are as follows: (a) accepting calibration of the new system, (b) ensuring traceability of standards for the unit of measurement under consideration, and (c) carrying out calibration of measurement periodically, depending on the usage or when it is used after storage. Calibration is achieved by comparing the measuring instrument with the following: (a) a primary standard, (b) a known source of input, and (c) a secondary standard that possesses a higher accuracy than the instrument to be calibrated. During calibration, the dimensions and tolerances of the gauge or accuracy of the measuring instrument is checked by comparing it with a standard instrument or gauge of known accuracy. If deviations are detected, suitable adjustments are made in the instrument to ensure an acceptable level of accuracy. The limiting factor of the calibration process is repeatability, because it is the only characteristic error that cannot be calibrated out of the measuring system and hence the overall measurement accuracy is curtailed.
Thus, repeatability could also be termed as the minimum uncertainty that exists between a measurand and a standard. Conditions that exist during calibration of the instrument should be similar to the conditions under which actual measurements are made. The standard that is used for calibration purpose should normally be one order of magnitude more accurate than the instrument to be calibrated. When it is intended to achieve greater accuracy, it becomes imperative to know all the sources of errors so that they can be evaluated and controlled.
