METHODS OF MEASUREMENT
When precision measurements are made to determine the values of a physical variable, different methods of measurements are employed. Measurements are performed to determine the magnitude of the value and the unit of the quantity under consideration. For instance, the length of a rod is 3m, where the number, 3, indicates the magnitude and the unit of measurement is metre. The choice of the method of measurement depends on the required accuracy and the amount of permissible error. Irrespective of the method used, the primary objective is to minimize the uncertainty associated with measurement. The common methods employed for making measurements are as follows:
Direct method In this method, the quantity to be measured is directly compared with the primary or secondary standard. Scales, vernier callipers, micrometers, bevel protractors, etc., are used in the direct method. This method is widely employed in the production field. In the direct method, a very slight difference exists between the actual and the measured values of the quantity. This difference occurs because of the limitation of the human being performing the measurement.
Indirect method In this method, the value of a quantity is obtained by measuring other quantities that are functionally related to the required value. Measurement of the quantity is carried out directly and then the value is determined by using a mathematical relationship.
Some examples of indirect measurement are angle measurement using sine bar, measurement of strain induced in a bar due to the applied force, determination of effective diameter of a screw thread, etc.
Fundamental or absolute method In this case, the measurement is based on the measurements of base quantities used to define the quantity. The quantity under consideration is directly measured, and is then linked with the definition of that quantity.
Comparative method In this method, as the name suggests, the quantity to be measured is compared with the known value of the same quantity or any other quantity practically related to it. The quantity is compared with the master gauge and only the deviations from the master gauge are recorded after comparison. The most common examples are comparators, dial indicators, etc.
Transposition method This method involves making the measurement by direct comparison, wherein the quantity to be measured (V) is initially balanced by a known value (X) of the same quantity; next, X is replaced by the quantity to be measured and balanced again by another known value (Y). If the quantity to be measured is equal to both X and Y, then it is equal to V X = Y
An example of this method is the determination of mass by balancing methods and known weights.
Coincidence method This is a differential method of measurement wherein a very minute difference between the quantity to be measured and the reference is determined by careful observation of the coincidence of certain lines and signals. Measurements on vernier calliper and micrometer are examples of this method.
Deflection method This method involves the indication of the value of the quantity to be measured directly by deflection of a pointer on a calibrated scale. Pressure measurement is an example of this method.
Complementary method The value of the quantity to be measured is combined with a known value of the same quantity. The combination is so adjusted that the sum of these two values is equal to the predetermined comparison value. An example of this method is determination of the volume of a solid by liquid displacement.
Null measurement method In this method, the difference between the value of the quantity to be measured and the known value of the same quantity with which comparison is to be made is brought to zero.
Substitution method It is a direct comparison method. This method involves the replacement of the value of the quantity to be measured with a known value of the same quantity, so selected that the effects produced in the indicating device by these two values are the same. The Borda method of determining mass is an example of this method.
Contact method In this method, the surface to be measured is touched by the sensor or measuring tip of the instrument. Care needs to be taken to provide constant contact pressure in order to avoid errors due to excess constant pressure. Examples of this method include measurements using micrometer, vernier calliper, and dial indicator.
Contactless method As the name indicates, there is no direct contact with the surface to be measured. Examples of this method include the use of optical instruments, tool maker’s microscope, and profile projector.
Composite method The actual contour of a component to be checked is compared with its maximum and minimum tolerance limits. Cumulative errors of the interconnected elements of the component, which are controlled through a combined tolerance, can be checked by this method. This method is very reliable to ensure interchangeability and is usually effected through the use of composite GO gauges. The use of a GO screw plug gauge to check the thread of a nut is an example of this method.
