Subject:- Measurement and Metrology:-Standard in Measurement ,Method of Measurement, Terms related to measuring instruments.

 Measurement:-

                        Measurement is a process of comparison between a pre-defined standard and an unknown physical quantity with the same unit.

Metrology:-

                   The word, metrology consists, of two words, one is metro which indicates metric which means measurement and the other is logy which indicates technology, which means science behind the measurement.

The essential character of Standard:-

 1. it should be easily readable and understandable.

 2. it should be properly defined and universally acceptable. 

 3. it should be by the size of the quantity to be measured.

 4. it should not change its character i.e.physical dimension concerning changes in place, time, and environmental condition.

Standard in Measurement

 →Standards are basically the references used in measurement from which we derive the value of an unknown physical quantity by comparing it with it.

→ Standards are of three types:-Line standard, End standard, and Wavelength standard.

 I. Line standard:-

                       When the distance between two engraved lines is used to measure the length, it is called line standard or line measurement. The most common examples are yards and meters. The rule with divisions marked with lines is widely used.

→ Accuracy is upto ±  0.2 mm

→ Measurement is quick and easy.

→ Scales and their markings are not subjected to wear.

→ They are simple in construction and less costly.

II. End standard:-

                            When the distance between two flat parallel surfaces is considered a measure of length, it is known as end standard or end measurement. The end faces of the end standards are hardened to reduce wear and lapped flat and parallel to a very high degree of accuracy. 

The end standards are extensively used for precision measurement in workshops and laboratories. The most common examples are measurements using slip gauges, end bars, ends of micrometer anvils, vernier calipers, etc.

→ Accuracy is upto ± 0.1mm.

→Measurement is time-consuming.

→Wear takes place on measuring surfaces.

→They are complex in operation and are costly.

Method of Measurement

There are basically two methods of measurement.

 I . Direct Measurement

 II. Indirect Measurement

Direct Measurement Method:-

                                        In this method, the quantity to be measured is directly compared with the primary or secondary standard. Scales, vernier calipers, 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.

→ It is also known as an absolute measurement.

Indirect Measurement 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. 

→ It is also known as comparative measurement.

→ Some examples of indirect measurement are angle measurement using a sine bar, measurement of strain induced in a bar due to the applied force, determination of the effective diameter of a screw thread, etc.

  Terms related to measuring instruments:-

 I. Accuracy:-

                    The dimension of the product produced after manufacturing is called standard or True Value, while the dimension of the product measured by the instrument is called measured value or actual value.

→ Thus the closeness of actual value /measured value with true value is called Accuracy.

→ More will be the closeness, the more accurate will be the instrument.

→ It is the ability of an instrument to produce correct results.

II. Precision:- 

                    It is defined as the repeatability of measuring instruments to obtain a particular value while taking a set of measurements. If for a particular value of reading, measurement is taken several times and the measured value thus obtained is varied by a very small amount. Then mean value of a set of values is called precise and the measurement is called precise measurement.

 III. Sensitivity:- 

                           It is defined as the ratio of changes in the output of the measuring instrument to the changes in the value of the measured quantity.

→It denotes the ability of measuring instruments to detect small variations in quantity to be measured.

→ Sensitivity is represented by S.

IV. Repeatability:-

                    It is the ability of measuring instruments to give almost the same value of reading every time when the measurement is repeated by the same operator and the same condition.

V. Reproducibility:- 

                            It is the ability of a measuring instrument to continuously produce a particular value of reading every time when the measurement is repeated by "different operators and different conditions.

VI. Range:-

                  The minimum to maximum value on a scale of measuring instrument for which the measurement is designed to measure any quantity.

VII. Hysteresis:- 

                     Hysteresis means basically the time lag between input given to the measuring instrument and output obtained from the measuring instrument.

VIII. Threshold:- 

                      The minimum value of input after which the instrument gives the output reading is called threshold.

 IX. Calibration:-

                         As we know that due to several uses, the parts of the measuring instrument gets aligned from its exact position due to which the instrument starts giving incorrect reading.

→ Therefore the measuring instrument is regularly checked by comparing it with predefined standards. if the instrument gives incorrect reading this indicates parts of the instrument get misaligned.

→Therefore we properly aligned of parts or components of the instrument, this process of rechecking and realignment of the measuring instrument is called calibration.