Introduction to Metrology

Introduction to Metrology

Metrology:

Metrology has come from Greek word “metron” (measure) and –logy, meaning science of measurement. Engineering metrology is the measurement of dimensions: length, thickness, diameter, taper angle, flatness, profiles, etc.

Objectives of metrology:

The basic objective of metrology is to provide the required accuracy at minimum cost while taking measurements. Further objectives-

1) To maintain accuracy and precision at the time of inspection.

2) It can be used for selection of proper measuring instruments in industries.

3) It is used for minimizing cost of inspection.

4) To find/decide tolerances.

5) To achieve standardization.

6) To achieve interchangeability and selective assembly.

Terms used in measurement:

1. Accuracy

2. Precision

3. Calibration

4. Sensitivity

5. Readability.

6. Magnification

1. Accuracy:-

“Accuracy is the degree to which measured value agrees with the true value of quantity of interest.” Accuracy denotes the closeness of measured value with the true value. For example if an instrument like micrometer measures dimension of part as 25mm and if stated accuracy is +0.01mm then the true dimension could lie between 24.99mm and 25.01mm.

2. Precision .

“Precision means the degree of repeatability of measuring process.” Precision has no meaning for only one measurement but it exists when set of measurements is carried out. Suppose 10 measurements are performed. Then, precision means how well these 10 measured values agree with each other i.e. how nearer the readings of these 10 measurements are and how less is the variation between each value.

Distinction between Precision and Accuracy:

Accuracy is very often confused with precision, but both are different.

Consider gun shots on a dart

In figure (a), since the gunshots have hit the target region and all the gunshots are nearer to each other, they are precise and accurate. In figure (b), gunshots are far away from the target region, so the gunshots are inaccurate but they are nearer to each other, so they are precise.

· represents the location of average position of shots.

In fig (c), the gunshots are much apart from each other and nor is their average position within the target region, so the gunshots are neither accurate nor precise. In figure (d), suppose the gunshots are divided in a sample. Each sample consists of five gunshots. Then the gunshots are nearer to each other in a sample, hence gunshots are precise within a sample. But the samples are far away from each other, hence the gunshots on the whole are not precise nor are they accurate

3. Calibration .

Calibration is the procedure in which the measuring instrument is checked against a known standard.” For e.g. Calibrating a Thermometer involves checking its reading in ice (of pure water). Instruments must be calibrated often since they lose accuracy on frequent uses.

4. Sensitivity

Sensitivity is the ability of measuring device to detect small differences in quantity being measured. The indication of this characteristic is the smallest variation of the quantity that can be detected by the instrument. Sensitivity is also called Resolution. The greater the resolution of device, the smaller will be the things it can resolve and greater will be magnification required to expand these measurements upto the point where they can be observed by naked eye. Sensitivity is also related to measuring instrument/device.

5. Readability .

Readability is the susceptibility of a measuring device to have its indications converted to a meaningful number.” It refers to ease with which the readings of a measuring instrument can be read.

Characteristics/Requirements of precision measuring instrument:

1) High degree of sensitivity.

2) High degree of accuracy.

3) There should be no errors, in transferring accurate measurements.

4) It must give same results, any number of times, in hands of different operators ( i.e. good precision)

5) Materials from which it is made must be seasoned and stabilized, to reduce minimum error in accuracy due to changes that take place normally in metals.

6) Abbe’s principle of alignment should be followed in the design.

“It states that the axis or line of measurement should coincide with the axis of measuring instrument or line of measuring scale.”