Introduction to Metrology


Introduction, Objective, Measurement, Accuracy, Precision, Absolute error, Static errors, Loading errors, Dynamic error, Loading error, Dynamic error, Ambient error, Avoidable errors, Stylus pressure, Random error in Metrology

Introduction to Metrology

Engineering Metrology
           Metrology is the science of measurement. Metrology may be divided depending upon the quantity to be measured like metrology of length, metrology of time. But for engineering purposes, it is restricted to measurement of length and angles and other qualities which are expressed in linear or angular terms. In the broader sense it is not limited to length measurement but is also concerned with industrial inspection and its various techniques.

Metrology is mainly concerned with;
Establishing the units of measurements, ensuring the uniformity of measurements
Developing methods of measurement.
Errors of measurement.
Accuracy of measuring instruments and their care.
Industrial inspection and its various techniques.

Necessity and importance of metrology:
In design, design engineer should not only check his design from the point of view of the strength or economical production, but he should also keep in mind how the dimensions specified can be checked or measured.
Higher productivity and accuracy can be achieved.
Improve the measuring accuracy, dimensional and geometrical accuracies of the product.
Proper gauges should be designed and used for rapid and effective inspection.
Also automation and automatic control, which are the modern trends for future developments, are based on measurement.
We can also use digital instruments for inspection.

Objectives of metrology:
The basic objective of a measurement is to provide the required accuracy at a minimum cost.
Complete evaluation of newly developed products.
Determination of Process Capabilities.
Determination of the measuring instrument capabilities and ensure that they are quite sufficient for their respective measurements.
Minimising the cost of inspection by effective and efficient use of available facilities.
Reducing cost of rejects and rework through application of statistical quality control techniques.
To standardise the measuring methods.
To maintain the accuracies of measurement.
To prepare design for all gauges and special inspection fixtures.

Measurand:
           Measurand is the physical quantity or property like length, angle, diameter, thickness etc. to be measured.
Reference:
           It is the physical quantity or property to which quantitative comparisons are made.
Comparator:
           It is the means of comparing measuring measurand with some reference.Suppose a fitter has to measure the length of M.S. plate- he first lays his rule along the flat. Then carefully aligns the zero end of his rule with one end of M.S.flat and finally compares the length of M.S. Flat with the graduations on his rule by his eyes. In this example the length of M.S. flat is a measurand, steel rule is the reference and eye can be considered as a comparator.

 Methods of Measurement 

           The methods of measurement can be classified as:
1.Direct method:
           This is a simple method of measurement in which the value of the quantity to be measured is obtained directly without the calculations. For example, measurements by scales, Vernier calipers, micrometers, bevel protectoretc. This method is most widely used in production. This method is not very accurate because it depends on human judgement.

2.Indirect method:
           In indirect method the value of quantity to be measured is obtained by measuring other quantities which are functionally related to required value. For example, angle measurement by sine bar, measurement of shaft power by dynamometer etc.

 Precision and Accuracy 

Accuracy:
           Accuracy is defined as the closeness of the measured value with true value. Practically it is very difficult to measure the true value and therefore a set of observations is made whose mean value is taken as the true value of the quantity measured.

Precision:
           A measure of how close the trials are repeated to each other. Precision is the repeatability of the measuring process. It refers to the group of measurements for the same characteristics taken under identical conditions. It indicates to what extent the identically performed measurements agree with each other. If the instrument is not precise it will give different results for the same dimension when measured again and again.
Precision and Accuracy

 Errors in Measurements 

           It is never possible to measure the value of a dimension, there is always some error. The error in the measurement is the difference between the measured value and the true value of measured dimensions. The error in measurement may be expressed either as an absolute error or as a relative error.
Absolute error:
1.True absolute error:
           It is the algebraic difference between the result of measurement and the conventional true value of the quantity.
2.Apparent absolute error:
           The series of measurement are made then the algebraic difference between one of the results of measurement and the arithmetical mean is known as apparent absolute error.
3.Relative error:
           The results of the absolute error and the value of comparison used for calculation of that absolute error. The comparison may be true value or conventional true value or arithmetic mean for series of measurement.

 Types of Error 

The errors can be classified into:
• Static errors
(i) Characteristic errors
(ii) Reading errors
(iii) Environmental errors
• Loading errors
• Dynamic error

Static error:
           It causes due to the physical nature of the various components of the measuring system. The static errors due to environmental effect and the other properties which influence the apparatus are also reasons for static errors.
(i) Characteristic error:
           The deviation of the output of the measuring system from the nominal performance specifications is called characteristic error. The linearity, repeatability, hysteresis and resolution are part of the characteristic error.
(ii) Reading error:
           It is exclusively applied to the read out device. The reading error describes the factors parallax error and interpolation error.
• The use of mirror behind the readout indicator eliminates the occurrence of parallax error.
• Interpolation error is a reading error resulting from the inexact evaluation of the position of index.
• The use of digital readout devices eliminates the subjective error.
(iii) Environmental error:
           Every instrument is manufactured and calibrated at one place and it is used in some other place where the environmental conditions such as temperature, pressure, and humidity change. The change in environment influences the readings of the instrument. This change in environment is called environment error. By following the below conditions, the environmental errors are eliminated.
• Monitoring the atmospheric conditions.
• By calibration of instrument at the place of use.
• Automatic devices are used to compensate the effects.

Loading error:
           When measured quantity looses energy due to the act of measurement, an error is introduced known as loading error. Loading means the measuring instrument always taking the input from the signal source. The signal source will always be altered by the act of measurement known loading. Example: If steam flows through the nozzle, it is very difficult to lied the perfect flow rate. This is called loading error.

Dynamic error:
           This is due to time variations in the measurand. The dynamic errors are caused by inertia, friction and clamping action The dynamic errors are mainly classified into:
• Systematic errors or Controllable errors
• Random errors.
Systematic error:
           The systematic errors are constant and similar in form. These are controllable in both their sense and magnitude. The systematic errors are easily determined and reduced, hence these are also called as controllable errors. Systematic errors includes:
• Calibration errors
• Ambient or Atmospheric conditions
• Avoidable errors, and
• Stylus pressure.
Calibration error:
           Calibration is a process of giving a known input to the measurement system and also taking necessary actions to see that the output of the measurement system matches with its input. If instrument is not calibrated, the instrument will show very high degree of error. Calibration errors are fixed errors.

Ambient error:
           This is due to variation in atmospheric conditions [Example:Pressure, Temperature and moisture] normally the instruments are calibrated at particular pressure and temperatures. Temperature will not be equal at all places. If the temperature and pressure vary, the ambient error will form. Standard temperature of 20°C and pressure of 760 mm of Hg are taken as ambient conditions.

Avoidable errors:
           This type of error is due to parallax, non-alignment of work piece centers, and improper location of measuring instrument. For example placing a thermometer in sunlight to measure air temperature will cause the instrument location error.

Stylus pressure:
           Whenever a component is measured under particular pressure, the deformation of
the work piece and surface deflection will occur. The pressure involved is generally small but this is sufficient to cause appreciable deformation on stylus and the work piece.
Stylus pressure

Random error:
           These types of errors occur randomly and reason for this type of errors cannot be specified.


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