Introduction of Metrology 2018-19 Vinod Kumar R Page 1 Module 1 STANDARDS OF MEASUREMENT DEFINITION OF METROLOGY: Metrology (from Ancient Greek metron (measure) and logos (study of)) is the science of measurement. Metrology includes all theoretical and practical aspects of measurement. Metrology is concerned with the establishment, reproduction, conservation and transfer of units of measurement & their standards. For engineering purposes, metrology is restricted to measurements of length and angle & quantities which are expressed in linear or angular terms. Measurement is a process of comparing quantitatively an unknown magnitude with a predefined standard. OBJECTIVES OF METROLOGY: The basic objectives of metrology are; 1. To provide accuracy at minimum cost. 2. Thorough evaluation of newly developed products, and to ensure that components are within the specified dimensions. 3. To determine the process capabilities. 4. To assess the measuring instrument capabilities and ensure that they are adequate for their specific measurements. 5. To reduce the cost of inspection & rejections and rework. 6. To standardize measuring methods. 7. To maintain the accuracy of measurements through periodical calibration of the instruments. 8. To prepare designs for gauges and special inspection fixtures. NEED OF INSPECTION: The need of inspection can be summarized as: 1. To ensure that the part, material or a component conforms to the established standard. 2. To meet the interchangeability of manufacture. 3. To maintain customer relation by ensuring that no faulty product reaches the customers.
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Introduction of Metrology 2018-19
Vinod Kumar R Page 1
Module 1
STANDARDS OF MEASUREMENT
DEFINITION OF METROLOGY:
Metrology (from Ancient Greek metron (measure) and logos (study of)) is the science of
measurement. Metrology includes all theoretical and practical aspects of measurement.
Metrology is concerned with the establishment, reproduction, conservation and transfer of units
of measurement & their standards. For engineering purposes, metrology is restricted to
measurements of length and angle & quantities which are expressed in linear or angular terms.
Measurement is a process of comparing quantitatively an unknown magnitude with a predefined
standard.
OBJECTIVES OF METROLOGY:
The basic objectives of metrology are;
1. To provide accuracy at minimum cost.
2. Thorough evaluation of newly developed products, and to ensure that components are
within the specified dimensions.
3. To determine the process capabilities.
4. To assess the measuring instrument capabilities and ensure that they are adequate for
their specific measurements.
5. To reduce the cost of inspection & rejections and rework.
6. To standardize measuring methods.
7. To maintain the accuracy of measurements through periodical calibration of the
instruments.
8. To prepare designs for gauges and special inspection fixtures.
NEED OF INSPECTION:
The need of inspection can be summarized as:
1. To ensure that the part, material or a component conforms to the established standard.
2. To meet the interchangeability of manufacture.
3. To maintain customer relation by ensuring that no faulty product reaches the customers.
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4. Provide the means of finding out shortcomings in manufacture. The results of inspection are
not only recorded but forwarded to the manufacturing department for taking necessary steps, so
as to produce acceptable parts and reduce scrap.
5. It also helps to purchase good quality of raw materials, tools, equipment which governs the
quality of the finished products.
6. It also helps to co-ordinate the functions of quality control, production, purchasing and other
departments of the organization.
To take decision on the defective parts i.e., to judge the possibility of making some of these parts
acceptable after minor repairs.
REQUIREMENTS OF MEASUREMENTS:
If the result of measurements has to be meaningful, then the following two important conditions
must be satisfied:
1. The standard used for comparison must be accurately accepted known and commonly
accepted.
For example: a length cannot be simply said it is too long but it must be said it is
comparatively longer than some standard.
2. The procedure and the apparatus used for comparison must be commonly accepted and
must be provable.
METHODS OF MEASUREMENTS
These are the methods of comparison used in measurement process. In precision measurement
various methods of measurement are adopted depending upon the accuracy required and the
amount of permissible error.
The methods of measurement can be classified as:
l. Direct method
2. Indirect method
1. Direct method of measurement:
This is a simple method of measurement, in which the value of the quantity to be measured is
obtained directly without any calculations. For example, measurements by using scales, vernier
callipers, micrometers, bevel protector etc. This method is most widely used in production. This
method is not very accurate because it depends on human insensitiveness in making judgment.
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2. Indirect method of measurement:
In indirect method the value of quantity to be measured is obtained by measuring other quantities
which are functionally related to the required value. E.g. Angle measurement by sine bar,
measurement of screw pitch diameter by three wire method etc.
LEAST COUNT:
In Metrology the least count of an instrument is the smallest change in the value that can be
measured with the measuring instrument.
A Vernier scale on caliper may have a least count of 0.02 mm while a micrometer may have
a least count of 0.01 mm.
The least count error occurs with both systematic and random errors. Instruments of higher
precision can reduce the least count error. By repeating the observations and taking the
arithmetic mean of the result, the mean value would be very close to the true value of the
measured quantity.
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ACCURACY
The closeness of a measured value to the actual value of the object being measured is
called as the accuracy of a substance. For instance, if in lab you obtain a weight
measurement of 3.2 kg for a given substance, but the actual or known weight is 10 kg,
then your measurement is not accurate.
Accuracy is the degree to which the measured value of the quality characteristic agrees
with the true value. The difference between the true value and the measured value is
known as error of measurement.
It is practically difficult to measure exactly the true value and therefore a set of
observations is made whose mean value is taken as the true value of the quality
measured.
PRECISION
The closeness of two or more measurements to each other is known as the precision of a
substance. From the above-given example, we can figure out that, if you weigh a given
substance five times, and get 3.2 kg each time, then your measurement is very precise.
Precision is independent of accuracy.
The terms precision and accuracy are used in connection with the performance of the
instrument.
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 (widely varying) results for the same
dimension when measured again and again. The set of observations will scatter about the
mean. The scatter of these measurements is designated as 0, the standard deviation. It is
used as an index of precision. The less the scattering more precise is the instrument.
Thus, lower, the value of 0, the more precise is the instrument.
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DIFFERENCE BETWEEN ACCURACY AND PRECISION:
Sl
No Accuracy Precision
1 Accuracy indicates how close a
measurement is to be correct or accepted
value
Precision indicates the closeness of two or
more measurements to each other
2 the measurement will be close to the
standard measurement
The measurement will be similar every time
you measure.
3 Accuracy can be improved Precise cannot be improved
4 Accuracy is not dependent on precision Precision is not dependent on accuracy.
ERRORS IN MEASUREMENT
It is never possible to measure the true value of a dimension, there is always some error. The
error in measurement is the difference between the measured value and the true value of the
measured dimension.
Error in measurement =Measured value - True value.
TYPES OF ERRORS
1. Gross Errors
This category of errors includes all the human mistakes while reading, recording and the
readings. Mistakes in calculating the errors also come under this category. For example while
taking the reading from the meter of the instrument he may read 21 as 31. All these types of error
are come under this category.
Gross errors can be avoided by using two suitable measures and they are written below:
A proper care should be taken in reading, recording the data. Also calculation of error
should be done accurately.
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By increasing the number of experimenters we can reduce the gross errors. If each
experimenter takes different reading at different points, then by taking average of more
readings we can reduce the gross errors.
2. Systematic Errors
Errors which occur due to changes in environment conditions, instrumental reasons or wrong
observations. These errors are of three types
Instrumental Errors
Environmental Errors
Observational Errors
Instrumental Errors:
These errors occur due to shortcomings in the instruments, improper use of instruments or
loading effect of the instrument. Sometimes improper construction, calibration or operation of an
instrument might result in some inherent errors. For example, weak spring in a Permanent
Magnet Instrument might result in too high readings. These errors can be easily detected or
reduced by applying correction factors, careful planning of measurement procedure or re-
calibrating the instrument.
Environmental Errors
This type of error arises due to conditions external to the instrument. External condition includes
temperature, pressure, humidity or it may include external magnetic field. Following are the
steps that one must follow in order to minimize the environmental errors:
Try to maintain the temperature and humidity of the laboratory constant by making some
arrangements.
Ensure that there should not be any external magnetic or electrostatic field around the
instrument.
Observational Errors
These errors occur due to a mismatch between a line of vision of the observer and the
pointer above the instrument scale. This is also termed as Parallax error which occurs
when the observer is unable to have a vision aligned with the pointer.
These errors can be minimized by using highly accurate meters (having the pointer and