METHODS OF MEASUREMENTS
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
3. Absolute
or Fundamental method
4. Comparative
method
5. Transposition
method
6. Coincidence
method
7. Deflection
method
8. Complementary
method
9. Contact
method
10. Contact less 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.
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.
3. Absolute or Fundamental method:
It is based on the measurement of the base quantities used to define the quantity. For example, measuring a quantity directly in accordance with the definition of that quantity, or measuring a quantity indirectly by direct measurement of the quantities linked with the definition of the quantity to be measured.
4. Comparative method:
In this method the value of the quantity to be measured is
compared with known value of the same quantity or other quantity practically
related to it. So, in this method only the deviations from a master gauge are
determined, e.g., dial indicators, or other comparators.
5. Transposition method:
It is a method of measurement by direct comparison in which the value of the quantity measured is first balanced by an initial known value A of the same quantity, and then the value of the quantity measured is put in place of this known value and is balanced again by another known value B. If the position of the element indicating equilibrium is the same in both cases, the value of the quantity to be measured is AB. For example, determination of amass by means of a balance and known weights, using the Gauss double weighing.
6. Coincidence method:
It is a differential method of measurement in which a very small difference between the value of the quantity to be measured and the reference is determined by the observation of the coincidence of certain lines or signals. For example, measurement by vernier calliper micrometer.
7. Deflection method:
In this method the value of the quantity to be measured is
directly indicated by a deflection of a pointer on a calibrated scale.
8. Complementary method:
In this 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 predetermined comparison
value. For example, determination of the volume of a solid by liquid
displacement.
9. Method of measurement by substitution:
It is a method of direct comparison in which the value of a
quantity to be measured is replaced by a known value of the same quantity, so
selected that the effects produced in the indicating device by these two values
are the same.
10. Method of null measurement:
It is a method of differential measurement. In this method
the difference between the value of the quantity to be measured and the known
value of the same quantity with which it is compared is brought to zero.
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