Page 20 - Personal Study Notes (Engineering Metrology - 22342)

Page 20 of 176

Q. Write short note on ‘Functional requirement of good comparators”.

A comparator has to fulfil many functional requirements in order to be effective in the
industry. It should not only provide a high degree of accuracy and precision but also be
convenient for use. It should withstand the rough and tough operating environment on
the shop floor and also have good sensitivity to detect minute changes in the parameter
being measured. We can summarize the major requirements of a comparator as
follows:

1. A comparator should have a high degree of accuracy and precision. We can safely
say that in general, comparison measurement provides better accuracy and precision
than direct measurement. In direct measurement, precision is dependent on the least
count of the scale and the means for reading it. In comparison measurement, it is
dependent on the least count of the standard and the means for comparing. Accuracy,
in contrast, is dependent on other factors, geometrical considerations being the most
important of them. Direct measurement instruments such as vernier calliper and
micrometer have the standard built into it, with the result that measurement is done by
the displacement method. It is the relationship between the distance displaced and a
standard that constitutes the measurement. On the other hand, comparison
measurement uses the interchange method for measurement. In this method, both ends
of the unknown feature are compared with both ends of the standard at the same time.
This enables comparators to have a more favourable geometry, which gives scope for
better accuracy.

2. The scale should be linear and have a wide range. Since a comparator, be it
mechanical, pneumatic, or electrical, has a means of amplification of signals, linearity of
the scale within the measuring range should be assured.

3. A comparator is required to have high amplification. It should be able to amplify
changes in the input value, so that readings can be taken and recorded accurately and
with ease. Amplification demands use of more number of linkages in a mechanical
system and a more elaborate circuit in an electrical system. This puts load on the
system, resulting in the system being unable to sense small changes in the input signal.
Therefore, one has to strike a compromise between the two. Alternately, the designer
can be biased in favour of one at the cost of the other, depending on the major objective
of measurement.

4. A comparator should have good resolution, which is the least possible unit of
measurement that can be read on the display device of the comparator. Resolution
should not be confused with readability, the former being one among many factors that
influence the latter. Other factors include size of graduations, dial contrast, and parallax.

5. There should be a provision incorporated to compensate for temperature effects.

6. Finally, the comparator should be versatile. It should have provisions to select
different ranges, attachments, and other flexible means, so that it can be put to various
uses.

Engineering Metrology (Only For Personal Use) 12