All measurements require the unknown quantity to be compared with a known quantity, called the standard. A measurement is generally made with respect to time, mass, and length. In each of these cases, three elements are involved: the unknown, the standard, and a system for comparing them. These instruments enable us to directly measure a linear dimension up to the given degree of accuracy. On the other hand, in certain devices the standards are separated from the instrument. It compares the unknown length with the standard. Such measurement is known as comparison measurement and the instrument, which provides such a comparison, is called a comparator. In other words, a comparator works on relative measurement. It gives only dimensional differences in relation to a basic dimension or master setting. Comparators are generally used for linear measurements, and the various comparators currently available basically differ in their methods of amplifying and recording the variations measured.
Figure 1 illustrates the difference between direct and comparison measurements. As can be seen in the figure, a calibrated standard directly gives the measured value in case of direct measurement. On the other hand, a comparator has to be set to a reference value (usually zero setting) by employing a standard. Once it is set to this reference value, all subsequent readings indicate the deviation from the standard.
The deviation can be read or recorded by means of a display or recording unit, respectively. Accuracy of direct measurement depends on four factors: accuracy of the standard, accuracy of scale, least count of the scale, and accuracy of reading the scale. The last factor is the human element, which depends on the efficiency with which the scales are read and the accurate interpretation of the readings.
Accuracy of comparison measurement mainly depends on four factors: accuracy of the standard used for setting the comparator, least count of the standard, sensitivity of the comparator, and accuracy of reading the scale. In distinction to direct measurement, the role of the sensing element is significant in a comparator. The sensitivity of the comparator to sense even a minute variation in the measured value is equally significant. The variation in the measured value may be in terms of change in displacement, pressure, fluid flow, temperature, and so on.
Fig.1 Direct measurement versus comparison measurement
FUNCTIONAL REQUIREMENTS OF COMPARATORS:
A comparator has to accomplish 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 favorable 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. Hence, one has to strike a compromise between the two. Alternately, the designer can be biased in favor 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 several factors that influence the latter. Other factors include size of graduations, dial contrast, and parallax.
5. There should be a establishment 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.
CLASSIFICATION OF COMPARATORS
We can classify comparators into mechanical device and electrical device on the basis of the means used for comparison. In recent times, engineers prefer to classify comparators as lowand high-amplification comparators, which also reflect the sophistication of the technology that is behind these devices. Accordingly, we can draw the following classification.
Fig.2 Typical Mechanical comparator
Fig.3 Electrical comparator
Fig.4 Pneumatic Comparator
With respect to the principle used for amplifying and recording measurements, comparators are classified as follows:
1. Mechanical comparators
2. Mechanical–optical comparators
3. Electrical and electronic comparators
4. Pneumatic comparators
5. Other types such as projection comparators and multi-check comparators Each of these types of comparators has many variants, which provide flexibility to the user to make an appropriate and economical selection for a particular metrological application.