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 low-

and high-amplification comparators, which also reflect the sophistication of the technology that

is behind these devices. Accordingly, we can draw the following classification.

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.

It is primarily used to compare workpieces against a master. The basic features of a dial gauge

consist of a body with a circular graduated dial, a contact point connected to a gear train,

and an indicating hand that directly indicates the linear displacement of the contact point.

The contact point is first set against the master, and the dial scale is set to zero by rotating

the bezel. Now, the master is removed and the workpiece is set below the contact point; the

difference in dimensions between the master and the workpiece can be directly read on the

dial scale. Dial gauges are used along with V-blocks in a metrology laboratory to check the

roundness of components. A dial gauge is also part of standard measuring devices such as

bore gauges, depth gauges, and vibrometers. Figure 6.2 illustrates the functional parts of a

dial indicator.

The contact point in a dial indicator is of an interchangeable type and provides versatility to

the instrument. It is available as a mounting and in a variety of hard, wear-resistant materials.

Heat-treated steel, boron carbide, sapphire, and diamond are some of the preferred materials.

Although flat and round contact points are commonly used, tapered and button-type contact

points are also used in some applications. The stem holds the contact point and provides the

required length and rigidity for ease of measurement. The bezel clamp enables locking of

the dial after setting the scale to zero. The scale of the dial indicator, usually referred to as

dial, provides the required least count for measurement, which normally varies from 0.01 to

0.05 mm. The scale has a limited range of linear measurements, varying from 5 to 25 mm. In

order to meet close least count, the dial has to be large enough to improve readability.

The dials are of two types: continuous and balanced. A continuous dial has graduations

starting from zero and extends to the end of the recommended range. It can be either clockwise

or anti-clockwise. The dial corresponds to the unilateral tolerance of dimensions. On the other

hand, a balanced dial has graduations marked both ways of zero. This dial corresponds to the

use of bilateral tolerance. Figure 6.3 illustrates the difference between the two types of dials.

Metrological features of a dial indicator differ entirely from measuring instruments such

as slide callipers or micrometers. It measures neither the actual dimension nor does it have

a reference point. It measures the amount of deviation with respect to a standard. In other

words, we measure not length, but change in length. In a way, this comparison measurement

is dynamic, unlike direct measurement, which is static. Obviously, the ability to detect and

measure the change is the sensitivity of the instrument.