What Is Comparator in Metrology? How It Differs from A Direct Measuring Device?

What is comparator in metrology?

The measurement of an unknown quantity is done by comparing it with a known quantity known as standard. If this measuring device has a built-in standard, it enables us the direct measurement of the unknown quantity. These are called direct measuring instrument. Example for direct measuring devices are Micrometers, Vernier Calliper etc.

In other hands, in certain measuring instrument, the standard is separated, such an instrument is called comparator. It will not give direct measurement; instead, it compares unknown with the standard. It indicates difference in measurement between the component and working standard/ master settings. So, comparator is a precision instrument used to compare the measurement of unknown (part under test) with a known working standard. The following figure illustrates the difference between direct measurement and comparator.

Difference between direct measuring device and comparator

Functional requirement of comparator

To be effective in industry, a comparator must fulfil some functional requirement. Essential requirements are listed below

1. Comparator is desirable to be versatile and has provision to select different ranges and other flexible means to be put to various use.
2. High degree of accuracy and precision.
3. The scale should be linear and have a wide range.
4. They should be free from backlash and lag between reading and recording mechanism.
5. It should have high amplification capability so that reading can be recorded easily.
6. It must have good resolution; resolution means the least measuring unit that can be read on the display unit.
7. Indication should be clear and free from the oscillation
8. Must have provision to compensate for the environmental effect (eg: temperature) on measurement.
9. comparator should withstand the rough and tough operating environment.

Types of comparator

With respect to the principle used, the comparator can be classified into the following

Mechanical comparator: It is most straightforward in fabricating and operating. It gives a cost-effective solution. E.g., Dial indicator, Johansson Mikrokator, Sigma Comparator

Mechanical–optical comparator: As the name suggest, it has a mechanical part and optical part. E.g., Zeiss Ultra-optimeter

Electrical comparator: Even though electrical comparator uses electrical and electronic means to get magnification and displaying output, it relays on mechanical contact with the work to be measured. The movement of mechanical contact is converted to the electrical signal. They are widespread because of their instantaneous response and convenience in amplifying. E.g., Linear Variable Differential Transformer (LVDT), Sigma Electronic Comparator.

Pneumatic comparator: Use air as a means for the measurement. These types working on the principle of Bernoulli’s theory. The pneumatic comparator’s main advantage is that it eliminates metal to metal physical contact between the gauge and component. E.g., Free Flow Air Gauge, Solex Pneumatic Gauge, Back Pressure Gauge

Other classifications are low and high-amplification comparators, projection comparators, multi-check comparators, Fluid Displacement Comparators etc.

Each of the above-mentioned types of comparator has many variants so that it gives users flexibility to make the appropriate and economical selection for a particular application.