---
title: "Classification and merits of comparators"
subject: "Metrology"
chapter: 7
key: "semester_3/metrology:classification-of-comparators"
source: https://www.bctengineer.com/q/semester_3/metrology/classification-of-comparators
site: BCT Engineering
---

# Classification and merits of comparators

Metrology · Chapter 7: Comparators

- Asked in: year not given, 6 marks
- Video: [Comparator | Principle, Characteristics & Classification | Hindi + English | Metrology Instrument](https://www.youtube.com/watch?v=gXif1B2a4IQ) — Tech Research Academy
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_3/metrology/classification-of-comparators
- Whole chapter: https://www.bctengineer.com/md/semester_3/metrology/chapter-7

## Question

What is a comparator? Classify comparators and compare the advantages and disadvantages of mechanical, optical, electrical and pneumatic types.

## Short answer

A comparator indicates the difference between a workpiece and a standard on a magnified scale. Types: mechanical (cheap, robust, limited magnification), optical (high magnification, dark room, bulky), electrical (very high magnification, remote recording, needs stabilised power), pneumatic (non-contact, self-cleaning, slow, requires clean dry air).

## Answer

A **comparator** is a precision instrument that compares the size of a workpiece with a known standard (master or slip gauge) and indicates the **difference** on a magnified scale. It does not give the absolute size. Principle: set the instrument to zero on a master of the basic size; the pointer reading when the work replaces the master gives the deviation.

#### Classification
1. **Mechanical** (dial indicator, reed-type, sigma comparator): amplification by levers, gears, reeds.
2. **Optical** (Zeiss Ultra-optimeter, optical projector): amplification by a light beam and mirrors.
3. **Electrical / electronic** (LVDT, capacitance, inductive): displacement converted to an electrical signal.
4. **Pneumatic** (Solex type): the change of air gap is measured through back pressure or flow.
5. **Combinations**: mechanical-optical, electro-pneumatic.

#### Comparison
| Type | Advantages | Disadvantages |
|---|---|---|
| Mechanical | Cheap, robust, no power needed, portable | Friction, backlash and wear; moving parts; limited magnification (up to about 1000) |
| Optical | Very high magnification (up to 1000-2000), no inertia, few moving parts | Needs power and a darkened room, costly, bulky, eye strain |
| Electrical | High magnification (up to 10 000), remote reading, easy to record and automate | Needs power supply and stabilisation, heat from supply affects result, costly |
| Pneumatic | Non-contact, high magnification, self-cleaning, can measure many dimensions at once | Needs clean, dry air at constant pressure, slow response, scale not linear over a wide range |

Comparators are used for the inspection of large batches where the tolerance is small.

## Check yourself

The answer's key points as multiple-choice questions, with the correct option and why.

1. A comparator indicates:
   A. The absolute size of the work
   B. The difference between the workpiece and a master or slip gauge
   C. The surface roughness
   D. The hardness of the work
   Answer: B. It is set to zero on a master and the reading when work replaces it gives the deviation, not the absolute size. (Key point: Comparator function)

2. A main advantage of mechanical comparators is that they are:
   A. Non-contact
   B. Cheap, robust and need no power
   C. Of highest magnification
   D. Insensitive to wear
   Answer: B. Mechanical comparators are cheap, robust, portable and need no power, but have friction, backlash and limited magnification. (Key point: Mechanical comparator)

3. Which comparator measures by the change of air gap through back pressure or flow and is non-contact?
   A. Pneumatic
   B. Electrical LVDT
   C. Mechanical dial
   D. Optical projector
   Answer: A. Pneumatic (Solex-type) comparators are non-contact and self-cleaning but need clean, dry air at constant pressure. (Key point: Pneumatic comparator)

4. Electrical comparators are noted for:
   A. No need for a power supply
   B. High magnification (up to 10 000) and remote reading, but needing a stabilised supply
   C. Needing a darkened room
   D. Being the cheapest type
   Answer: B. Electrical/electronic types give very high magnification and easy recording, at the cost of power supply, heat effects and expense. (Key point: Electrical comparator)

## Related in Metrology

- [Optical lever comparator magnification](https://www.bctengineer.com/question.md?key=semester_3%2Fmetrology%3Aoptical-lever-comparator-numerical) — Metrology, chapter 7

## Related in other subjects

- [Communication system; optical fiber merits](https://www.bctengineer.com/question.md?key=semester_2%2Fbasic_electronics_engineering%3Adefine-comm-optical-merits) — Basic Electronics Engineering, chapter 5
- [Electrical and mechanical requirements of insulators](https://www.bctengineer.com/question.md?key=semester_4%2Fpower_system_analysis_i%3Ainsulator-requirements) — Power System Analysis I, chapter 2
- [Civil, mechanical and electrical components](https://www.bctengineer.com/question.md?key=semester_8%2Fpower_plant_design%3Ahydropower-civil-mech-elec) — Power Plant Design, chapter 3
- [Comparators used in static relays](https://www.bctengineer.com/question.md?key=semester_6%2Fswitchgear_and_protection%3Acomparators-static-relay) — Switchgear and Protection, chapter 8, asked 2×