Sigma Comparator: Construction, Working Principle, Applications, Advantages, and Disadvantages [PDF]

A comparator works on the relative dimension, which means it can measure the difference between basic dimensions with the actual dimension. Sigma comparator is a type of mechanical comparator developed by the Sigma Instrument Company, USA.

Construction of Sigma Comparator:

A Sigma Comparator is constructed by the following components, and those are:

  • Plunger
  • Knife Edge
  • Slit Diaphragm
  • Sapphire Bearing Block
  • Moving Block
  • Fixed Block
  • Cross Strip Hinge
  • Y-arm
  • Phosphor Bronze Driving Band
  • Driving Drum
  • Pointer Spindle
  • Pointer
  • Scale


It is the sensing unit that senses any uneven surface texture. This component of the comparator is directly in touch with the surface.

Knife Edge:

It is a knife-edge shaped metallic strip, which is fitted with the plunger.

Slit Diaphragm:

Generally, a diaphragm is made of rubber-type material (polymer). In the Sigma comparator there are 2 slit diaphragm provided to have frictionless linear movement of the plunger.

Saphire Bearing Block:

This component is directly in touch with Knife-edge and moving block.

Moving Block:

A moving block is a moving arm or member which is directly connected with knife-edge member, and it is the bottom part of Cross Strip.

Fixed Block:

It is an upper part of Cross Strip.

Cross Strip Hinge:

It is a pivoted portion of the moving block and fixed block arms.


It is a simple Y shape arm connected between the driving drum through a Phosphor Bronze driving band and moving block [Cross Strip].

Phosphor Bronze Driving Band:

This arm is made of Phosphor Bronze alloy having high spring qualities, high fatigue resistance, superior formability, and high corrosion resistance.

Driving Drum:

It used to link the pointer with the Phosphor Bronze driving band [Y-arm] with the help of pointer spindle.

Pointer Spindle:

It is used to attach the pointer with the driving drum.


A pointer is used for displaying the measured value on a linear scale.


The pointer and scale are a part of the display unit, the scale can be linear or circular in shape.

diagram of sigma comparator
Movement of Sigma Comparator

Working Principle of Sigma Comparator:

The working of a Sigma Comparator is stated below:

  • The plunger is fitted through the slit diaphragm to give a frictionless surface.
  • When the plunger is pushed upward due to the uneven element it displace the knife-edge either upward or downward.
  • The knife-edge is fitted with the moving block using a Sapphire bearing block.
  • Due to the upward or downward movement of the knife-edge, the moving block respond accordingly as it is hinged.
  • Y-arm is directly connected with the cross-strip, so when there is a rotating movement on the cross strip that movement is transferred to the driving drum with help of Y-arm.
  • The driving drum is fitted with spindle and pointer.
  • Any movement in driving drum results in the pointer movement, and we get the magnified result on the display unit or scale.
  • The first magnification of this instrument depends on the Y-arm and the length of the Y-arm, and the second magnification depends on the ratio of the radius of the pointer and the radius of the spindle.

Applications of Sigma Comparator:

Sigma Comparator is used for various purposes like:

  • It is used in the selective assembly.
  • In mass production, we can see this type of instrument.

Advantages of Sigma Comparator:

Following are some advantages of Sigma Comparator:

  • It has a bold scale and larger indicating pointer.
  • Cheaper in Price
  • This type of comparator does not need any external source of energy to work.
  • It is robust, compact and easy to handle instrument.

Disadvantages of Sigma Comparator:

Following are some disadvantages of Sigma Comparator:

  • Design is complicated.
  • Due to many mechanical components, there is a chance of wear and tear.
  • Parallax error can occur in this type of instrument.
  • It is not a sensitive instrument as of mechanical or optical due to the chances of friction. 


What is a Sigma comparator?

Sigma comparator is a type of mechanical comparator developed by the Sigma Instrument Company, USA.

Sigma Comparator is which type of comparator?

Sigma Comparator is Mechanical Comparator.

More Resources:

Johansson Mikrokator Overview: In-Depth


Media Credit:

  • Comparator image: Engineering Metrology by R. K Jain- Khanna Publishers
  • Feature image: Designed by Author
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