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 one 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 Diaphram
  • Saphire 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 which 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 Diaphram:

Generally, a diaphram is made of rubber type material (polymer), in Sigma comparator there are 2 number of slit diaphram is 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:

Moving block is a moving arm or member which is directly connected with knife-edge member, and it is a 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 simple Y shape arms connected between the driving drum through Phosphor Bronze driving band and moving block [Cross Strip].

Phosphor Bronze Driving Band:

This arm is made of Phosphor Bronze alloy it has 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 diaphram to give a frictionless surface.
  • When the plunger is pushed upward due to the uneven element it displaced the knife-edge either upward or downward.
  • The knife-edge is fitted with the moving block using a Saphire bearing block.
  • Due to the upward or downward movement of the knife-edge, the moving block 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 moving, 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 also.

Advantages of Sigma Comparator:

Here 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 the instrument.

Disadvantages of Sigma Comparator:

Here 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. 

Some FAQ:

What is a Sigma comparator?

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

Sigma Comparator is which type of comparator

Sigma Comparator is Mechanical Comparator.

I hope you like this article, feel free to comment down your thoughts reagaring this article, I will love to see those in the comment section. One more thing DO NOT forget to share this article on your favourite social media.

Some More Resources for You:

Johansson Mikrokator Overview: In-Depth

References (External Links):

Media Credit:

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

A Mechanical Engineer with 19 years of working experience in various renowned MNCs.

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