Reed Type Mechanical Comparator

what is reed type mechanical comparator & how does it work? (Pdf)

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Reed type mechanical comparator: Definition

Reed type mechanical comparator is a precision instrument used to compare the dimensions of the workpiece with standard dimensions. As the name indicates the reed type comparator is definitely a  mechanical comparator. The other comparators like sigma comparator, Johansson mikrokator etc also comes under the category of the mechanical comparator. 

In reed type mechanical comparator the external appearance might be the same as what other mechanical comparator looks like. But its internal construction and the working principle is very simple while comparing other comparators.

Now let’s see the construction, working principle, applications, advantages and disadvantages of reed type mechanical comparator.

Parts of Reed type mechanical comparator

Reed Type Mechanical Comparator
Reed Type Mechanical Comparator

The parts and their construction are very simple in this mechanical comparator. In reed type mechanical comparator there are five major parts used for its construction and they are listed below.

  • Plunger
  • Blocks
  • Reeds
  • Pointer or Needle
  • Scale


The plunger is a small metal component that acts as a sensing probe in this mechanical comparator. In other words, the plunger is called as spindle. It is a necessary component that helps in sensing the dimensional variations in the workpiece to measure.


There are two blocks placed inside this comparator. Among two blocks, one is fixed and another one is movable. The movable block is attached with the spindle or plunger at the bottom. Mostly the blocks are made up of metals.


Like blocks, there are two pairs of reed placed inside the comparator. A pair of the reed is placed horizontal and the remaining pair is placed vertically. The horizontal reed bridges the fixed block and the movable block in the middle position. The upper end of the movable block and the fixed block is attached with the vertical reeds.

Needle or Pointer

The pointer or needle is placed in the junction of the vertical reeds. Pointer is usually made up of plastic, aluminium, acryl material or other lightweight materials.


A graduated scale is used to give output readings as per the deflection of the pointer. The measurement readings are printed on a graduated plate in the comparator case and the whole setup is enclosed by the fibreglass.

Working principle of reed type mechanical comparator

Now let’s see its working principle and how it is operated?

Reed Type Mechanical Comparator Working
Reed Type Mechanical Comparator Working

The read type mechanical comparator consists of a spindle, blocks, reeds, needle and a graduated scale. Initially, with the help of slip gauges or gauge blocks, the whole comparator is calibrated to zero scale. Then the work to be measured or analysed is kept under the plunger contact point. If there is any dimensional variation compared to standard dimensions (slip gauges), the movement of the plunger or spindle will be in an upward or downward direction.

The movable block also moves since it is attached to the plunger. Simultaneously the vertical reed fixed to the movable block also moves upward. The fixed parameter and linear motion of the movable parameter make the needle or pointer deflect. The deviation in the dimension of the workplace may result either in positive or negative value.

Magnification of reed type mechanical comparator

  • The deflection of the pointer or needle is directly proportional to the displacement of the movable or floating block.
  • Therefore the large angular movement $(X)$ or displacement of the needle is produced by the small displacement of $(x)$ the plunger.
  • The magnification of the reed type comparator is actually less than 100. If any optical lens system has been used then the magnification is multiplied as per the lens system.
  • The magnification range of Reed type mechanical comparator is from $ \displaystyle \times $  500 to $ \displaystyle \times $ 1000.
  • The sensitivity of the reed type comparator is $\displaystyle 0.25\times {{10}^{{-3}}}$ mm/scale division.

Advantages of reed type mechanical comparator

  • Reed type comparator has only fewer components.
  • It is a frictionless device due to a very less number of internal components.
  • It is very lightweight and portable.
  • It is very cheap and easy to use.
  • It is highly sensitive.

Disadvantages of reed type mechanical comparator

  • Decrease in activity of any movable parameter inside the comparator can result in poor accuracy of the instrument.
  • Since it is very sensitive, it is not suitable for any vibration conditions.
  • The range of the measuring scale in this comparator is limited.

Applications of reed type mechanical comparator

  • It is used to measure the dimensional deviation of the workpiece with the standard dimensions.
  • It is used in mass production and testing laboratories for analysis purpose.
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