Pneumatic Comparator : Design, Working, Types & Its Applications

The comparator in Metrology is a precision instrument, used for measuring the accuracy of a given component by comparing the given component dimension with the actual working standard. It is an indirect type of accuracy measurement because it will not measure the dimension, but it will specify the dissimilarity within measurement between the specified component & working standard. Generally, comparators are available in different types which are used based on the requirement in different fields like electrical & electronics, optical, electromechanical, mechanical-optical, pneumatic, multi-check, fluid displacement, projection, automatic gauging  & mechanical comparator. So this article discusses one of the types of comparator namely – pneumatic comparator, working, types & its applications.

What is a Pneumatic Comparator?

A comparator that uses compressed air (or) pneumatic system as a means of magnification is known as a pneumatic comparator. The term ‘Pneumatic’ in pneumatic comparator means ‘air’. The main function of this comparator is to analyze the dimensional variation between the standard workpiece & workpiece to be measured. So these comparators are extremely preferred in most cases over other types of comparators due to many benefits.

Pneumatic Comparator
Pneumatic Comparator

Working Principle

The working principle of the Pneumatic comparator is the variation of pressure generated within the airflow. When the air escapes under stable pressure by flowing through the two orifices then the pressure of air among them mainly depends on the cross-sectional regions of the two orifices. So the dimension of one is kept consistent, and the pressure of the remaining one changes based on its dimension.

Features & Specifications

The specifications of the pneumatic comparator are discussed below.

  • It is manufactured by Solex Air Gauges Ltd industry, USA.
  • Its weight is approximately 2.2kg.
  • It uses compressed air as a working medium.
  • Its cost ranges from 40k to 65k.
  • It is manufactured & used for Laboratory.
  • Its color is Silver.
  • This comparator works through high-pressure air, back pressure, valves, etc.
  • The pressure range is approximately 0 to 300 psi or 20 bar.
  • Its vacuum range is approximately 0 to 24 inHg or 800 mbar.
  • Adjust resolution is 0.1millibar or 10 Pascals.
  • The material used in this comparator is stainless steel or aluminum for the body.
  • Its dimensions are approximately 420L×280W×160H (mm).

Pneumatic Comparator Design

The pneumatic comparator is designed with some important parts like a compressor, water tank, air filter, pressure regulator, dip tube, manometer tube, control orifice, flexible tube, gauging head, and scale which are explained below.

Pneumatic Comparator Design & Working
Pneumatic Comparator Design & Working

The compressor in this comparator is the most significant device. Generally, a compressor increases the substance pressure generally a gas by decreasing the substance volume. So, the compressor is used in the comparator, natural gas processing plants, Petroleum refineries, and different plants like petrochemical, chemical & large industrial plants. The main function of this compressor in this comparator is to produce & continuously supply compressed air within the comparator.

An air filter is used to prevent any engine from being damaged with different contaminants like dust, sand, and dirt, but if it is blocked, your engine performance will be reduced. Similarly, the filter in this type of comparator is allied to the CU (compressor unit). It is very useful in filtering the dirt particles coming from the air. Here the air is compressed through the compressor.

A pressure regulator unit is on type of valve that simply controls the gas or fluid pressure to a preferred value. This regulator unit in this comparator is arranged by using the filter to control the compressed air pressure coming from the filter.

A dip tube or withdrawal tube is an internal pipe that is arranged within the airline next to the pressure regulator of the pneumatic comparator. These tubes are made with polypropylene or polyethylene. This tube is dipped directly towards the metal cylinder or water tank. The connection of this tube as well as the airline is known as an upper chamber. These tubes are mainly used for the withdrawal of liquid phase products from liquefied products cylinders like propane, carbon dioxide, liquid hydrocarbon blends, ammonia, or different refrigerants.

The water tank is a type of cylinder made of metal that is arranged at the upper chamber’s base in this comparator so that it holds the dip tube within it. Once the air supplies through the air filter, the air pipe splits into two paths so some amount of air supplies into the water tank & some amount will go into the orifice.

The manometer tube is a vertical type or inclined U-tube column. So this tube in this comparator is arranged vertically and is parallel to the base of a water tank. The second chamber like an orifice has an exit to a manometer tube. This tube is filled with a reference liquid to display the level of pressure.

The control orifice is a flow restrictor that controls the flow of air, liquid & gas through different systems like ventilators, comparators, coffee makers, etc. This orifice in this comparator is located in the airline between two intersections of the dip & manometer tube. This orifice allows the Air to supply at a stable pressure. These orifices are available normally in different materials, configurations & diameters to meet the specific requirements of a variety of airflow & liquid control applications

A Flexible tube is normally used for connecting taps to stop cocks. These are available in a variety of lengths. These are made with synthetic rubber that is covered through a stainless steel intertwining. A flexible tube is used within the airline for holding the measuring head. So the tube’s initial point is fixed with a next chamber that is a manometer as well as an airline junction whereas the endpoint is connected through a gauging or measuring head.

The measuring head in this comparator is connected by an orifice through a flexible tube. The measuring head in this comparator is simply allied to the pneumatic hose or flexible tube. This measuring head includes two restricting jets inside that help in checking irregularities or unevenness within the workpiece.

A measuring scale is an object or device used to calculate, count, or compare the unknown length by the standard. The measuring scale in this comparator is calibrated & connected to the manometer tube in parallel. The main function of this scale is for measuring the dislocation of fluid that takes place within the tube.

Pneumatic Comparator Working

The pneumatic comparator working is; it includes a water tank where the water is filled up to a particular level. After that, a calibrated manometer is connected to the tank vertically. In the water tank, the water level, as well as the manometer, must be similar & it is adjusted with a workpiece. A compressor is used at maximum pressure to compress the air. So, the air is filtered as well as regulated with pressure with the help of a pressure regulator & air filter.

The air which is filtered will be supplied through a dip tube where this tube is submerged within a tank. At the same time, the compressed air by the equivalent pressure flows throughout the opening of the control orifice. Once air flows throughout the CO or control orifice, then velocity will be increased & at the same time, pressure will become stable. The air including vast velocity supplied through the pneumatic hose or flexible tube and lastly it arrives at the measuring head.

The head of water is maintained stably in the dip tube because of the expansion of air within. The surplus air will run away within the tank like bubbles of air. At stable pressure, the air runs away using a measuring jet in the measuring head.

When the workpiece is standard then the air escapes continuously through the measuring jet. At the same time, the water level within the tank & manometer tube will be matched. If there is any irregularity within the workpiece, especially for actual flow within the measuring jet, then a certain back force will be formed.

In the manometer, the water level will be dropped because of the induced back pressure through the limits within the workpiece. So, the level of water change can be signified as the dimensional variation or any abnormalities within the workpiece as compared to the normal workpiece.

Pneumatic Comparator Types

These comparators are normally available in three types which are discussed below.

  • Flow/Velocity type Pneumatic Comparator.
  • Back Pressure type Pneumatic Comparator.
  • Differential type Pneumatic Comparator.

Flow type Pneumatic Comparator

The flow-type pneumatic comparator is also known as the velocity-type pneumatic comparator. This comparator is mainly designed with different parts like a compressor, air filter, pressure regulator, shut-off valve, glass column, float, screw with zero adjustments, scale, flexible tube, air bleed, measuring head & workpiece.

Flow Type Pneumatic Comparator
Flow Type Pneumatic Comparator

In this comparator, the glass column must be calibrated first in the necessary dimension using a zero adjustment screw and air bleed. Like a pneumatic comparator, the air in this type of comparator is first compressed, then filtered & regulated. Afterward, the air flows through the shut-off valve & moves toward the glass column where this column has a metal float. This valve is used to shut off the air supply when it is not used. The air supplies throughout the air column & moves away finally from the measuring head.

Here, the measuring head in this comparator is placed within a workpiece to analyze. If there is any restriction or irregularities will be present above the workpiece, then airflow will be controlled which results in a little metal float displacement within the glass column. So, the metal float movement can be easily controlled through the airflow speed within the glass column and it is decided by the permission of the workpiece as well as the measuring head. Thus, the rate of airflow is proportional directly to the permission.

Back Pressure type Pneumatic Comparator

This type of comparator is mainly designed by using different parts like a compressor, pressure regulator, filter, scale, adjustable restrictor & measuring head. This comparator includes two orifices like control office ‘O1’ & measuring orifice ‘O2’.

Back Pressure Type Comparator
Back Pressure Type Comparator

Similar to the above-discussed comparator, the air which is compressed in this comparator is filtered through an air filter and then supplies through the pressure regulator as shown in the above diagram. The pressure is decreased to two bars to pass throughout the control orifice. Lastly, the air within the measuring head moves away by the measuring orifice.

The two orifices’ diameter within the measuring head is denoted with D1 & D2 whereas the pressures of orifices are denoted with P1 & P2. The air including stable pressure ‘P1’ passes throughout the control office orifice into the middle chamber. At last, it moves away from the measuring head throughout the measuring orifice. The distance between the measuring orifice as well as the workpiece can be signified with ‘d’.
At first, the measuring orifice will be completely closed. Once it is closed then both the orifices pressures will be equivalent. Similarly, once this orifice is completely opened then these pressures will be zero at two orifices. Once the distance between the measuring orifice and the workpiece changes, the pressures of the two orifices can be changed. The difference within pressure is measured by a pressure-indicating device in the comparator.

Differential type Pneumatic Comparator

The differential type pneumatic comparator is designed with different parts like a pressure regulator, compressor, air filter, control orifice, zero setting valve or reference jet, pressure indicating device & measuring head.

Differential Pneumatic Type
Differential Pneumatic Type

The air in this comparator is first compressed & allowed to flow throughout the air filter as well as the pressure regulator. At last, the pressure of air is generated & made stable. The air flows at stable pressure throughout the split channel. The air flows at one end through control orifice1 (OC1) & arrives at the measuring head through the ‘P’ channel. Likewise, the air supplies throughout the control orifice2 to reach a zero adjusting valve through the channel.

In this comparator, the pressure-indicating device is simply located within the airline. Here the airline simply connects both the channels like P & Q. Firstly; it is adjusted through a normal workpiece. Once this measuring head moves toward the workpiece then the pointer within the measuring device begins to turn aside.

The pressure within the system will be increased when there is any permission or constraint decreases between the workpiece & measuring head, so deflection occurs finally. The formula to convert the readings of pressure into the linear dimensions from the indicating device, P = ρgh is used.

Where, P = Pressure indicated through the device, ρ= density of air, g = particular gravity, and h= the workpiece dimensions to be analyzed. The magnification range of this comparator ranges from 1250x to 20000x.

Difference between Mechanical Comparator and Pneumatic Comparator

The difference between the mechanical comparator and the pneumatic comparator is discussed below.

Mechanical Comparator Pneumatic Comparator
It is a measuring instrument used to magnify the means’ movement to improve the accuracy of the instrument. It is used to evaluate the dimensional variation between the workpiece to be measured & the standard workpiece.
A mechanical comparator is also known as microcator. A pneumatic comparator is also known as a Solex air gauge or Solex pneumatic comparator.
Mechanical comparator works on pinions, gears linkages, springs, levers, etc. A pneumatic comparator simply works through high-pressure air, back pressure, valves, etc.
As compared to the pneumatic comparator, this comparator is not faster. This comparator is normally faster
It is less precise. It is more precise than a mechanical comparator.
It is not expensive compared to the pneumatic type. It is more expensive.
It needs less maintenance. It needs more maintenance.

How to Select Pneumatic Comparator?

To select a pneumatic comparator, the following points need to consider.

  • The comparator must be compact.
  • It should respond very quickly.
  • It must be easily handled.
  • The comparator must be reliable while using.
  • The comparator must be linear in scale so that it is simple to read & obtain a uniform response.
  • It should give quick results.
  • Its weight should be low.
  • It must be low cost.
  • There must be no effects from the surroundings.
  • Its maintenance must be low.
  • Its design must be strong and must be available very easily in the market.


The advantages of a pneumatic comparator include the following.

  • Its operation is very simple.
  • It is very appropriate to measure the hole’s diameter.
  • It has a high magnification equivalent to 30000: 1
  • There is no intrusion when both the indicating device & measuring head are arranged at different locations.
  • This comparator doesn’t have any contact between the workpiece & the measuring head.
  • An air jet in this comparator helps clean up the workpiece.
  • These are very accurate because of the extremely less number of moving parts so there is extremely less friction & low inertia.
  • While operating this comparator, there is no vibration takes place.
  • This comparator is best to determine the taperness & ovalness of the circular bores.
  • The measuring is extremely small & does not cause any damage to the workpiece. But it assists in cleaning dust from the measured part of the jet of air.


The disadvantages of pneumatic comparators include the following.

  • These are not handy.
  • This comparator is not appropriate for all ecological conditions.
  • It needs extra Auxiliary components like a pressure regulator, air filter, etc.
  • This comparator’s response speed is low compared to the optical comparator.
  • It needs various measuring heads for checking different types of workpieces.
  • This type of comparator needs a compressor.
  • Meniscus error occurs when glass tubes are used like an indicating device.
  • The accuracy of this comparator mainly depends on temperature & humidity change.
  • Different gauging heads are required in this comparator or different dimensions.
  • The scale in this comparator is not consistent.
  • This comparator needs elaborate auxiliary equipment like a precise pressure regulator.
  • It is not economical as compared to mechanical type comparators.


The uses or applications of pneumatic comparators include the following.

  • The pneumatic comparator detects the inside diameter of cylinder workpieces.
  • This comparator is mainly used to discover how much the length changes from the original dimensions.
  • This comparator is utilized once a large number of workpieces or cylinders need to be tested.
  • These comparators are mainly used for fast checking of a huge number of equal dimensions.
  • These are widely used in controlling the component’s size automatically like plug gauges.
  • The workpiece inside & outside diameters can be detected by using this comparator.
  • This comparator is used to analyze Tappers & workpiece ovality.
  • The workpiece’s straightness & flatness can be easily detected.
  • This is very useful for checking the squareness & roundness of the workpiece.
  • These comparators can be applied to inside & outside diameter measurements and thickness measurements.
  • The pneumatic comparator applications changes depending on the gauging head types & the number of orifices.
  • These comparators are very useful in checking the angular parts concentricity, blind hole depth, center distance of hole, parallelism flatness, etc.

Thus, this is brief information on pneumatic comparator – types, advantages, disadvantages, and applications. The most frequently used pneumatic comparator is Solex pneumatic comparator which is designed & marketed by Solex Air Gauges Ltd industry in the USA. So this is one of the most popular pneumatic comparators. Here is a question for you, what is a comparator in electronics?