Home Measuring instruments Pneumatic Comparators: Types, Principle, Characteristics, Advantages

Pneumatic Comparators: Types, Principle, Characteristics, Advantages

In this post, you’ll learn what is pneumatic comparator and types. It’s working principle, characteristics, advantages and more.

Pneumatic Comparators Principle

In Pneumatic comparators air is utilised as a means of magnification. Pneumatic comparators work on the principle that of an air jet is in close proximity with a surface the flow of air out of that jet are restricted. Which results in the change of pressure in the system supplying the jet.

The set up for explaining the principle of the pneumatic comparator is as shown below:

Principle of Pneumatic Comparator
Principle of Pneumatic Comparator

The arrangement shows a chamber is fitted with control orifice (C) and a gauging orifice (G) through which air flows from a supply at constant pressure (P1).

If the size of the control orifice ‘C’ remains constant, any variation in the size of the gauging orifice ‘G’ will cause an alteration in the pressure (P2) in the chamber. This pressure variation is measured by a pressure gauge of suitable sensitivity either using manometer, Bourdon gauge etc. The size of the gauging orifice (G) varies as the distance of the workpiece from the gauging orifice (G) alters.

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Characteristics of Pneumatic Comparators

  1. Very high amplification of order into 30,000 is possible. Hence it can be used to measure diameters length, squareness, parallelism, concentricity, taper, centre distance between holes etc.
  2. There is no wear of gauge because there is no physical contact either with the setting gauge or part being measured.
  3. The story of size, taper, straightness, camber and bell mouth etc., will be given whenever internal dimensions are measured.
  4. It is independent of operator skill.
  5. High-pressure air gauging can be done with cleaning of the parts which helps to eliminate errors due to dirt and foreign matter.
  6. To prevent part deflection gauging pressures can be kept low.
  7. It is an accurate, flexible, reliable, universal and speedy device for inspecting parts in mass production,
  8. The total cost of the gauging heads is less.

Types of Pneumatic Comparator

Following are the types of pneumatic comparator:

  1. Flow or velocity type
  2. Backpressure type

Classified on the basis of the physical phenomenon on which pneumatic comparator works.

1. Flow or Velocity Type Comparator/Force Flow Air Gauges

This types of pneumatic comparator operate by sensing and indicating the momentary rate of airflow. The flow could be sensed by a glass tube with tapered bore, mounted over a graduated scale, inside the bore afloat is liſted by the airflow.

Schematic of Flow or Velocity Type Comparator
Schematic of Flow or Velocity Type Comparator

In this setup, the compressed air after filtering and pressure regulating unit flow through a glass tube containing small metal float.

The compressed air then flows through a plastic tube, which is located in the opposite direction with two identical holes to escape the air.

The position of the float depends upon the amount of air flowing through the gauge head, which in turn depends upon the clearance between the bore to be measured and the gauging head.

2. Back Pressure Types Pneumatic Comparator

The principle of back pressure pneumatic comparator can be explained by the following setup.

Principle of Back Pressure Type Pneumatic Comparator
Principle of Back Pressure Type Pneumatic Comparator

The air from a constant pressure source lows to the atmosphere through two orifices ‘Oc’ and ‘Om’ as shown in the abo

If P1 is the pressure of the upstream of the first orifice and P2 is the pressure between two orifices both measured with reference to the atmospheric pressure as a datum.

The relationship between P1 and P2 depends upon the relative sizes of two orifices. P2 becomes equal to P1 when Om is blocked and tends to zero as Om is increased indefinitely.

Back Pressure Circuit
Back Pressure Circuit

The figure below shows the basic back pressure circuit, in which a bourdon tube, bellows or diaphragm deflects according to the backpressure change built up in the cult when the workpiece is placed over the measuring head. The deflection is amplified rever and gear arrangement and indicated on a dial.

Solex Pneumatic Gauge

This instrument is produced by Solex Air Gauges Ltd, which is designed for internal measurement, but with suitable measuring head, it can be utilised for external gauging also.

Solex Pressure Gauge

In the above setup, the high-pressure air or compressed air after filtering, through the flow valve. There is a lank in which water is filled up to a certain level and a dip tube is immersed into it up to a depth corresponding to the air pressure required, which is represented by ‘H’.

As the air is sent at a higher pressure than required, some air will leak out from the dip tube and bubble out of the water and the air moving towards control orifice is at desired constant pressure ‘H’.

The air at reduced pressure then passes through the control orifice and escapes from the measuring jets. The backpressure in the circuit is indicated by the head of water displaced in the manometer tube; which is denoted as h. This instrument is able to measure the accuracy of microns.

To measure the concentricity, the workpiece may be revolved around measuring gauge, if there is no change in reading, then it is a perfectly round hole.

Similarly, the diameter can be noted down at several places along the length of bore and thus tapering of a hole is determined.

Thus it is best suited for measuring roundness and taperness of cylinder bores and gun barrel bores.

Advantages of Pneumatic comparators

  1. In pneumatic comparators, the gauging member does not come into contact with the part to be measured and hence practically no wear and tear take places on the gauging member.
  2. These have very less number of moving parts and in some cases none. Thus accuracy is high due to the absence of friction and less inertia.
  3. It is possible to have a very high degree of magnification.
  4. The measuring pressure is very small and the jet of air helps to clean dust, if any, from the part measured by the jet of air.
  5. Very much suitable to measure the diameter of holes when diameters are small as compared to the lengths.
  6. The best method of measuring orality and taperness of a circular bore.
  7. It is possible to have a very high degree of magnification.

Disadvantages of Pneumatic comparators

  1. It requires elaborate auxiliary equipment such as an accurate pressure regulator.
  2. The scale is generally not uniform.
  3. The device is not portable and is rather involved in many industrial applications.

That’s it,

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Saif M
Saif M. is a Mechanical Engineer by profession. He completed his engineering studies in 2014 and is currently working in a large firm as Mechanical Engineer. He is also an author and editor at theengineerspost.com