Pneumatic Comparator (Solex Air Gauge)

Pneumatic comparator (Pdf): Parts, types, working principle, applications, advantages & disadvantages

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What is pneumatic comparator?

A pneumatic comparator is a precision device operated using a pneumatic system or compressed air. Like other mechanical, optical, and electrical comparators, pneumatic comparators are also used to analyze the dimensional difference between the workpiece to be measured and the standard workpiece. In a pneumatic comparator, compressed air or pressurized air is used as a working medium. Therefore it has a lot of advantages when compared to other comparators. Since it has a lot of advantages it is highly preferred over other comparators in most cases.

Solex pneumatic comparator (or) Solex air gauge

Solex air gauge is the most popular pneumatic comparator and it has been developed and introduced commercially by a USA instrument manufacturing company called Solex air gauge Ltd. Among other pneumatic comparators, this is the most popular one.

In this article, we are going to see about their parts & construction, working principles, types, applications, advantages, and disadvantages.

Now let’s see about the parts and construction of pneumatic comparator.

Parts & construction of pneumatic comparator

Pneumatic Comparator (Solex Air Gauge)
Pneumatic Comparator (Solex Air Gauge)

The Solex air gauge or pneumatic comparator is constructed using some major parts. These important parts are listed below.

  • Compressor
  • Cylinder (or) water tank
  • Air filter
  • Pressure regulator
  • Dip tube
  • Manometer tube
  • Control orifice
  • Flexible tube
  • Gauging head
  • Scale


The compressor is the main component and it plays a major role in the pneumatic comparator. The compressed air in the pneumatic comparator is produced and continuously supplied by the compressor.

Air filter

The air filter is fitted next to the compressor unit. The main purpose of an air filter is to filter the dust particles from the air which is compressed by the compressor. This is the main function of the air filter in a pneumatic comparator.

Pressure regulator

The pressure regulator unit is placed along with the air filter. It’s main function is to regulate the pressure of compressed air which comes from the air filter.

Dip tube

The dip tube in the airline is placed after the pressure regulator and it is dipped straight towards the water tank or metal cylinder. The junction of the dip tube and airline is called an upper chamber.

Water tank

The water tank in the pneumatic comparator is a metal cylinder placed at the bottom of the upper chamber which holds the dip tube in it.

Manometer tube

The manometer tube is fitted at the bottom of a water tank or metal cylinder. This tube is placed vertically parallel to the water tank.

Control orifice

The control orifice in the airline is placed between two junctions of the dip tube and manometer tube. Air is supplied at constant pressure through the orifice.

Flexible tube

A flexible tube or pneumatic hose is used in the airline to hold the measuring head. The starting point of the flexible tube is fitted with a second chamber (junction of manometer & airline) and the ending point is connected with a measuring or gauging head.

Gauging head

A measuring gauge or gauging plug is connected to the flexible tube or pneumatic hose. It is used to measure the irregularities in the workpiece. There are several types of measuring gauges available.

Types of gauging head
Types of gauging head


The measuring scale is calibrated and placed parallel to the manometer tube. It is used to measure the fluid displacement that occurs in the manometer tube.

Principle of pneumatic comparator

Pneumatic comparator works on the basic principle of pressure difference produced in the airflow. The air at constant pressure flow through the workpiece and creates back pressure. This back pressure variation helps to find the dimension of the workpiece.

Now let’s see how a pneumatic comparator works?

Working of pneumatic comparator

The working of the Solex pneumatic comparator is discussed below.

  • The pneumatic comparator consists of a water tank or metal cylinder filled with water. The water in the cylinder is filled up to a certain level.
  • Along with a water tank, a Calibrated manometer is fitted vertically parallel to the water tank.
  • The level of water in the water tank and manometer should be the same and it is calibrated using a standard workpiece.
  • Air is compressed at high pressure using a compressor and it is filtered and pressure regulated by passing through the air filter and pressure regulator.
  • The filtered air passes through a dip tube which is immersed in a water tank filled with water.
  • Simultaneously the compressed air with the same pressure passes through the control orifice gap.
  • After air passing through the control orifice velocity is increased and pressure becomes constant.
  • The air with huge velocity passes through the flexible tube or pneumatic hose and finally reaches the measuring head.
  • Due to air expansion in the dip tube the head of water (H) is maintained constant. The excess air will escape as air bubbles in the water tank.
  • The air at constant pressure escapes from measuring jet in measuring gauge or measuring head.
  • If the workpiece is standard or there is no restriction in the workpiece, the air will continuously escape through the measuring jet. Simultaneously the level of water in the manometer tube and water tank will coincide.
  • If there is any irregularities or restriction in the workpiece for real flow in measuring jet, certain back pressure will be created.
  • The water level in the manometer will fall due to the back pressure induced by the restrictions in the workpiece.
  • The change of water level in the manometer is denoted as the dimensional difference or any irregularities in the workpiece being measured when compared to the standard workpiece.

Types of pneumatic comparator

Basically, the pneumatic comparators are classified into three types.

  • Flow (or) velocity type pneumatic comparator
  • Back pressure type pneumatic comparator
  • Differential pneumatic comparator

Flow (or) velocity type pneumatic comparator

Flow (or) Velocity Type Pneumatic Comparator
Flow (or) Velocity Type Pneumatic Comparator

The parts of flow type pneumatic comparator are compressor, filter, shut off valve, pressure regulator, glass column, zero adjustment screw, air bleed, scale, float, rubber hose, and gauging head. Initially, the tapered glass column is calibrated to the required dimension with the help of zero adjustment screw air bleed, and slip gauges.

The air is compressed, air filtered and regulated as same as in the Solex air gauge. Then it passes through the shut-off valve and reaches a glass column that contains a metal float. The shut-off valve in this pneumatic circuit is used to shut off the air supply when not in use. The air flows through the air column and finally escapes from measuring or gauging head.

The measuring head is inserted in a workpiece to be analyzed and there should be a small clearance between the measuring head and workpiece. If any irregularities or restrictions exist on the workpiece, the airflow will get restricted and this results in a small displacement of the metal float inside the class column.

The movement of the metal float can be controlled by the rate of airflow in the glass column which is determined by the clearance between the measuring head and workpiece. Therefore the airflow rate is directly proportional to the clearance.

The accuracy of this comparator is up to $ \displaystyle 1\mu m$. The magnification of this comparator is 1000,000:1.

Back pressure type pneumatic comparator

Back Pressure Type Pneumatic Comparator
Back Pressure Type Pneumatic Comparator

The back pressure type pneumatic comparator is constructed using the following parts such as compressor, filter, pressure regulator, adjustable restrictor, scale, and measuring head. This pneumatic comparator consists of two orifices $O_{1} $ and $O_{2} $. The orifice $O_{1} $ is called as control office and the orifice $O_{2} $ is called as measuring orifice.

Like the previous type, the compressed air is filtered by an air filter and passes through the pressure regulator in the circuit. Here the pressure is reduced to 2 bars and it passes through the control orifice. Finally, the air escapes from the measuring orifice in the measuring head or gauging head.

In measuring head, the diameter of two orifices is denoted as $D_{1} $ & $D_{2} $ and the pressure of orifice $O_{1} $ and $O_{2} $ is denoted as $P_{1} $ and $P_{1} $. The air with constant pressure $P_{1} $ passes through the orifice $O_{1} $ into the intermediate chamber. Finally, it escapes from the measuring head through orifice $O_{2} $. The distance between $O_{2} $ and the workpiece is denoted as $’d’$.

Initially, the measuring orifice $O_{2} $ will be fully closed. When it is fully closed the pressure $P_{1} $ and $P_{2} $ will be equal. When the measuring orifice is fully opened the pressure $P_{1} $ and $P_{2} $ at orifice $O_{1} $ and $O_{2} $ are equal to zero.

The pressure $P_{1} $ and $P_{2} $ at orifice $O_{1} $ and $O_{2} $ varies when the distance between measuring orifice and workpiece varies. The variation in pressure $P_{1} $ and $P_{2} $ is measured with the help of a pressure indicating device. The magnification achieved in this comparator is up to 7500:1.

Differential type pneumatic comparator

Differential Type Pneumatic Comparator
Differential Type Pneumatic Comparator

This comparator is a type of backpressure pneumatic comparator. The main parts used for its construction are pressure regulator, air filter, compressor, control orifice, reference jet or zero setting valve, pressure indicating device, and measuring head. In this comparator, the air is compressed and allowed to pass through the air filter and regulator. Finally, the air pressure is produced and made constant.

The air at constant pressure passes through the split channel. At one end, the air flows through orifice $OC_{1}$ and reaches the measuring head via channel $P$. Similarly, the air flows through an orifice $OC_{2}$ and reaches a zero setting valve via channel $Q$.

The pressure indicating device is placed in the airline which interconnects both channel $P$ and $Q$. Initially, it is calibrated with a standard workpiece. When the measuring head approaches the workpiece the pointer in the measuring device starts to deflect. If there is any restriction or the clearance between the measuring head and workpiece decreases, the pressure within the system increases. Finally, deflection takes place.

$\displaystyle P=\rho gh$ is used to convert pressure readings from the indicating device into the linear dimensions.

  • $P =$ Pressure indicated by the device
  • $ \displaystyle \rho =$ Air density
  • $g =$ Specific gravity
  • $h =$ Dimensions of the workpiece to be analyzed

The magnification range of differential back pressure pneumatic comparator is from 1250 $ \displaystyle \times $ to 20000$ \displaystyle \times $.

Advantages of pneumatic comparator

  • Pneumatic comparators are easy to operate
  • There is no contact between the measuring head and the workpiece.
  • Wear does not take place on the measuring head because there is no contact.
  • It is very accurate and precise.
  • High magnification up to 30000: 1 is possible in a pneumatic comparator.
  • Since indicating device and measuring head are placed at different places, there is no interference.
  • They are self-cleaning because a jet of air helps in cleaning the workpiece.
  • It has a tendency of self-aligning and centring.
  • No vibration occurs while operating a pneumatic comparator.

Disadvantages of pneumatic comparator

  • Pneumatic comparators are not portable.
  • This comparator requires a compressor.
  • It is not suitable for all environmental conditions.
  • The change in temperature and humidity can affect their accuracy.
  • While comparing electronic comparators, pneumatic comparators are slow in response.
  • Meniscus error can occur while using glass tubes as an indicating device.
  • In a pneumatic comparator, different measuring heads are required for inspecting different kinds of workpieces.
  • Axillary components such as air filter, pressure regulator, etc additional required.
  • It is not cheap in cost when compared to mechanical comparators.
  • The scale is not uniform in the pneumatic comparator.

Applications (or) uses of pneumatic comparator

  • It is used to detect the bore of cylinder workpieces.
  • The inner and outer diameters of the workpiece can be detected.
  • The straightness and flatness of the workpiece can be detected.
  • Tappers and ovality of the workpiece can be easily analyzed.
  • It is also used to check the roundness and squareness of the work[iece.
  • Based on the types of gauging head and number of the orifice, the applications of pneumatic comparator varies.
Type of gauging headNo of orificesApplications/Uses
Type A1To analyze height, length, squareness, straightness, flatness
Type B2To analyze roundness, taperness, inner diameter, and bellmouth
Type C3To analyze pseudo roundness
Type D4To analyze the average diameter of a workpiece in a single setup
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