Pneumatic Comparator

Contents

1. What is pneumatic comparator ?

2. Parts and construction of pneumatic comparator

    2.1 Compressor 
    2.2 Cylinder (or) water tank 
    2.3 Air filter 
    2.4  Pressure regulator 
    2.5 Dip tube 
    2.6 Manometer tube 
    2.7 Control orifice 
    2.8 Flexible tube 
    2.9 Gauging head 
    2.10 Scale 

3. Principle of pneumatic comparator

4. Working of pneumatic comparator
5. Characteristics of pneumatic comparators
6. Types of pneumatic comparator

    6.1 Flow (or) velocity type pneumatic comparator 

    6.2 Back pressure type pneumatic comparator 

    6.3 Differential pneumatic comparator

7. Advantages  of pneumatic comparator 

8. Disadvantages of pneumatic comparator 

9. Applications of pneumatic comparator

 

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. 
• A comparator is an instrument that is used to measure precision of a given component by comparing the dimension of a given component with the actual working standard. 
• Pneumatic means air and in pneumatic comparators, the air is used for magnification of measured reading. 
• Pneumatic comparator work on the principle that if an air jet is in close proximity with a surface then the flow of air out of that jet is restricted which changes the air pressure in the system supplying the jet. 

Animation of Pneumatic Comparator

Parts and Construction of Pneumatic Comparator :- 

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 

 

Fig.1 Pneumatic Comparator

1. Compressor : 

Compressor is the heart of a pneumatic comparator.The compressor is the main component and it plays a major role in the pneumatic comparator.  Compressor is used to compress the air and compressed air is present inside the compressor. 

2. Cylinder (or) 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. After the air passes through the filter, the air pipe divides into two paths. Some air goes into the tank and some air goes to the orifice. 

3. 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.  

4. 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.  

5. 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.  

6. 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. 

7. 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. 

 

8. 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. 

9. 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.  

10. Scale : 

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 : 

It works on the principle of pressure difference generated by the air flow. Air is supplied at constant pressure through the orifice and the air escapes in the form of jets through a restricted space which exerts a back pressure. The variation in the back pressure is then used to find the dimensions of a component.  

Working of pneumatic comparator : 

The working of the 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. 

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.

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Types of pneumatic comparator : 

Basically, the pneumatic comparators are classified into three types : 

1.  Flow (or) velocity type pneumatic comparator 

 
2.  Back pressure type pneumatic comparator 

 
3.  Differential pneumatic comparator 
 

1. Flow (or) velocity type pneumatic comparator :

Fig.2 Flow or velocity type pneumatic comparator

This types of pneumatic comparator (Fig.2) 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.

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 type pneumatic comparator :

Fig.3 Back pressure type pneumatic comparator

The back pressure type pneumatic comparator (Fig.3) 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 O1 and O2. The orifice O1 is called as contro; office and the orifice O2 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 D1 & D2  and the pressure of orifice O1 and O2 is denoted as P1 and P2.The air with constant pressure P1 passes through the orifice O1 into the intermediate chamber. Finally, it escapes from the measuring head through orifice O2. The distance between O2 and the workpiece is denoted as 'd'.

Initially, the measuring orifice O2 will be fully closed. When it is fully closed the pressure P1 and P2 will be equal. When the measuring orifice is fully opened the pressure P1 and P2 at orifice O1 and O2 are equal to zero.

$P_{1}$$O_{2}$$O2 and P_{1}$Initially, the measuring orifice 

$O_{2}$

3.  Differential pneumatic comparator :

Fig.4 Differential 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 OC1 and reaches the measuring head via channel P. Similarly, the air flows through an orifice OC2 and reacges a zero settings valve via channel Q.

$OC_{2}$$P$$OC_{1}$OC1 

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.

Advantages of Pneumatic Comparator :

1. In Pneumatic Comparator, high magnification can be achieved.
2. The gauging member does not come into direct contact with the part to be measured and hence no wear and tear takes place on gauging member or the workpiece.
3. The measuring is very small and does not cause any harm to the workpiece. Instead it helps to clean dust if present from the part measured from the jet of air.
4. Due to very less number of moving parts the accuracy is high as there is very less friction and less inertia.
5. Pneumatic comparator is best for determining the ovalness and taperness of the circular bores.

Disadvantages of Pneumatic Comparators :

1. Different gauging heads are required for different dimensions.
2. The pneumatic comparator can require elaborate auxiliary equipment such as an accurate pressure regulator.
3. The scale is generally not uniform.
4. They are very sensitive to temperature and humidity changes.
5. This device is not easily portable and is rather involved in many industrial applications.

Applications of Pneumatic Comparators :

1. It is used to detect the bore of cylinder workpieces.
2. The inner and outer diameters of the workpiece can be detected.
3. The straightness and flatness of the workpiece can be detected.
4. Tappers and ovality of the workpiece can be easily analyzed.
5. It is also used to check the roundness and squareness of the workpiece.
6. Based on the types of gauging head and number of the orifice, the applications of pneumatic comparator varies.

For better understanding "What is Pneumatic Comparator" plz watch this video carefully.

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