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.
Parts of Pneumatic Comparator:-
Compressor is the heart of a pneumatic comparator. Compressor is used to compress the air and compressed air is present inside the compressor.
After the compressor, the filter is present. In this filter, all the dust particles present in the air are separated from the air and clean air passes through this filter.
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.
This orifice leads to the second chamber.
The second chamber has an outlet to a glass manometer tube.
The second chamber is also connected by a flexible tube to the gauging head. This gauging head has two restricting jets that are used to check unevenness or irregularities in the workpiece.
Working of Pneumatic Comparator:-
- In this comparator, compressed air is used as an operating medium. At first, the air is compressed in the air compressor,
- Then the compressed air from the compressor passes through the air filter.
- All the dust particles present in the air are separated from the air in the air filter and only pure passes through the filter.
- After passing from the filter, the air pipe is divided into two ways and some air goes to the tank and some air goes to the restricted jet or orifice.
- As air enters the water tank, bubbles will be created in the tank and the rest of the air which passes through the orifice reaches the gauging head through a flexible tube.
- A controlled amount of air pass through the orifice with the required pressure in it.
- At this time the manometer shows the initial pressure by which the air passes to the gauging head.
- The workpiece or the job is reciprocated along the gauging head.
- This gauge head is generally used to examine the internal diameter of a cylinder whether it has irregularities or not in its internal diameter.
- This gauge head is inserted into the cylinder and both jets in the gauge head face towards the internal wall of the cylinder.
- The internal wall of the cylinder restricts the air coming out of the jet of the gauging head.
- Based on the gap between the jet and internal wall of the cylinder, back pressure will be created as the internal wall restricts the air coming from the jet.
- When the backpressure is created, the air which is passing through the gauging head will return back and it will go inside the manometer tube and this backflow of air will push down the water inside the manometer.
- At first, a standard workpiece is taken in which the internal wall is perfectly flat. This standard workpiece is used to calibrate the pneumatic comparator. Using this standard workpiece the water height is the manometer is adjusted so that it is equal to the height of water in the tank. This standard workpiece is known as a reference workpiece because it is used to get the reference pressure in the manometer.
- When the gauge head of the pneumatic comparator is placed inside a cylinder which is to be examined. The air hits the internal wall of the cylinder and if there are any irregularities in the internal wall the backpressure in the comparator is created and the water inside the manometer is pushed down. Hence, the water level inside the manometer falls and we get a height difference between the water level of tank and water level of the manometer. This height difference is denoted by ΔH.
- So if the ΔH value is high then we can say that more amount of roughness is there at the position of the cylinder where the gauge head is placed.
- After that the gauge head is placed in different positions inside the cylinder and ΔH is noted for different points so that roughness throughout the cylinder can be measured.
- If ΔH is high for different points inside the workpiece then this workpiece needs to be rejected.
- Pneumatic comparator is used when a large number of cylinders or workpiece needs to be tested i.e when there is the mass production of the workpiece and quality inspection needs to be done.
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.