Page 14 - Personal Study Notes (Engineering Metrology - 22342)

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of a krypton 86 atom in vacuum. This standard can be reproduced with an accuracy of
about 1 part in 109 and can be accessible to any laboratory.

Advantages

1. It is more reliable than metallic standards.

2. It could be access and adopted at any time in all countries without the risk of being
error.

3. It reduces the uncertainty by a factor 50-55.

4. It does not change length and hence provides more accuracy.

5. It provides higher accuracy than material standards for comparative measurement.

6. It can be kept in all standard rooms and physical laboratories by “identical copies”.

Q. Explain the process of wringing of slip gauges.

Wringing is the phenomenon of adhesion of two flat and smooth surfaces when they are
brought into close contact with each other. The force of adhesion is such that the stack
of a set of blocks will almost serve as a single block, and can be handled and moved
around without disturbing the position of individual blocks. More importantly, if the
surfaces are clean and flat, the thin layer of film separating the blocks will also have
negligible thickness. This means that stacking of multiple blocks of known dimensions
will give the overall dimension with minimum error.

Wringing Phenomenon

When two surfaces are brought into contact, some amount of space exists between
them. This is because of surface irregularities and presence of dirt, oil, grease, or air
pockets. Let us assume that the two surfaces are perfectly flat with highly finished
surfaces, free from dirt and oil, and firmly pressed together. Now the air gap becomes
so small that it acts in the same way as a liquid film. The thickness of this film can be as
low as 0.00001 mm. Now a question arises as to why the blocks stick together so firmly

Engineering Metrology (Only For Personal Use) 6