READABILITY,CALIBRATION & FLATNESS MEASUREMENT

READABILITY Readability refers to the susceptibility of a

measuring device to having it’s indications converted to a meaningful number .

Readability is defined as the closeness with which the scale of analog instrument can be read .

Eg:A micrometer instrument can be made more readable by using vernier .

Very finely spaced lines may make a scale more readable when a microscope is used , but for the unaided eye , the readability is poor .

Contd…For getting better readability , the instrument

scale must be as high as possible . Then the reader can observe the reading accurately.

The size of the pointer also should be larger with more accurate end conditions i.e, at the end pointer should be sharp .Due to which the parallax error is minimized .

CALIBRATION

Calibration is the process of determining and adjusting an instrument’s accuracy to make sure its accuracy is within the manufacturer’s specifications.

If a known input is given to the measurement system the output deviates from the given input, the corrections are made in the instrument and then the output is measured. This process is called calibration.

It is carried out by making adjustments such that readout device produces zero output for zero measurand input.

ELEMENTS OF METROLOGY(SWIPE)

1) Measuring Instruments 2) Calibration Standards 3) Work piece 4) Person who is carrying out the measurement 5) EnvironmentThe above said five elements composed into the acronym SWIPE.Where, S —Standard W —Work piece I —Instrument P— Person E —Environment

The factors affecting these five elements:

1. Standard: - Affected by Temperature, time, thermal expansion and elasticity. 2. Work piece: - Surface finish, cleanliness, supporting elements, and elastic properties. 3. Instrument: - Friction, error. mechanical parts. 4. Person: - Ability to measure, training, cost estimation. 5. Environment: - Light. Temperature, Humidity.

FLATNESS TESTING USING INTERFEROMETER

The essential equipment for measurement by light wave interference is a monochromatic light source and a set of optical flats.

An optical flat is a circular piece of optical glass or fused quartz having its two planes faces flat and parallel and the surfaces are finished to an optical degree of flatness.

If an optical flat placed upon another flat reflecting surface (without pressure) it will not for an intimate contact, but it will lie on some angle θ making an inclined plane.

If the optical flat be now illuminated by monochromatic source of light, the eye if placed in proper position will observe a number of bands. These are produced by the interference of the light rays reflected fro the lower surface of the top flat and the top surface of the lower flat through a very thin layer of air between the flats.

S is the source of monochromatic light. At point A, the wave of incident beam from S is partially reflected along AB and is partially transmitted across the air gap along AC. At C, again the ray is reflected along CD and passes out towards the eye along CDE. Thus two reflected components, reflected at A and C are collected and recombined by the eye, having travelled paths whose lengths differ by an amount ACD.

AC and FH = 3 λ/4 - λ/4 = λ/2

Checking of flatness by above method

In practice, on the surface to be tested, we have large number of hills and valleys and in these circumstances following figure will show the behaviour of optical flat if it is made to rest on the hills. If optical flat is resting on a hill then it will just behave as if it were placed on some spherically convex surface. In such case the contact is made at the central high point and in centre a bright circle will be visible. Around it there will be concentric dark and bright circular fringes.

As the distance from the centre increases the separation between the optical flat and the surface keeps on increasing and the fringes become narrow and more closely spaced.

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