Definition:

“Calibration is the comparison of measurement values
delivered by a device under test with those of a calibration standard of known
accuracy. Such a standard could be another measurement device of known
accuracy, a device generating the quantity to be measured such as a voltage, or
a physical artefact, such as a metre ruler.” (Wikipedia)

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When is it needed?

Calibration is needed when we manufacture new instruments or
when an instrument has been repaired. It is also needed after certain hours of
operation of an instrument. It maybe needed if the observations are
questionable.

Why is it Important?

Calibration shows the accuracy and quality of readings
recorded using an instrument. Certain quantity like length tend to change with
temperature. So, we need to make sure that we get correct values of same
quantity over time and reliable readings are needed for industry standards. The
main aim of calibration is to assure the accuracy of the instruments and
readings.

For example:
If we need the length of a particular component to be 50mm with tolerance of ±.05mm
but our instrument gives different readings, then we cannot get accurate results.
Thus, calibration is required.

(Tempcon Instrumentation, 2012)

Calibration of Outside Micrometer:

Purpose:

The aim of this procedure is to give general instructions for
the calibration of outside micrometers along with digital, Vernier, or
combination scales.

Prior Instructions and
Notes:

We conduct calibration in a place that adheres to given specifications.
We will now refer the outside micrometer as micrometer.

The first step is to assure that the micrometer doesn’t have
any dust. Then we check the micrometer for any condition that could create
errors in the process. If there is a need to make adjustment, we do it and make
sure that there is no damage done to the instrument.

If there happens to be a defect or malfunction while calibrating,
we stop the calibration and make necessary corrective steps. If corrective steps
affect a measurement that we took before, the value maybe recalibrated before
the rest of the procedure is done.

 

Figure 1: Outside Micrometer

(Source: http://www.starrett.com/metrology/product-detail/Outside-Micrometers/Micrometers/Precision-Hand-Tools/Precision-Measuring-Tools/v230mxfl
)

Equipment Required:

The following standard equipment is used: Hard Arkansas stone,
Lint free clean cloth, cleaning solution, Gage oil and Gage block set.

Detailed Procedure:

1)The First step is to use a clean leant free cloth along with
cleaning solution to clean the outside surfaces of the micrometer.

2) The micrometer is closed by a friction stop

3) A source of light is kept nearby to visually inspect the
micrometer.

4) The spindle is then disassembled.

5) The screw and oil spindle is removed carefully.

6) We have to clean the nut which is fixed in the barrel and
oil it.

7) Now we can assemble the micrometer back.

8) The measuring screw can be checked for damage by moving the
measuring screw forward and backward. For a perfect movement they should be
aligned.

9) We can compensate for the error if found by pressing the nut
on barrel to make it tight. This adjustment should be made to get a perfect
fit.

10) Now the micrometer can be closed and check for zero fit.

11) The accuracy of the micrometer can be checked by using a gauge
block. A variety of sizes in gauge block can be used to do this. Readings might
be taken multiple times for accuracy.

12) For using the gauge blocks we first combine the gauge blocks
to a required size and press them tightly, then measure them with micrometer
and compare the values.