ERRORS AND CALIBRATION OF INSTRUMENTS

Errors in measurement

 

1   Measurement error:

Measurement error is the difference between the indicated and actual values of the measurements. During  measurement, several types of errors may arise and these errors could be classified into two categories.

1. Systematic errors (controllable errors )
2. Random errors.
   
   

2   Systematic error

These  are  controllable in both their magnitude and sense. These can be determined and reduced, if attempts are made to analyse them. These are also known as controllable errors. These can be due to

        1.calibration errors

        2.Ambient conditions

        3.stylus pressure

        4.Avoidable errors

2.1.   Calibration errors

The actual length of standards such as slip gauges and engraved scales will vary from nominal value by a small amount

2.2   Ambient conditions

Variations in the ambient conditions from internationally agreed standard value of temperature 200C, barometric pressure 760 mm of mercury, etc. can give rise to errors in the measured size of the component. Temperature is by far the most significant of these ambient conditions and due correction is needed to obtain error free results.

2.3   Stylus pressure

Error induced due to stylus pressure is also appreciable. Whenever any  component is measured under a definite stylus  pressure both the deformation of the work piece surface and  defection of the work piece  shape will occur

2.4   Avoidable errors

These errors include the error due to parallax and the effect of misalignment of the work piece centers. Instrument location errors such as placing a thermometer in sunlight when attempting to measure air temperature also belong to this category.

3   Random Errors

    These occur randomly and specific cases of such errors cannot be determined. Eg: Errors due to slight displacement of  lever joints, fluctuation in friction in the instrument and operators error in reading scale. Random errors are those, which are identical, and whose magnitude and size cannot be predicted. From the above, it is clear that systematic errors are those, which are repeated consistently with repetition of the experiment, where as random errors are those, which are accidental, and whose magnitude and sign cannot be predicted from knowledge of measuring system and conditions of measurements

4   Calibration

Every measuring instrument must be provable i.e. it must be caused to prove it’s ability reliably. The procedure for this is calibration. In order to maintain the precision and accuracy of measuring device it’s periodical calibration is essential because the moment an instrument is put into use it begins to deteriorate in it’s accuracy. It is also important that the calibration standard for the system should be at least one order of magnitude more accurate than the desired measurement system accuracy. I.e. Ratio of 1 : 10

5   Calibration Procedure

    In order to maintain the accuracy of the measuring instruments the following procedures should be followed.

1. Each instrument should be numbered.

2. A card record should be established for each instrument.

3. Checking interval should be established.

4. Some system should be adopted providing adherence to the checking schedule.

5. The record of the findings of the check should be maintained.

6. The record of the checks should be further studied and analysed to improve upon the system.

6   Calibration of Vernier Caliper

    The following table gives the allowable deviation in the parameters

Parameter	Permissible error
	Least Count – 0.02	Least Count – 0.05
Zero error	0.02	0.05
Flatness of measuring jaws	0.003	0.004
Parallelism of measuring jaws	0.01	0.015
Error in reading	0.02	0.05

The zero error is checked by bringing the jaws in contact and the shift of zero of main scale is observed with respect to zero of vernier scale.

The flatness of the measuring jaws is checked using a straight edge of class 1 accuracy. The straight edge is put over the surfaces and the light gap is observed.

The parallelism is checked by inserting a slip gauge of any value between the jaws at various positions and determining the out of parallelism using slip gauges.

Error in readings along the entire range is also found out using slip gauges. In case of vernier calipers having spherical inside measuring jaws, the width of spherical portion is checked using a pass meter.

7   Calibration of micrometer :

   

In case of micrometers the following are the main points to be checked.

1. General appearance and relative movement of moving parts.

2. Checking initial zero setting for micrometer.

3. Flatness of measuring surfaces.

4. Parallelism of measuring surfaces.

5. Error at different positions.

In general appearance, the micrometer is thoroughly checked for presence of scratches, dents etc. on measuring jaws as well as for corrosion marks, scratches, dents etc. on the surfaces of measuring drums, for proper working of ratchet system.

      

The relative movement of moving parts is also checked which should be smooth. The working of lock system is also checked.

      

The zero error of micrometer is checked and if it is found wrong it is adjusted easily for micrometers of size 25 - 50mm and more. The size of the setting piece is checked on interferometer or any other comparator set to read up to 0.0001mm.The permissible error allowed in its size is 0.001mm for micrometers up to size of 100mm.

       

The flatness error is checked by keeping optical flat on each jaw. The maximum permissible error is 0.0009mm.

      

The parallelism error is also checked using optical flat .The permissible error in parallelism is 0.002mm for micrometers up to 100mm size and 0.004mm for micrometers above 100mm and up to 200mm size.

       

The error in readings is checked using slip gauges so as to cover the entire range. The maximum permissible error is 0.004mm for class one micrometers and 0.008mm for class two micrometers.