Calibration of the Measuring Instruments

Calibration of the measuring instrument is the process in which the readings obtained from the instrument are compared with the sub-standards in the laboratory at several points along the scale of the instrument. As per the results obtained from the readings obtained of the instrument and the sub-standards, the curve is plotted. If the instrument is accurate there will be matching of the scales of the instrument and the sub-standard. If there is deviation of the measured value from the instrument against the standard value, the instrument is calibrated to give the correct values.

All the new instruments have to be calibrated against some standard in the very beginning. For the new instrument the scale is marked as per the sub-standards available in the laboratories, which are meant especially for this purpose. After continuous use of the instrument for long periods of time, sometimes it loses its calibration or the scale gets distorted, in such cases the instrument can be calibrated again if it is in good reusable condition.

Even if the instruments in the factory are working in the good condition, it is always advisable to calibrate them from time-to-time to avoid wrong readings of highly critical parameters. This is very important especially in the companies where very high precision jobs are manufactured with high accuracy.

All the measuring instruments for measurement of length, pressure, temperature etc should be calibrated against some standard scale at the regular intervals as specified by the manufacturer. There are different methods or techniques of calibration, which are applied depending on whether it is routine calibration or if it is for special purpose where highly accurate calibration of the instruments is desired. In many cases different methods of calibration are applied for all the individual instruments. No what type of calibrations is being done, all of them are done in the laboratory.

The calibration of the instrument is done in the laboratory against the sub-standard instruments, which are used very rarely for this sole purpose. These sub-standards are kept in highly controlled air-conditioned atmosphere so that there their scale does not change with the external atmospheric changes.

To maintain the accuracy of the sub-standards, they are checked periodically against some standard which is kept in the metrological laboratories under highly secured, safe, clean and air conditioned atmosphere. Finally, standards can be checked against the absolute measurements of the quantity, which the instruments are designed to measure.

Here is the procedure for the calibration of mechanical instruments:

1) Firstly, the readings obtained from the scale of the instrument are compared with the readings of the sub-standard and the calibration curve is formed from the obtained values. In this procedure the instrument is fed with some known values (obtained from the sub-standard). These are detected by the transducer parts of the instrument. The output obtained from the instrument is observed and compared against the original value of the substandard.

2) A single point calibration is good enough if the system has been proved to be linear (that is readings from instrument are linear with the substandard), but if it is not, then readings will have to be taken at multiple points.

3) In most of the cases the static input is applied to the instruments and its dynamic response is based on the static calibration.

In some instruments it is not feasible to introduce the input quantity for the calibration purpose like in bonded strain gauges. In such cases the spot calibration is done by the manufacturer. The procedure applied for different types of such instruments is different, which shall be discussed with the individual instruments.

1) Book: Mechanical and Industrial Measurements by R. K. Jain, Khanna Publishers