How Calibration of the Instruments is done? How Instruments are Calibrated?
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.