Performance Characteristics

Performance Characteristics

The performance characteristics of an instrument are mainly divided in two categories

i) Static characteristics and

ii) Dynamic characteristics

Some applications involve measurands which are either constant or varying very slowly with time.

Key Point The set of criteria defined for the instruments, which are used to measure the quantities which are slowly varying with time or mostly constant, i.e. do not vary with time, is called static characteristics.

While, when the quantity under measurement changes rapidly with time, it is necessary to study the dynamic relations existing between input and output. These relations are generally expressed with the help of differential equations.

Key Point The set of criteria defined based on such dynamic differential equations is called dynamic characteristics.

The various performance characteristics are obtained in one form or another by a process called calibration.

1. Calibration.

Key Point  calibration is the process of making an adjustment or making a scale so that the readings of an instrument agree with the accepted and the certified standard.

In other words, it is the procedure for determining the correct values of measurand by comparison with the measured or standard ones. The particular instrument is compared with either a primary standard, secondary standard with higher accuracy or an instrument with known accuracy.

The calibration offers a guarantee to the device or instrument that it is operating with required accuracy, under the stipulated environmental conditions. It creates the confidence of using the properly calibrated instrument, in user's mind. The periodic calibration of an instrument is very much necessary.

The calibration procedure involves the steps like visual inspection for various defects, installation according to the specifications, zero adjustment etc.

The calibration characteristics can be determined by applying known values of quantities to be measured and recording the corresponding output of the instrument. Such output values are then compared with the input, to determine the error. Such a record obtained from calibration is called calibration record. It is generally recorded in the tabular form. If it is represented in the graphical form, it is called calibration curve. Such a calibration record or calibration curve is useful to obtain the performance characteristics of an instrument. The performance of the instrument is not guaranteed by the calibration. It only indicates whether the performance of the instrument is meeting the accuracy and range specification or not. If the device has been repaired, aged, adjusted or modified, then recalibration is carried out.

Review Question

1. Define and explain the terrm 'Calibration'.