Reproducibility:

reproducibility

                    It is the degree of closeness with which a given value may be repeatedly measured. It may be specified interms of units for a given period of time. The perfect reproducibility indicates no drift in the instrument.

The repeatability is defined as variation of scale reading and in random in nature. Both reproducibility and the repeatability are measure of the closeness with which a given input may be measured again and again. The fig shows the input and output relationship with positive and negative repeatability.

Stability

                    The ability of an instrument to retain its performance throughout its specified operating life and storage life is defined as its stability.

Tolerance:

                    The maximum allowable error in the measurement is specified interms of some value which is called tolerance. This is closely related to accuracy.

The tolerance indicates the maximum allowable deviation of a manufactured component from a specified value.

Rang of span

       

                    The minimum and maximum value of a quantity for which an instrument is designed to measure is called its range or span. Sometimes the accuracy is specified interms of range or span of an instrument.

Bias

                    The constant error which exists over the full range of measurement of an instrument is called bias. Such a bias can be completely eliminated by calibration. The zero error is an example of  bias which can be moved by calibration

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Hysteresis

If the input to the instrument is increased from a negative value, the output also increases. But the curve is now decreased steadily. The output does not follow the same curve but lags by certain value. It traces the curve 2. The difference between the two curves is called hysteresis. The maximum input Hysteresis and maximum output Hysteresis are shown. These are generally expressed as the percentage of the full scale reading. 

Instrument with hysteresis

                                                                                                          

Dead space

                    In some instruments, it is possible that till input increases beyond certain value, the output does not change. So for certain range of input values there is no change in output. This range of input is called dead space. There is possibility that instrument without hysteresis may show the dead space in their output characteristics. Backlash in gears is a good example which causes the dead space.

Span drift or sensitivity drift

                    If there exists a proportional change, in the indication, all long the upward scale then the drift from nominal characteristics is called span drift or sensitivity drift.

Span drift