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

Performance Characteristics of an Instrument

                    Selecting the proper instrument for a particular type of measurand needs the knowledge of the performance characteristics of an instrument.      

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

1.    Static characteristics

2.    Dynamic characteristics 

Static characteristics:

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. The various Static characteristics are accuracy, precision, resolution, error, sensitivity, threshold, reproducibility, zero drift, stability, and linearity.

Accuracy

                    It is the degree of closeness with which the instrument reading approaches the true value of the quantity to be measured. It denotes the extent to which we approach the actual value of the quantity. It indicates the ability of instrument to indicate the true value of the quantity. 

Precision

                    It is the measure of consistency or repeatability of measurements. It denotes the closeness with which individual measurements are departed or distributed about the average of number of measured values. This confirms the fact that high degree of precision does not guarantee the accuracy. It is the accurate calibration that makes the accurate measurement possible.

                    The precision is composed of two characteristics.

1.    Conformity.

2.    Number of significant figures.

Error

                    The most important Static characteristics of an instrument is its accuracy, which is generally expressed in terms of the error called static error.

                    The algebraic difference between the indicated value and the true value of the quantity to be measured is called an error.

Mathematically it can be expressed as,

E=At-Am

                     

Where          e      = Error

                    Am  = Measured value of the quantity

                    At    = True value of the quantity

In this expression, the error denoted as ‘e’ is also called absolute error .

Sensitivity 

                    The sensitivity denotes the smallest change in the measured variable to which the instrument responds. Ti is defined as the ratio of the changes in the output of an instrument to a change in the value of the quantity to be measured.

                    Mathematically it is expressed as

                                              Infinitesimal change in output

                        Sensitivity =  

                                               Infinitesimal change in input  

Sensitivity

                      

                       

Resolution

                    It is the smallest increment of quantity being measured which can be detected with certainty by an instrument. Thus, the resolution means the smallest measurable input change.

                    So if a non-zero input quantity is slowly increased, output reading will not increase until some minimum change in the input takes place. This minimum change which causes the change in the output called resolution. The resolution of an instrument is also referred to as discrimination of the instrument. The resolution can affect the accuracy of the measurement.

Threshold

                    If the input quantity is slowly varied from zero onwards the output   does not change until some minimum value of the input is exceeded. This minimum value of the input is called threshold.

                    Thus the resolution is the smallest measurable input change while the threshold is the smallest measurable input.

Linearity

                    The instrument requires the property of linearity that is the output varies linearly, according to the input. The linearity is defined as the ability to reproduce the input characteristics symmetrically and linearly. Graphically such relationship between input and output is represented by a straight line.

Linearity

                    The graph of output against the input is called the calibration curve. The linearity property indicates the straight line nature of the calibration curve.

The linearity is defined as the maximum deviation of the actual calibration curve (output) from the idealized straight line, expressed as a percentage of full scale reading or percentage of the actual reading.

It is desirable to have an instrument as linear as possible as the accuracy and linearity are closely related to each other.                   

Zero drift

                    The drift is the gradual shift of the instrument indication, over an extended period during which the value of the input variable does not change.

The zero drift is defined as the deviation in the instrument output with time, from its zero value, when the variable to be measured is constant. The whole instrument calibration may gradually shift by the same amount.