Tuesday, 25 December 2012

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
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
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.

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