Static Characteristics of Instruments

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:

1. Accuracy 
2. Sensitivity
3. Repeatability
4. Reproducibility
5. Precision
6. Resolution 
7. Threshold

1. Accuracy :- It is closeness with which an instrument reading approaches the true value of the variable being measured.The measured quantity may be different from the true value due to the effects of temperature, humidity etc.
2. Sensitivity :- It is the instrument of the output signal (or response) of the instrument to a change of input measured value & can be expressed as:

S = dy/dx

where, 

                S =  Sensitivity

               dy = Change instrument output value 

               dx = Change in input true value 

3. Repeatability :- Repeatability is the variation in measurement taken on the same item under the same conditions. 

4. Reproducibility :- It is the degree of closeness with which a given value may be repeatedly measured. It is specified in terms of scale readings over a given period of time.

5. Precision :- Precision is define as the ability of instrument to reproduce a certain set of readings within given accuracy. Precision describes an instruments’ degree of random variations in its output when measuring a constant quantity. Precision depends upon repeatability.

6. Resolution :- It is the minimum change or smallest increment in the measured value that can be detected with certainty by the instrument. It can be least count of instrument. 

7. Threshold :- If the instrument input is increased very gradually from zero there will be some minimum value below which no output change can be detected. This minimum value defines the threshold of the instrument.

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