Characteristics of an Measuring Instrument
characteristics of an measuring instrument are mainly divided into two parts,
- Static characteristics
- Dynamic characteristics
The set of criteria defined for the instruments, which are used to measure the quantities which are slowly varying with time is called ‘static characteristics’.
Accuracy :- the accuracy of a measurement indicates the nearness value to the actual value of quantity. It can be expressed by different ways are following,
Percentage of actual value – the accuracy is expressed in terms of the percentage. Example… 0.3% of the actual value.
Pony accuracy – it is defined at only the instruments scale. Hence, it does not give info abovo the accuracy at other point on the scale.
Precision :- it is the measure the the degree to which successive measurement differ from each other. By this we can get fixed value of variable.
Error :- It is the algebraic different between the actual value and measured value.
Its involved the comparison of unknown quantity with an standw quantity. There are different types of error. Gross error, systematic error, random error, schematic error.
Repeatability :- it is the defined as the variation of the scale readings. It is random in nature. Repeatability is measure of closeness with which a given input can be measure over and over again.
Reproducibility :- it is defined as the degree of closeness by which a given value can be repeatedly measured.
Drift :- drift is defined as the gradual shift in the induction over a period of time where in the input variable does not change.
Its divided in three parts, zero drift, sensitivity drift, zonal drift.
Sensitivity :- it is the ratio of Change in output of an instruments to the change input.
The sensitivity of an instruments should be as high as possible.
Dead Zone :- it’s largest change in input quantity for which there is no output.
Dead time :- Is the time before which instruments starts to responds after the output has been changed.
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.
Resolution :- If we measured some value and we do minor change in that measure value, instruments will respond. it is smallest increments in the input value that can be detected by the instrument.
Stability :- It is the ability of an instrument to retain its performance throughout is
specified operating life.
Tolerance:- The maximum allowable error in the measurement is specified in terms of some value which is called tolerance.
The set of criteria defined for the instruments, which are changes rapidly with time, is called ‘dynamic characteristics’.
Speed of response :– its defined as the when we changed in input value and how much rapidly we get the change in output value by instruments. by this we can get the system is how much fast and active.
Measuring lag:- It is the retardation or delay in the response of a measurement system to changes in the measured quantity. The measuring lags are of two types:
1) Retardation type:- In this case the response of the measurement system begins immediately after the change in measured quantity has occurred.
2) Time delay lag:- In this case the response of the measurement system begins after a dead time after the application of the input. Fidelity: It is defined as the degree to which a measurement system indicates changes in the measured quantity without dynamic error.
Dynamic error:- It is the difference between the true value of the quantity changing with time & the value indicated by the measurement system if no static error is assumed. It is also called measurement error.
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