Important Instrumentation Definitions

There are two different terms which should be considered in the instrumentation, they are static terms and dynamic terms.

Static Terms:


An instrument is calibrated to read values of physical variable being measured between two values. One is the higher calibration value Hc and the other is the lower calibration value Lc. This region between which the instrument is to operate is called range.

Range = Lc to Hc.

We can say that the range of the instrument (thermometer) is 0’C to 100’C.


Span is the algebraic difference between the higher calibration value and the lower calibration value.

Span = Hc to Lc.

Example: if the range of the instrument is 100’c to 150’c, its span is 150’-100’ = 50’c.


if an instrument does not reproduce the same reading at different times of measurement for the same input signal, it is said to have drift. It an instrument has perfect reproducibility, it is said to have no drift.


A known input is given to the measurement system and the systems output is noted. if the systems output deviates with respect to the given known input, corrections are made in the instrument so that the output matches the input. This process is called calibration.


All the energy put into the stressed component when loaded is not recovered upon unloading. Hence the output of a measurement system will partly depend o its previous input signal and this is called as hysteresis.

Dead Zone:

Dead zone id the largest change in the physical variable to which the instrument doesn’t respond. That is, the region upto which the instrument doesn’t respond for an input is called the deadzone.


Sensitivity = change in the output signal/change in the input signal.

Threshold Value:

The minimum value of input signal that is required to make a change or start from zero is called as threshold value.


The minimum value of the input signal required to cause an appreciable change or an increment in the output is called resolution.

Back lash:

It is the maximum distance through which one part of the instrument may be moved without disturbing the other part.

Relative error:

The ratio of the static error to the true value expressed as a percentage is called as relative error.

Dynamic Terms:

Dynamic Measurement

Dynamic Measurement means that the measuring system (instrument) is required to measure an input which is varying with time.

Speed of Response or Responsiveness

The speed of response of measuring instrument is defined as the quickness with which an instrument responds to a change in the output signal.

Measuring Lag:

It is delay in the response of an instrument to a change in the input signal.


It is the ability of a measurement system to reproduce the output in the same form as the input.

Example: if the input to the system is a sine wave, the system is said to have 100% fidelity if the output also is a sine wave.

Dynamic error:

The difference between the indicated quantity and the true value of the time varying quantity is called the dynamic error.

Over Shoot:

A moving part of an instrumrnt (for example pointer) will not assume its final deflection position due to its mass and inertia. It usually moves beyond the steady state. The maximum amount by which the moving parts moves beyond the stedy state is called the over shoot.

Frequency Response:

It is defined as the maximum frequency of the measured variable that the measurement system is capable of following without error.

These are the must known important terms with respect to Instrumentation.

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