Characteristics of Instrument

ByAbhishek Gedam

INSTRUMENTATION CHARACTERISTICS• Shows the performance of instruments to be

used.

• Divided into two categories• A)Static characteristics: Involves measurement

of constant or slowly varying quantities.

• B)Dynamic characteristics: Involves measurement of quantities that varies with time.

Accuracy• It is the degree of closeness with which an

instrument reading approaches the true value of the quantity being measured.

• Accuracy is the ability of an instrument to show the exact reading.

• Normally shown in percentage of error which of the full scale reading percentage.

PrecisionPrecise = Clearly defined

• Defined as the capability of an instrument to show the same reading when used each time (reproducibility of the instrument).

• Measure of consistency in measurement.

Accuracy & Precision

Sensitivity• Sensitivity is the ratio of change in output of an

instrument to the change in input.

Sensitivity

Dead Zone• It is the largest changes of input quantity

for which there is no output.

• For e.g. the input that is applied to an instrument may not be sufficient to overcome friction. It will only respond when it overcomes the friction forces.

Dead Zone

Dead Space

OutputReading

MeasuredVariables

- +

Range

The region between the limits within which an instrument is designed to operate for measuring, indicating or recording a physical quantity is called the range of the instrument.

• The range is expressed by stating the lower and upper values.

• Range -100°C to 100°C

Span• Span represents the algebraic differences

between the upper and lower range values of the instrument.

• An instrument which has a reading range of –100°C to 100 °C span is 200 °C.

Linearity

• The linearity is defined as the maximum deviation from the linear characteristics as a percentage of the full scale output.

• Linearity is actually a measure of nonlinearity of the instrument.

• Input/output characteristic of the instrument to be approximately linear

Full Range of GageReference Value

No Bias

Bias

Reproducibility

• Reproducibility is defined as the degree of closeness by which a given value can be repeatedly measured.

• Perfect reproducibility signifies that the given readings that are taken for an input, do not vary with time.

• Means no drift

Consistency of measurement by various inspectors or gauge

Repeatability

• Reproducibility is defined as the degree of closeness by which a given value can be repeatedly measured.

• How consistent one inspector/ Gauge?

Resolution • If the i/p is slowly increased from some

arbitrary value (non zero), it will found that o/p does not changes at all until certain increment is increased.

• This increment is called Resolution• Resolution indicates the

minimum change in input variable that is detectable.

1 2 3 4 5

Poor Discrimination

1

1 2 3 4 5

Better Discrimination

Drift• The drift is defined as the gradual shift in the

indication over a period of time where in the input variable does not change.

• Because of environment factors like stray electric fields, stray magnetic fields, thermal e.m.fs, changes in temperature, mechanical vibrations etc.