Various characteristics (Static and Dynamic) of measuring systems

Various characteristics (Static and Dynamic) of measuring systems

Sensitivity

Sensitivity is defined as the ratio of the magnitude of response (output signal) to the magnitude of the quantity being measured (input signal).

In other words, the relationship between a change in the output reading for a given change of the input. (This relationship may be linear or non-linear.)

If the calibration curve is liner as shown fig i, the sensitivity of the instrument is the slope of the calibration curve.

 If the calibration curve is not linear as shown, then the sensitivity varies with the input.

Readability

            It refers to the ease with which the readings of a measuring instrument can be read. Generally used in analog instruments.

            Readability difficulties may often occur due to parallax errors when an observer is noting the position of a pointer on a calibrated scale.

Parallax errors

A parallax error is the apparent displacement of an observed object’s position as it is viewed from different angles.

            The error is most easily noticed by looking at a nearby object with one eye closed, then looking at it through the other eye.

Repeatability

            It is the closeness value of same output for same input under same operating condition

Reproducibility

            It is the closeness value of same output for same input under same operating condition over a period of time

Drift

            If an instrument does not reproduce the same reading at different times of measurement for the same input signal, it is said to be measurement drift. It is commonly caused by internal temperature changes and component instability. Sensitivity drift defines the amount by which instrument’s sensitivity varies as ambient conditions change.

Resolution

This is defined as the smallest input increment change that gives some small but definite numerical change in the output.

Threshold

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

Range

            The Range is the total range of values which an instrument is capable of measuring.

Span

The algebraic difference between higher calibration values to lower calibration values.

Uncertainty

            The word uncertainty casts a doubt about the exactness of the measurement results

True value = Estimated value + Uncertainty

            The range about the measured value within the true value of the measured quantity is likely to lie at the stated level of confidence

Hysteresis

            This is the algebraic difference between the outputs for an input, when approached from opposite directions.

            Hysteresis is caused by energy storage/ dissipation in the system.

Zero stability

            The ability of the instrument to return to zero reading after the measured has returned to zero.

Dead band

            This is the range of different input values over which there is no change in output value.

Linearity

            The ability to reproduce the input characteristics symmetrically and linearly

Backlash

            Lost motion or free play of mechanical elements is known as backlash.

True value

            The errorless value of measured variable is known as true value.

Response

            A measurement system’s reaction to changes in the measured quantity

Measuring lag:

            It is the retardation or delay in the response of a measurement system to changes in the measured quantity.

Retardation type: Begins immediately after the change in measured quantity

Time delay lag: Begins after a dead time after the application of the input

Fidelity:

            The degree (faithfulness) of a measurement system which indicates changes in the measured quantity without error.

Correction
            Correction is defined as a value which is added algebraically to the uncorrected result of the measurement to compensate to an assumed systematic error.
            Ex : Vernier Caliper, Micrometer

Calibration
            Calibration is the process of determining and adjusting an instruments accuracy to make sure its accuracy is within manufacturing specifications.

Interchangeability

            A part which can be substituted for the component manufactured to the same shape and dimensions is known a interchangeable part.

            The operation of substituting the part for similar manufactured components of the shape and dimensions is known as interchangeability.