Measurement And Instrumentation

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___Measurement And Instrumentation___

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• What is a meter?

          A meter is any device built to accurately detect and display an electrical quantity in a form readable by a human being. Usually this ”readable form” is visual: motion of a pointer on a scale, a series of lights arranged to form a ”bargraph,” or some sort of display composed of numerical figures.

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• What is Instrument?

          A device or mechanism used to determine the present value of the quantity under measurement.

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• Measurement: The process of determining the amount, degree, or capacity by comparison (direct or indirect) with the accepted standards of the system units being used.
• Accuracy: The degree of exactness (closeness) of a measurement compared to the expected (desired) value.
• Resolution: The smallest change in a measured variable to which an instrument will respond.
• Precision: A measure of the consistency or repeatability of measurements, i.e. successive readings does not differ. (Precision is the consistency of the instrument output for a given value of input).
• Range or span: The range or span of an instrument defines the minimum and maximum values of a quantity that the instrument is designed to measure.
• Expected value: The design value, i.e. the most probable value that calculations indicate one should expect to measure.
• Error: The deviation of the true value from the desired value.
• Sensitivity: The ratio of the change in output (response) of the instrument to a change of input or measured variable.
  scale deflection

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  value of measurand producing deflection
• Threshold : If the input to an instrument is gradually increased from zero, the input will have to reach a certain minimum level before the change in the instrument output reading is
  of a large enough magnitude to be detectable. This minimum level of input is known as the threshold of the instrument.
• Dead space:  Dead space is defined as the range of different input values over which there is no change in output value. Any instrument that exhibits hysteresis also displays dead space.

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• Active and passive instruments

Instruments are divided into active or passive ones according to whether the instrument output is entirely produced by the quantity being measured or whether the quantity being measured simply modulates the magnitude of some external power source.

An example of a passive instrument is the pressure-measuring device.

An example of an active instrument is a float-type petrol tank level indicator.

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• Analogue and digital instruments

An analogue instrument gives an output that varies continuously as the quantity being measured changes. The output can have an infinite number of values within the range that the instrument is designed to measure. The deflection-type of pressure gauge is a good example of an analogue instrument.

A digital instrument has an output that varies in discrete steps and so can only have a finite number of values. The rev counter is an example of a digital instrument.

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