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Instrument Accuracy and Measurement Terms

This document defines and explains key terms related to instrument accuracy and performance. It discusses accuracy as the difference between the indicated and actual values, and defines related terms like absolute accuracy, range, span, precision, tolerance, sensitivity, zero drift, and sensitivity drift. It also classifies instruments as zero-order, first-order, or second-order based on their response type. Finally, it distinguishes between passive and active instruments based on whether the measured quantity directly produces the output or modulates an external power source.

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57 views16 pages

Instrument Accuracy and Measurement Terms

This document defines and explains key terms related to instrument accuracy and performance. It discusses accuracy as the difference between the indicated and actual values, and defines related terms like absolute accuracy, range, span, precision, tolerance, sensitivity, zero drift, and sensitivity drift. It also classifies instruments as zero-order, first-order, or second-order based on their response type. Finally, it distinguishes between passive and active instruments based on whether the measured quantity directly produces the output or modulates an external power source.

Uploaded by

Kibwe Trim
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© © All Rights Reserved
We take content rights seriously. If you suspect this is your content, claim it here.
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The accuracy of an instrument or device is the difference between the indicated

value and the actual value.


Alternate Definition: The accuracy of an instrument is a measure of how close
the output reading of the instrument is to the correct /actual value.
The absolute accuracy of an instrument is the deviation from true as a number
NOT as a percentage.
i.e., if a voltmeter has an absolute accuracy of 3 V in the 100-volt range, the
deviation is 3 V at all the scale readings, e.g., 10 3 V, 70 3 V and so on.

The range of an instrument specifies the lowest and highest readings it can
measure, i.e., a thermometer whose scale goes from 40C to 100C has a range
from 40C to 100C.
The span of an instrument is its range from the minimum to maximum scale
value, i.e., a thermometer whose scale goes from 40C to 100C has a span of
140C.
Precision is a term that describes an instruments degree of freedom from
random errors.
Tolerance is a term that is closely related to accuracy and defines the maximum
error that is to be expected in some value.

Sensitivity is a measure of the change in the output of an instrument for a


change in the measured variable, and is known as the transfer function, i.e., when
the output of a pressure transducer changes by 3.2 mV for a change in pressure of
1 psi, the sensitivity is 3.2 mV/psi.
Zero drift is sometimes known by the alternative term, bias.
Zero drift or bias describes the effect where the zero reading of an instrument
is modified by a change in ambient conditions.
Sensitivity drift (also known as scale factor drift) defines the amount by which
an instruments sensitivity of measurement varies as ambient conditions change.
The Potentiometer is a good example of a Zero-order instrument.

The thermocouple is a good example of a first-order instrument.


An example of a second order linear instrument is a galvanometer and U-tube
manometer.
Passive - instrument output is produced entirely by the quantity being measured

Active - the quantity being measured modulates the magnitude of some external
power source.

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