Instrumentation and Measurement
Lecture# 03
Measurement Terminologies
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Lecture outline
 Characteristics of instruments
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
Static characteristics of instruments
Dynamic characteristics of instruments
 Measurements terminologies including (static x-ics)
 Resolution
 Sensitivity
 Accuracy
 Uncertainty etc.
 Engineering units and standards.
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Characteristics of instruments
 Static characteristics
 Dynamic characteristics
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Characteristics of instruments
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Static characteristics of instruments
 The static characteristics of measuring instruments are
concerned only with the steady-state reading that the
instrument settles down to, such as the accuracy of the
reading etc.
 If we have a thermometer in a room and its reading shows a
temperature of 20°C, then it does not really matter whether
the true temperature of the room is 19.5°C or 20.5°C. Such
small variations around 20°C are too small to affect
whether we feel warm enough or not.
 Our bodies cannot discriminate between such close levels
of temperature and therefore a thermometer with an
inaccuracy of ±0.5°C is perfectly adequate.
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Static characteristics of instruments
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Error
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Tolerance

Range or span

Stability

Accuracy and
inaccuracy

 Precision/repeatability
/reproducibility
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Linearity
Threshold
Resolution
Sensitivity of
measurement
Sensitivity to
disturbance
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Static characteristics of instruments
 Error:
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Error = Measured value – True value
Measured value: Practical observation
True value: Theoretical observation
An RTD may give a value of 27°C or 23°C but
the actual (true) value is 25°C.
• Error for 23°C would be -2
• Calculate for 27°C by yourself
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Static characteristics of instruments
 Tolerance: (Margin)
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Tolerance is a term that is closely related to
accuracy and defines the maximum error that is to
be expected in some value.
The accuracy of some instruments is sometimes
quoted as a tolerance figure.
When used correctly, tolerance describes the
maximum deviation of a manufactured component
from some specified value.
A resistor of 100Ω value with 5% tolerance may
give resistance between 95 & 105.
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Static characteristics of instruments
 Range or span:
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The range or span of an instrument defines the
minimum and maximum values of a quantity
that the instrument is designed to measure.
Range defines the limits between which the
input can vary. e.g 0~40Km
Span is the maximum value of the input minus
the minimum value. e.g. 40Km
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Static characteristics of instruments
 Stability:
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The ability of a transducer to give the same
output when used to measure a constant input
over a period of time.
Your system would be stable if it gives you the
desired output/response over a certain period of
time.
A system might be stable, unstable, marginally
stable or conditionally stable.
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Static characteristics of instruments
 Accuracy:

The accuracy of an instrument is a measure of the
extent for which the output reading of the
instrument is the correct value.
 Inaccuracy:
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Inaccuracy is the extent to which a reading might
be wrong, and is often quoted as a percentage of
the full-scale (f.s.) reading of an instrument.
The term measurement uncertainty is frequently
used in place of inaccuracy.
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Static characteristics of instruments
 Precision:
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Precision is a term that describes an instrument’s
degree of freedom from random errors.
If a large number of readings are taken of the same
quantity by a high precision instrument, then the
spread of readings will be very small.
Precision is often, though incorrectly, confused
with accuracy.
High precision does not imply anything about
measurement accuracy.
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Static characteristics of instruments
 Precision:
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A high precision instrument may have a low
accuracy. Low accuracy measurements from a
high precision instrument are normally caused
by a bias in the measurements, which is
removable by recalibration.
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Comparison of accuracy and
precision.
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Static characteristics of instruments
 Repeatability (Precision):
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It describes the closeness of output readings
when the same input is applied repetitively over
a short period of time, with the
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Same measurement conditions
Same instrument and observer
Same location
Same conditions of use maintained throughout
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Static characteristics of instruments
 Reproducibility (Precision):
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It describes the closeness of output readings for
the same input when there are changes in
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Method of measurement
Observer
Measuring instrument, location
Conditions of use and time of measurement.
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Static characteristics of instruments
 Linearity:
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It is normally desirable that the output reading
of an instrument is linearly proportional to the
quantity being measured.
Most of the sensors give non-linear response
(not all).
The advantage of linearity is the easiness in the
context of measured value to be judged at some
distant point.
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Static characteristics of instruments
 Threshold:
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Minimum value of input required to cause a
detectable change is known as threshold.
Minimum value below which no output
appeared is known as the threshold.
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Static characteristics of instruments
 Resolution:
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Smallest possible increment between measured values.
Smallest input change that is detectable by instrument.
Resolution of an instrument is how finely its output
scale is divided into subdivisions
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Static characteristics of instruments
 Sensitivity of measurement:
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It is a measure of the change in instrument
output that occurs when the quantity being
measured changes by a given amount.

The sensitivity of measurement is therefore the
slope of the straight line drawn on figure next.
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Static characteristics of instruments
• Sensitivity of measurement:
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If, for example, a pressure
of 2 bar produces a
deflection of 10 degrees in
a pressure transducer
Sensitivity of instrument is
5 degrees/bar (assuming
that the deflection is zero
with
zero
pressure
applied).
Solve ex#2.1 of book.
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Static characteristics of instruments
 Sensitivity to disturbance:
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It is a measure of the magnitude of stimuli
changed (like pressure or temperature) beyond
the given range.
Such environmental changes affect instruments
in two main ways, known as
• Zero drift (Bias)
• Sensitivity drift (scale factor drift ).
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Static characteristics of instruments
 Sensitivity to disturbance:

Zero drift or Bias:
• It describes the effect where the zero reading of an
instrument is modified by a change in ambient
conditions.
• This causes a constant error that exists over the full
range of measurement of the instrument.
• If someone of known weight 70 kg were to get on
the scale, the reading would be 71 kg.
• Zero drift is normally removable by calibration.
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Static characteristics of instruments
 Sensitivity to disturbance:

Zero drift or bias:
• Zero drift is also commonly found in instruments
like voltmeters that are affected by ambient
temperature changes.
• Typical units by which such zero drift is measured
are volts/°C.
• This is often called the zero drift coefficient related
to temperature changes.
• An instrument may have several zero coefficients
subjected to more environmental parameters.
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Static characteristics of instruments
 Sensitivity to disturbance:

Sensitivity drift or scale factor drift:
• It defines the amount by which an instrument’s
sensitivity of measurement varies as ambient
conditions change.
• It is quantified by sensitivity drift coefficients that
define how much drift there is for a unit change in
each environmental parameter that the instrument
characteristics are sensitive to.
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Static characteristics of instruments
 Sensitivity to disturbance:

Sensitivity drift or scale factor drift:
• Many components within an instrument are affected
by environmental fluctuations, such as temperature
changes: for instance, the modulus of elasticity of a
spring is temperature dependent.
• Sensitivity drift is measured in units of the form
(angular degree/bar)/°C.
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References
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Chapter#02, Alan S Moris “Measurement and
Instrumentation Principles” 3rd edition.
Chapter#02, W. Bolten “Mechatronics, Electronic
control systems in mechanical and electrical
engineering” 3rd edition.
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