The performance of an instrument is described by means of a quantitative qualities termed as characteristics. They are characterized into two types static and Dynamic.

1. Static and Dynamic
Characteristics of Measuring
Instruments
Mrs.K.V.Archana
Assistant Professor
Department of Electronics and Communication
Engineering
Avinashilingam Institute For Home Science and Higher
Education For Women, Coimbatore

2. Characteristics of Instruments
The performance of an instrument is described by means of a
quantitative qualities termed as characteristics. These are broken down
into:
1.Static Characteristics: These characteristics pertain to a system
where the quantities to be measures are constant or vary slowly with
time
2.Dynamic Characteristics: Performance criteria based on dynamic
relations (involving rapidly varying quantities)
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3. Static Characteristics
Accuracy: It is the closeness with which an instrument reading approaches
the true value of the quantity measured.
Precision : The degree to which repeated measurements show the same
results.
Low Accuracy
Low Precision
Low Accuracy
High Precision
High Accuracy
Low Precision High Accuracy
High Precision
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4. Accuracy and Precision
Accuracy may be specified in terms of inaccuracy or limit of errors and can be
expressed in the following ways:
1.Point Accuracy
2.Accuracy as “Percentage of Scale Range”
3.Accuracy as “Percentage of True value”
Indication of Precision
Significant Figures: It is an indication of precision of measurement. It convey the actual
information regarding the magnitude and the measurement precision of a qty. The more the
significant figures, the greater the precision.
e.g. 302 A = 3S.F
302.10 V = 5 S.F 0.00030 = 5 S.F
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5. Static Characteristics
Resolution or Discrimination: The smallest detectable incremental change of the input
parameter that can be detected in the output signal. Eg; Scale, Multi range meters.
Sensitivity: For an instrument or sensor with input x and output y. Sensitivity = dy/dx
output
Static sensitivity = Infinitesimal change in output /infinitesimal
change in input
output
input
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6. Continued
Repeatability: Closeness of output reading when the same input is applied repeatedly
over a short period of time with the same measurement conditions same instrument and
observer , same location and same conditions of use maintained throughout.
 Reproducibility: Closeness of output readings for the same input when there are
changes in method of measurement , observer, location , conditions of use, and time of
measurement.
Span & Range:
• Range : High measurement possible
• Span : Difference between max. and min measurement possible
• E.g. Thermocouple (700 0C to 1200 0C) Ammeter (0 to 10 A)
Dead zone : The largest of a measured variable for which the instrument
• does not respond Cause: friction in mechanical measurement system
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7. Continued
• Dead Time :The time before the instrument begins to respond after the
measured quantity has been changed. E.g: Camera, Data acquisition
card,Ammeter
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8. Continued
 Drift : It is an undesired gradual departure of the instrument o/p over a period of time that
is unrelated to changes in i/p , operating conditions or load.
• The drift may be caused by the following factors:
1) Mechanical vibrations
2) Temp. changes
3) Wear and Tear etc.
Classification:
1) Zero drift : If the whole of instrument calibration/ characteristics gradually shifts one by same amount. It
may be due to presence set or slippage and can be corrected by shifting pointer position.
Zero
output
Characteristics with zero drift
Normal characteristics
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9. Continued
2) Span or sensitivity Drift : If the calibration from zero upwardschanges proportionally
output
Span Drift
Normal Characteristics
3) Zonal Drift : When the drift occurs only over a portion of span of an instrument.
output
Zonal Drift
Normal Characteristics
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10. Continued
Linearity: If the calibration from zero upwards changes proportionally. If
input-output relationship is a straight line passing through origin
• Nonlinearity cause lot of problem during signal conditioning even though it is
more accurate in some cases e.g. LVDT (linear) , Thermistor (Non-linear)
Error: Error is the degree to which a measurement conforms to the expected
or true value .Errors are due to measuring instruments (causing the change in
the value of the parameter being measured) or due to persons carrying out the
measurements (human errors).Errors may be expressed as absolute or
percentage.
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11. Error Types
Types of Errors
Gross errors
Human errors
Systematic errors
Instrument errors
Environmental errors
Observational errors
Random errors
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12. Dynamic Characteristics
1) Speed of Response: It is defined as the rapidity with which a
measurement system responds to changes in the measurement
quantity.
2) Measurement Lag: It refers to retardation or delay in the response
of measurement system to changes in measured quantity . The lag is
caused by conditions such as capacitance, inertia or resistance.
• Measuring lag are of two types:
a) Retardation type lag
b) Time delay type lag
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13. Continued
3)Fidelity: It is defined as the degree to which a measurement system
indicates changes in the measured quantity without any dynamic
error.
4)Dynamic error or measurement error : It is the difference between
true value of the quantity changes with time and the value indicated
by the measurement system if no static error is assumed.
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14. THANK YOU
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