Ch 01 (Measurement Systems).pptx

IPE 301
Instrumentation and Control
Chapter 1
Measurement Systems

Systems
Different
Components and
Parts
Boundary
Input(s) Output(s)
System = Arrangement of some parts within some boundary
 Some input(s) and output(s) must be there
Environment

Example of Systems
Electrical
Motor
Electrical
Energy
Mechanical
Energy
Amplifier
Input
Voltage = v
Output
Voltage = G×v
Fig: Block Diagrams of Two Different Systems

Interconnected Systems
A B
Input to A
Final
Output
Output from A is
input to B
 Linked systems
 Arrows generally represent physical connection
 Arrows might represent flow of information instead
of physical connection

Case Study of Interconnected Systems:
Processing Sequence for Frutika Fruit Drink

Measurement System
Measurement
System
Input:
True value
of a variable
Output:
Measured value
of the variable
 Give the user a numerical value corresponding to
the variable being measured
 The numerical value may not actually be the true
value of the variable.

Example of Measurement System
Anemometer
Input:
Wind
(speed)
Output:
Value for
wind speed

Ch 01 (Measurement Systems).pptx

The Constituent Elements of an
Instrumentation System
A C
B
Temperature
Signal
Resistance
Change
Current
Change
Movement of
pointer
across a
scale
Fig: System Elements of a Resistance Thermometer

1.Sensor
 Detects events or changes in its environment
 Provide a corresponding output
 The output depends in some way on the value of the
input variable
 This output is used by the rest of the measurement
system to give a value to it
Example: Thermocouple

2.Signal Processor
 Takes the output from the sensor
 Converts the output into a form which is suitable
for display
Example: Amplifier

3.Data Presentation
 Presents the measured
value in a form which
enables an observer to
recognize it
Example: Display

Transducers
 Converts a change in some physical variable into a related
change in some other physical variable
 It is generally used for an element that converts a
change in some physical variable into an electrical signal
change
Example: Galvanometer, Electrical Motor
Are sensors TRANSDUCERS?

Performance Terms
Accuracy:
 Maximum overall error to be expected from a measurement
system
 Accuracy is expressed as the inaccuracy and appears in
several forms.
(1) Measured variable
(2) Percentage of the instrument full-scale (FS)
(3) Percentage of instrument range or span
(4) Percentage of the actual reading
*FS=maximum value of the span or Reference

Example: A temperature sensor has a span of 10°-300°C. A
measurement results in a value of 100°C for the temperature. Specify the
error if the accuracy is (a) ± 4°C of measured value, (b) ± 0.5% FS, (c) ±
0.75% of span, and (d) ± 0.8% of reading.
Solution:
(a) Error = ± 4°C
(b) Error = ± (0.5% of 300°C) = ± 1.5°C
(c) Error = ± {0.75% of (300 ˗ 10)°C} = ± 2.175°C
(d) Error = ± (0.8% of 100°C) = ± 0.8°C
 Magnitude of Error = |Measured Value – True Value|

Errors in Specifications of
Instrumentation Systems
Hysteresis Error:
Value Measured
Instrument
Reading
Increasing
Decreasing
Hysteresis Error

Errors in Instrumentation Systems
Non-linearity Error:
Input
Output
Assumed Linearity
Actual Relationship
Non-linearity Error

Insertion Error:
Most of the time the act of attempting to make the
measurement modified the value of the variable being
measured. This effect is called loading and the consequence
as an insertion error.
Example:
 Cold thermometer in a hot liquid
 Ammeter inserted into a circuit
 Voltmeter connected across a resistor

Problem:
Two voltmeters are available, one with a resistance of 1 kΩ
and the other 1 MΩ. Which instrument should be selected
if the indicated value is to be closest to the voltage value
that existed across a 2 kΩ resistor before the voltmeter
was connected across it?

Performance Terms
Dead Band or Dead Space:
Output
Input
Dead Band
 A range of input values
for which there is no
output

Performance Terms
Precision:
 Describes the degree of freedom of a measurement system
from random errors
 High precision measurement instrument will give only a small
spread of readings if repeated readings are taken of the same
quantity.
 A low precision measurement system will give a large spread of
readings.
Example:
Readings from instrument 1: 17.5, 18.3, 19, 18.5, 20.2
Readings from instrument 2: 18.2, 18.3, 18.5, 18.5, 18.2

Performance Terms
Repeatability:
 The ability of a measurement system to give the same value for
repeated measurements of the same value of a variable.
 Cause of non-repeatability: Random fluctuations in the
environment, e.g. changes in temperature, humidity etc.
Reproducibility:
 Ability of the system to give the same output when it and/or
elements of the system are disconnected from the input and then
reinstalled.
 The resulting error (from both repeatability and
reproducibility) is usually expressed as a percentage of the
full range output.

Performance Terms
Difference between Accuracy and Precision:

Performance Terms
Sensitivity:
 Indicates how much the output of an instrument system
or system element changes when the quantity being
measured changes by a given amount.
 Example: A thermocouple having a sensitivity of 20 µV/°C
gives an output of 20 µV for each 1°C change in
temperature.
 Also used to indicate the sensitivity to inputs other than
that being measured.
 A pressure measurement sensor might be quoted as having
a temperature sensitivity of ±0.1% of the reading per °C
change in temperature.

Performance Terms
A pressure measurement system is stated as having the
following characteristics. Explain the significance of the
terms:
Range: 0 to 125 kPa and 0 to 2500 kPa
Accuracy: ±1% of the displayed reading
Temperature sensitivity: ±0.1% of the reading per °C

Reliability
Reliability of a measurement system:
 The probability that the system will operate to an
agreed level of performance, for a specified period,
subject to specified environmental conditions.
 The reliability is likely to change with time.
 A high reliability system will have a low failure rate.
 Failure rate is the number of times during some period of
time that the system fails to meet the required level of
performance.

Requirements
Requirements of a measurement system:
 The main requirement is fitness for purpose.
 In order to deliver the required accuracy, the measurement
system must have been calibrated to give that accuracy.
 Calibration is the process of comparing the output of a
measurement system against output from a standard
measurement system of known accuracy.
 Standard measurement systems are kept specially for
calibration duties or some means of defining standard values.

Ch 01 (Measurement Systems).pptx

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Ch 01 (Measurement Systems).pptx