MEASUREMENT

AND
INSTRUMENTATION
SHARIL C. TAMIAT
MAED-GEN. SCI.
Topic Outline:
• Elements of Measurement System
• Choosing Appropriate Measuring
Instruments
• Review of Instrument Types
• Static Characteristics of
Instruments
 ELEMENTS OF A MEASUREMENT
SYSTEM
• A measurement system provides information about the physical
value of a variable.
• This can be a single unit in simple systems, but more complex
systems involve multiple elements, which might be contained within
one or more boxes.
The measuring instrument is commonly used to describe a
measurement system, whether it contains only one or many elements,
and this term will be widely used throughout this text.
These elements include:
1. Primary Sensor: The initial element that outputs a signal based
on the measurand (the variable being measured). Examples
include thermometers and strain gauges. Some sensors, like the
mercury-in-glass thermometer, are complete measurement
systems.
2. Variable Conversion Elements: Convert the sensor output to a
more convenient form. For instance, a strain gauge's resistance
change is converted to voltage by a bridge circuit. When combined
with the primary sensor, this is called a transducer.
3. Signal Processing Elements: Improve the output quality, such as
amplifiers that enhance sensitivity and filters that reduce noise.
These elements are crucial for low-output sensors like
thermocouples.
4. Signal Transmission: Transmits the signal to a remote location
using cables or fiber-optic lines, which minimizes signal corruption
and loss.
5. Signal Presentation and Recording: Utilizes the measured
signal, either through direct control system input or for manual
observation, using presentation or recording units.
Measurement systems can be simple
or complex, depending on the
application and the need for accurate,
reliable data transmission and
processing.
 CHOOSING APPROPRIATE MEASURING
INSTRUMENTS
When selecting measuring instruments for a manufacturing plant or system, it is crucial to consider the
following:

1. Specification of Instrument Characteristics:

1. Desired measurement accuracy, resolution, sensitivity, and dynamic performance.
2. Environmental conditions affecting instrument performance and protection needs.

2. Expert Input:
1. Knowledge from plant personnel and skilled instrument engineers is essential for evaluating and
choosing suitable instruments.

3. Performance vs. Cost:

1. Instruments should be as insensitive as possible to the operating environment while balancing cost
and performance.
2. Better characteristics generally lead to higher costs, so instruments should meet minimum required
specifications without unnecessary expense.
4. Durability and Maintenance:
1.Consider durability, maintainability, and constancy of performance
over time.
2.Evaluate total purchase cost and estimated maintenance costs over
the instrument’s life.
5. Continuous Learning:
1.Instrumentation engineers must stay updated on new techniques and
instruments through technical journals and literature.
In summary, instrument choice involves balancing performance,
durability, maintenance needs, and cost, requiring extensive knowledge of
available instruments and their suitability for specific conditions.
Guess me right…..
Dial gauge Pressure gauge Temp. gun

Tachometer
Digital Feeler Gauge

Micrometer
Vernier caliper
Bore gauge
 REVIEW OF INSTRUMENT TYPES

Instruments can be subdivided into separate

classes according to several criteria. These
subclassifications are useful in broadly establishing
several attributes of particular instruments such as
accuracy, cost, and general applicability to different
applications.
A. Active and Passive Instruments
B. Null-type and deflection-type instruments
C. Analogue and digital instruments
D. Indicating instruments and
instruments with a signal output
E. Smart and Non-smart Instruments
 STATIC CHARACTERISTICS OF
INSTRUMENTS
"Instrument static characteristics, such as accuracy,
sensitivity, linearity, and response to ambient
temperature, define their suitability for specific
applications. These attributes, detailed in instrument data
sheets, ensure reliable measurements under standard
calibration conditions, though variations may occur in
other operational environments."
 A. Accuracy and inaccuracy (measurement
uncertainty)
•Accuracy: This refers to how close the output reading of an instrument is to the
true or correct value of the quantity being measured. It quantifies the
instrument's ability to provide correct measurements.
•Inaccuracy: This is the extent to which a measurement reading might deviate
from the true value. In practice, inaccuracy is often quoted as a percentage of the
full-scale reading (f.s.) of the instrument. It represents the maximum possible
error in a measurement. The term measurement uncertainty is frequently used in
place of inaccuracy.
In summary, accuracy measures the accuracy of the instrument's readings, while
inaccuracy quantifies the potential deviation or error in those readings.
 B. Precision, Repeatability, and
Reproducibility
•Precision: Precision refers to how close multiple measurements of the same quantity are to
each other, indicating low random errors. For example, if a scale consistently measures a
10g weight as 10.1g, 10.0g, and 10.2g, it is precise because the readings are close to
each other, even if not accurate.
•Repeatability: Repeatability is the consistency of measurements taken by the same
instrument, under the same conditions, over a short period. For example, if a thermometer
reads the same temperature three times in a row in the same environment, it
demonstrates good repeatability.
•Reproducibility: Reproducibility is the consistency of measurements taken under varying
conditions, such as different instruments, observers, or locations. For example, if two
different thermometers give similar readings for the same temperature in different labs,
they have good reproducibility.
C. Tolerance defines the maximum allowable error or deviation from a specified value. It is
closely related to accuracy but is not a static characteristic of measuring instruments. For
example, a crankshaft might be machined with a diameter tolerance of a few microns,
meaning its diameter can vary within that range. Similarly, a resistor with a nominal value
of 1000Ω and a tolerance of 5% can have an actual resistance between 950Ω and
1050Ω.
D. 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.
E. Linearity and Non-Linearity: Linearity refers to the desirable condition where an
instrument's output is directly proportional to the input quantity. For example, if doubling
the input doubles the output, the instrument is linear. Non-Linearity: Non-linearity is the
maximum deviation of an instrument's output from a straight line fitted to the actual
output readings. It is usually expressed as a percentage of the full-scale reading. For
example, if an instrument's output deviates significantly from the expected straight line as
the input varies, it exhibits non-linearity.
F. Sensitivity of measurement
G. Threshold. The threshold of an instrument is the minimum input
level required for a detectable change in output. For example, a car
speedometer with a threshold of 15 km/h won't show any reading
until the car's speed reaches 15 km/h. Manufacturers specify this
threshold either as an absolute value or as a percentage of the full-
scale reading.
H. Resolution. The resolution of an instrument is the smallest change
in input that produces a detectable change in output. For instance, a
car speedometer with 20 km/h subdivisions has a resolution of 5
km/h, meaning speed can only be estimated to the nearest 5 km/h.
Manufacturers specify resolution as either an absolute value or a
percentage of full-scale deflection.
 I. Sensitivity to Disturbance
➢is a measure of the magnitude of this change. Such environmental changes affect instruments in
two main ways, known as zero drift and sensitivity drift. Zero drift is sometimes known by the
alternative term, bias.
Zero Drift (Bias):
•Definition: A constant error over the full range of measurement caused by changes in ambient
conditions, affecting the zero reading of an instrument.
•Example: A bathroom scale showing 1 kg when unloaded. If a person weighing 70 kg stands on it,
it shows 71 kg. This drift is corrected by recalibration.
Sensitivity Drift (Scale Factor Drift):
•Definition: The change in an instrument's sensitivity to measurement due to variations in ambient
conditions.
•Example: A pressure gauge where temperature changes affect the modulus of elasticity of its
spring, altering its sensitivity. This drift is measured in units like (angular degree/bar)/°C.
Both drifts are quantified by their respective drift coefficients and can affect instruments like
voltmeters and pressure gauges.
 J. hysteresis effects
•Hysteresis refers to the difference in an instrument's output when the input
is increased versus when it is decreased. This non-coincidence between
loading and unloading curves is hysteresis. For example, in a pressure gauge
with a spring, the output reading differs depending on whether the input
pressure is being increased or decreased. Hysteresis is often expressed as a
percentage of the full-scale input or output and is common in instruments
with springs or friction, as well as those with magnetic components.
•Two quantities are defined, maximum input hysteresis and maximum output
hysteresis, as shown in Figure 2.8. These are normally expressed as a
percentage of the full-scale input or output reading respectively.
 K. Dead Space
Dead space is the range of input values where there
is no change in output. For example, in gear systems
with backlash, there can be a range of input motions
where the output doesn't change. Instruments with
hysteresis also show dead space, but it can occur
independently of hysteresis.
Reference:
Morris, A. S. (2001). Measurement and instrumentation principles. Measurement Science
and Technology, 12(10), 1743-1744.