Basic Instrument

ELET 241
Process Instrumentation
Chapter#1.Part#2

DISCLAIMER
• These powerpoint slides act only as a tool in delivering
lectures to the students.
• The materials presented in these slides are not
comprehensive as most of the materials are explained
to the students verbally with the guide of these
PowerPoint slides, smartboard and ELET241 reference
book.
• Hence, the students are reminded that the main
reference for ELET241 is Instrumentation and Process
Control by Franklyn W. Kirk text book
2

What is Measurement?
• Measurement is defined as the determination
of amount of a variable for monitoring and
controlling purposes.

How Measurement is done?
• An instrument may provide the information
about the value of a quantity under
measurement, in the form commonly known
as an indicating function.

Instruments Performance Characteristics
• Measuring instruments such as sensors are subject to performance
limitations.
• For example, a temperature-sensor primary element takes long
time to respond to changes.
• Thus, its performance can be described by its inherent
characteristics.
• Understanding the instrument performance characteristics is very
important when selecting a sensor for a process control.

1. Range
The range of an instrument is the minimum and maximum values of the measured
variable that the instrument is capable of measuring. The values of process
measurement supposed to be 0% and 100% of a transmitter’s calibrated range.
For example, A temperature transmitter is calibrated to measure a range of
temperature starting at –50°F and ending at 200°F, Determine its LRV and URV?
Answer:
a. Lower Range Value(LRV) would be –50°F at 0%
b. Upper Range Value (URV) would be 200°F at 100%.
2. Zero
It is the value of the measured variable at a datum or reference point.
For example, the zero value of the above example is -50 °F

3. Span
It is the difference between the maximum and minimum numbers in
the range.
Example: A thermometer can measure temperature between –50°F
and 200°F.
The span of the thermometer is 200 – (-50) =250 °F

•

• Example
Temperature measurement with a Pt100 Platinum Resistance Thermometer
When temperature is changed from 0 C to 50 C - the resistance in a Pt100
thermometer changes from 100 ohm to 119.4 ohm. The sensitivity for this
range can be calculated as
s = (119.4 ohm - 100 ohm) / (50 C – 0 C)
= 0.388 ohm/C

5. Resolution
It is the minimum detectable change in the measured variable
which is being measured by the sensor.
For example, if a voltmeter has a resolution of 1mV, then a
change lesser than 1mV will not be detected by it. i.e., if it reads
9.999 V, then it will become 10.000V only if a change of 1mV is
made and not less than that.

6. Time Response
The time taken by a sensor to approach its true output when
subjected to a step input is sometimes referred to as its response
time.
For example, if the temperature of a material changes, response
time determines how quickly a temperature sensor indicates that
change.
Performance Characteristics

7. Accuracy
It is the closeness to which an measured value matches the actual value
of a measurement over a specified range.
8. Precision
it is the closeness with which repeated measurements of the same
quantity matches with each other..

Accuracy and Precision
Sensors are designed to be both accurate and precise.
A sensor that is accurate but imprecise may come very close to
measuring the actual value of the process variable, but it will not be
reliable in its measurements.
A sensor that is precise but inaccurate may not come as close to
measuring the actual value of the controlled variable, but its
measurements will differ from the actual value by nearly the same
amount every time. This consistency makes it
possible to compensate for the sensor error.

Example:
• The true length of a steel beam is 6 m. Three repeated
readings with a laser meter has range of 0 to 40 m
indicates a length of 6.01 m, 6.0095 and 6.015 m
• Calculate the accuracy of the leaser meter?
• Calculate the precision of the leaser meter?

9. Linearity
It is the closeness to which multiple measurements approximate a
straight line on a graph.
A straight line connecting the minimum and maximum input and
output operating points, would represent perfect linear operation of
the instrument.
In practice, exact linearity is difficult to achieve, thus, most sensors
show small changes of slope over their work range which is expressed
as non-linearity.
Non- linearity can be determined be measuring the maximum
deviation of the output to the ideal line, as shown in Figure 1.
Non-linearity is then expressed as a percentage of the maximum
output value.

• Example
A level sensor has an input range of 0.0 to 3.0 m. Using the calibration results given in
the table, calculate:
a) The input and output span.
b) The maximum non-linearity percentage
Level m 0 0.5 1.0 1.5 2.0 2.5 3.0
Output
mV
0 16.5 32 44 51.5 55.5 58

• Input span = 3-0 = 3 cm and output span = 58-0 = 58 mV
• The non-linearity is approximately at (48 – 33 = 15 mV).
• Non-linearity expressed as percentage = (15/58)×100 ≈ 26 %.

10. Stability
It is the ability of the sensor to keep its performance unchanged after
a period of time.
11. Hysteresis
It is when the sensor shows different output behavior while
input increases (loading) and decreases (unloading) over the
same range.
For example,An RTD may exhibit hysteresis while heating and
cooling

12. Drift
It is the change in sensor reading over time while the input and
ambient conditions are constant

Drift can be causes of the following:
• Mechanical vibrations
• Stresses due to high pressure
• Temperature effects
• Electric and magnetic fields

Calibration
Every instrument has at least one input and one output.
The measurement of the output of a sensor must be in response to an accurately
known input.
This process is known as calibration, and the devices that produce the inputs are
described as calibration standards.
To calibrate an instrument means to check and adjust (if necessary) its response
so the output accurately corresponds to its input throughout a specified range.
It is usual to provide measurements at a number of points of the working range
of the sensor, so that a ratio of output to input may be determined from the
measured points by calculation.

Calibration
This graph shows how any given percentage of input should correspond to the same
percentage of output, all the way from 0% to 100%.

Input Percentage
%
Level Input in m Level Transmitter
Output in mA
100 30 20
75
50
25
0 0 4
Calibration
Every Process Variable has a range to be measured. This range will always be represented
by Percentage of its span. This percentage is very helpful when calibrating the instrument
to its original values is required.
Example:
Electronic level transmitter is calibrated with a range of 0 to 30 m, and its output signal has
a range of 4 to 20 mA. Calculate the full corresponding calibration readings of the level
transmitter based on 25% of the input span.

Calibration
Equation to calculate Instrument output/input from the process
variable range to any standard signal used by transmitter
Input Value= the value of the process variable measured by the sensor
LRVs= Lower range value of the sensor
SPANs= The span of sensor
LRVt=Lower Range Value of Transmitter
SPANt=The span of Transmitter

Instrument Selection
• Instruments selection can be done by specifying the minimum
performance characteristics required in order to suit the
required performance of the process control.
• Performance characteristics, maintenance requirements ,and
consumption cost effect the instrument selection process.

Instrument Selection
• In order to evaluate the instrument selection, performance characteristics
are considered.
• Generally, the greater the requirements for good performance, the higher
the cost for purchase and maintenance requirements.
• For example, the higher the accuracy of sensor, the higher of its
manufacturing cost.
• Therefore, finding the appropriate performance and cost is preferred
more than specifying the best available preforming instrument.

Basic Instrument

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