INTRODUCTION TO INDUSTRIAL
Instrumentation is the science of automated
measurement and control.
Applications of this science be plentiful in modern
research, industry, and everyday living.
This chapter explains some of the fundamental
principles of i d t i l i t
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f industrial instrumentation.
In Oil & Gas industry, the first step, naturally, is
measuring the process variables such as; pressure,
flow, level , temperature, analytical, …etc.
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Once the process variable measured, transmit a
signal representing this quantity to an indicating or
device where either human or
automated action then takes place.
If the controlling action is automated, the
computer sends a signal to a final controlling
device which then influences the quantity being
Both the measurement device and the final control
device connect to some physical system which we
call the process To show this as a general block
INSTRUMENTATION TERMS AND THEIR DEFINITIONS:
Process: The physical system we are attempting to
control or measure Examples: oil refinery unit
water filtration system, steam boiler, power
Process Variable (PV): The specific quantity we
are measuring in a process. Examples: pressure,
level, temperature, flow, electrical conductivity, pH,
position, speed, vibration.
Setpoint (SP): The value at which we desire the
process variable to be maintained at. In other
words the “target” value of the process variable
Primary Sensing Element (PSE): A device that
directly senses the process variable and translates that
sensed quantity into an analog representation (electrical
voltage, current, resistance; mechanical force, motion,
o oup ,
, bou do ub ,
etc.). Examples: thermocouple, RTD, bourdon tube,
potentiometer, electrochemical cell.
Transducer (Converter/ Relay): A device that
converts one standardized instrumentation signal into
another standardized instrumentation signal, and/or
performs some sort of processing on that signal
Examples: I/P converter, P/I converter, square-root
Transmitter: A device that translates PSE signal
into a standardized instrumentation signal such as;
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pneumatic 3-15 psi, electrical 4-20 mA DC, Fieldbus
digital signal packet, etc.,
Lower- and Upper-range values (LRV & URV):
e a ues o p ocess easu e e t dee ed
The values of process measurement deemed to be
0% and 100% of a transmitter’s calibrated range.
For example, if a temperature transmitter is
calibrated to measure a range of temperature
starting at 3000C and ending at 5000C; the 300 0C
degrees would be LRV and the 500 0C degrees would
o Zero and Span: alternative descriptions to LRV and
URV for the 0% and 100% points of an instrument’s
“Zero” the beginning-point of an instrument’s range
(equivalent to LRV),
“Span” the width of its range (URV − LRV).
o Controller: A device that receives a process variable
(PV) signal from transmitter, then compares that signal
to the desired value (SP), and calculates an appropriate
output signal value to be sent to a final control element
such as an electric motor or control valve
Final Control Element (FCE): A device that receives
signal from the controller to directly influence the
process. Examples: variable-speed electric motor, control
valve electric heater.
Manipulated Variable (MV): The output signal
generated b the controller. This is the signal
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commanding “manipulating” the final control element to
influence the process.
Automatic mode: When the controller generates an
output signal based on the relationship of process
variable (PV) to the setpoint ( )
Manual mode: When the controller’s decision-making
ability is bypassed to let a human operator directly
determine the output signal sent to FCE.
ESSENTIAL ELEMENTS OF A WATER LEVEL CONTROL SYSTEM, SHOWING
TRANSMITTER, CONTROLLER, AND CONTROL VALVE:
EXAMPLE: CHEMICAL REACTOR TEMPERATURE CONTROL
Indicators provide a human- readable indication
of an instrument signal.
Indicators give a convenient way of seeing what
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the output of the transmitter is without having to
connect test equipment
Indicators may be located far from their
respective transmitters, providing readouts in
locations more convenient than the location of the
Chart recorder or a trend recorder used to draw a
graph of process variable(s) over time.
i bl ( )
Recorders usually have indicators for showing the
instantaneous value of the instrument signal(s)
simultaneously with the historical values.
CIRCULAR CHART RECORDER USES A ROUND SHEET OF PAPER
CHART RECORDER ON THE RIGHT AND A
PAPERLESS CHART RECORDER ON THE LEFT
EXAMPLE OF A TYPICAL
“TREND” SHOWING THE RELATIONSHIP BETWEEN
PROCESS VARIABLE, SETPOINT, AND CONTROLLER OUTPUT IN AUTOMATIC MODE,
AS GRAPHED BY A RECORDER:
PROCESS SWITCHES AND ALARMS
Process switch is used to turn on and off with
varying process conditions
Usually, switches are used to activate alarms to
alert human operators to take special action.
In other situations, switches are directly used as
FOLLOWING P&ID OF A COMPRESSED AIR CONTROL
SYSTEM SHOWS BOTH USES OF PROCESS SWITCHES:
Process alarm switches may be used to trigger a special type of
indicator device known as an annunciator.
An annunciator is an indicator lights designed to secure a
human operator s attention by blinking and sounding an audible
buzzer when a process switch actuates into an abnormal state.
The alarm state may be then “acknowledged” by an operator
pushing a button, causing the alarm light to remain on (solid)
rather than blink, and silencing the buzzer.
The indicator light does not turn off until the actual alarm
condition (the process switch) has returned to its regular state.
ANNUNCIATOR LOCATED ON A CONTROL
PANEL FOR A LARGE ENGINE-DRIVEN PUMP
A SIMPLE LOGIC GATE CIRCUIT ILLUSTRATES THE ACKNOWLEDGMENT
(HERE IMPLEMENTED BY AN S R LATCH
CIRCUIT) COMMON TO ALL PROCESS ALARM ANNUNCIATORS:
Most instruments contain a f ilit f
M t i t
facility for making t
The RANGE adjustment.
The ZERO adjustment.
In order to calibrate the instrument an accurate gauge is
required This is likely to be a SECONDARY STANDARD
Instruments calibrated as a secondary standard have
themselves been calibrated against a PRIMARY STANDARD.
An input representing the minimum gauge setting should be
applied. The output should be adjusted to be correct. Next
the maximum signal is applied. The range is then adjusted to
give the required output. This should be repeated until the
gauge is correct at the minimum and maximum values.
RANGE AND ZERO ERRO
After obtaining correct zero and range for the instrument, a
calibration graph should be produced. This involves plotting the
indicated reading against the correct reading from the standard
gauge. This should be done in about ten steps with increasing
signals and then with reducing signals. Several forms of error could
show up. If the zero or range is still incorrect the error will appear
HYSTERESIS and NON LINEAR ERRORS
Hysteresis is produced when the displayed values are too
small for increasing signals and too large for decreasing
signals This is commonly caused in mechanical instruments
by loose gears and linkages and friction. It occurs widely with
things involving magnetisation and demagnetisation.
Th calibration may b correct at th maximum and minimum
t t the
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values of the range but the graph joining them may not be a
straight line (when it ought to be). This is a non linear error.
The inst ment ma ha e some adj stments fo this and it
instrument may have
may be possible to make it correct at mid range as shown.
A transmitter sends representative signal of the
value of measured variable from the sensor to the
indicator or controller. This has the advantage of
keeping hazardous process fluids outside the
control room and allows the use of a common
signal range. Transmitter picks up the
measurement provided by the sensor and converts
it to a standard signal range. Sensors and
transmitters are combined in to one device.
The two most common types of transmission used in
Air systems operate in the range of 3 to 15 psi (0.2 to 1.0
Bar) and make use of small – bore piping to transmit the
signal around the plant.
The main advantages of this system are:
1. Freedom to a certain extent from the fear of electrical
Ability to t a
transmit a d send signals th o gh
hazardous areas without the fear of explosion.
3. Noise immunity from external sources
Unfortunately as the transmission distances increase
lags the measurement system increase and some
distortion of the signal occurs.
Electronic systems make use of cables to send
current signals in range of 4-20 mA around the use
of a li zero used in current t
t transmission, allows
for an easy method to detect a loss of signal due to
cab e damage. u t e the current s the same
cable da age. Further t e cu e t is t e sa e at
all points the loop.
Electrical signals do not suffer from lag and signal
distortion problems when long transmission
distances are encountered. Unfortunately they can
Suffer from noise problems and all signals are lost
when the power fails unless an Uninterruptible
power Supply (UPS) Is available. The main
problem with the electrical Signals is the explosion
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risk in hazardous areas.
The traditional favorite means communicating a process
signal from the field to the control room is via the 4-20 mA
analogue current loop
This is a fast reliable industry standard but it leaves a lot
to be desired in terms of maintenance capabilities,
performance and diagnostics.
Analogue field instruments can be expensive to install and
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Smart field instruments may not only address this issues
but can provide further benefits the features of Smart
transmitters issues but can provide further benefits.
The features of Smart transmitters can provide substantial
benefits to users in terms of time and labor savings as well
as providing an increase in plant operating safety.
HIGHWAY ADDRESSABLE REMOTE TRANSDUCER (HART) PROTOCOL
HART field communications protocol is widely accepted in the
industry as the standard for digitally enhanced 4-20 mA
communication with smart field instruments. The HART protocol
was designed specifically for use with intelligent measurement
and control instruments that traditionally communicate using 44
20 mA analogue signals HART preserves the 4-20 mA signal and
enables two–way digital communications to occur without
disturbing th i t
di t bi
the integrity of th 4 -20 mA signal. U lik other
l Unlike th
digital communication technologies the HART protocol maintains
compatibility with existing 4 -20 mA systems and in doing so
provides users with uniquely compatible solution the HART
protocol permits the process variable to be transmitted by the 4 20 mA analogue signal and additional information about other
variables parameters , device configuration , calibration and
device diagnostics to be transmitted digitally at the same time .
HART makes use the technic of frequency shift keying (FSK) standard to
superimpose digital communications at a low level on top of the 4 -20 mA
This enables two-way field communications to take place and makes it
possible for additional information beyond just the normal process variable to
be communicated to from a smart field instrument the HART protocol allows
a host application (master) to get two or more digital updates per second from
a field device which is not fast enough for most applications.
HART is a master slave protocol, that means the field (slave) device only
speaks when spoken to by master sends out command signal (C) and the slave
sends back a response (R).
As with most protocol-based systems the manufacturer tries to “tieyou
you” into his equipment. The therefore if you use a HART based
system you cannot attach Honeywell equipment and expect it to work.
Hopefully with the final introduction of the Field-bus foundation
protocol this will change and a g
greater flexibility will emerge.
The HART protocol permits all digital communication with field
devices in installation saving are possible with the multi-drop
networking capabilities of HART.
(microprocessor based) measurement
diagnostic information as well as Process
in hybrid 4-20 mA mode.