Has a very high temperature coefficients - produce large changes in resistance in response to change in temperature. Hence, particularly good for small span & difficult to handle large spans. Has a Non-Linear temperature vs resistance curve. Stability greater than 300 degrees is a problem. Application - Temperature Control Circuits as safety device example: on windings of transformer to detect overheating
What are the most commonly sold types of T/C’s? (Type J and K)
Sensor elements for industrial applications must be packaged ruggedly. RMD standard sensors are all of mineral insulated cable design with 6 mm sheath diameter. RTD ( Resistance Temperature Detector) sensors using platinum resistance elements provide the most stable and reliable measurement over the range -200 to 500 C and should always be specified by choice. Sheath material is AISI 321 stainless steel. Model 65, using platinum film type element, has its’ range reduced to meet the requirements of the international specification IEC 751:1983 INDUSTRIAL PLATINUM RESISTANCE THERMOMETER SENSORS. ( including Amendments A1:1986 and A2:1995). Model 75, using a wire-wound element will meet IEC 751 over an extended temperature range. Grade B tolerance class is offered as standard with Grade A as an option. Thermocouples must be offered for temperatures in excess of 500 C. Up to 1100 C type K should be specified. Sheath material must be suitable for these high temperatures and Inconel is available as standard. Thermocouples meet the requirements of IEC 584 :1995 THERMOCOUPLES, and are available to tolerance class 2 as standard. Other tolerance classes, requiring specially selected materials, are available as specials. For temperatures above 1100 C thermocouples , such as types R or S, are available. These are always supplied as specials as the high temperature environment determines the many thermocouple and sheath material options that will be required.
Question - How is the transmitter powered? EXCERCISE: If the transmitter has a range of 0-200°C (and we know that the mA signal is linear with temperature), what must the mA signal be?
Basics Of Instrumentation
• What is Instrumentation?
• Basic Terminologies
• Process & its Control
• Field Instruments & its principles
• Valves & its working
What is Instrumentation?
• Instrumentation is about measurement and
• Instrumentation engineering is the
engineering specialization focused on the
design and configuration of process systems.
• Instruments are devices which are used in
measuring attributes of process systems.
Series of continuous or regularly recurring steps or actions
intended to achieve a predetermined result, as in heat treating
metal, or manufacturing acid.
Element which converts one form of Energy to Other form.
• Primary Transducer:
Transducer which converts the Process parameter to a form
readable by Secondary Transducer.
Eg: Orifice plate
• Secondary Transducer:
Transducer or transmitter which responds to a measured
variable and converts it to a standardized transmission signal which
is a function only of the measurement.
Eg: DP Transmitter
The signal is the event or phenomenon that conveys data from one
point to another.
A Loop is a combination of one or more interconnected instruments
arranged to measure a process variable. It shall comprises the whole chain
from Primary element to Correcting Element.
A device that operates automatically by use of some established
algorithm to regulate process variable(PV) according to the set point(SV).
The controller input receives information about the status of the process
variable and then provides an appropriate output signal(MV-manipulated
variable) to the final control element(eg-valves etc.,).
It refers to the set of plant conditions(eg. Level of a tank, temp of furnace,
position of furnace or a valve, flow of a fluid, etc) which are to be satisfied before
operating(starting, stopping, opening ,closing ,etc)of any instrument or equipment.
•Monitor pollutant gas emissions from industrial processes.
•Gas analyzer is a gas comparator providing high linearity of signal
•Controller multiplies the signal from the load cell(belt load,kg/m) with that
from the speed transducer(belt speed,m/s) to get the feed rate.
•The controller then either changes the belt speed or belt load to get the set
Conveyors are used as components in automated distribution and
warehousing. A belt conveyor consists of two or more pulleys, with a
continuous loop of material - the conveyor belt - that rotates about them.
One or both of the pulleys are powered, moving the belt and the material on
the belt forward. The powered pulley is called the drive pulley while the
unpowered pulley is called the idler.
• Material flowing over the belt may be weighed in transit using a
beltweigher.A belweigher or belt weigher is a piece of industrial control
equipment used to gauge the mass or flow rate of material travelling over a
Process & its Control
• Process Parameters:
A force applied to or distributed over a surface. The
pressure (P) of a force (F) distributed over an area(A) is
defined as :
P = F / A
Standard Unit of Pressure is Pascal
Other units of pressure are psi
1 Pa = 1 N/m2
• Primary Pressure Measuring Devices:
• Pressure measurements can be divided into three
– absolute pressure
– gauge pressure and
– differential pressure
• Gauge pressure is the pressure relative to the local atmospheric or ambient
In measurements a gauge is used to record the pressure difference between the
system and the atmospheric pressure. This is called gauge pressure and can be stated
by the following equation:
Pg= gauge pressure
Po = atmospheric pressure
• If the pressure of a system is below atmospheric, it is called vacuum pressure.
•When pressure is measured by a gauge, the quantity obtained usually
ambient atmospheric pressure and is therefore called overpressure,
Poverpressure = Pgauge
If atmospheric pressure is included, then the resulting pressure is
called absolute pressure
Pabsolute = Patmospheric + Pgauge
The absolute pressure is measured relative to the absolute zero pressure - the pressure
that would occur at absolute vacuum.
Differential pressure is the difference in pressure between two points.
•The atmospheric pressure is the pressure in the surrounding air at or "close" to the surface
of the earth.
•The atmospheric pressure varies with temperature and altitude above sea level.
•Atmospheric pressure is the pressure exerted at the surface of a body by a
column of air in an atmosphere.
1 atmosphere on Earth = 760 millimeters of mercury (760 Torr) and
STANDARD ATMOSPHERIC PRESSURE :
The Standard Atmospheric Pressure (atm) is used as a reference for gas
densities and volumes.
The Standard Atmospheric Pressure is defined at sea-level at 273o
and is 1.01325 bar or 101325 Pa (absolute). The temperature of 293o
C) is also used.
Types of Pressure Measuring Devices
Manometer Bourdan Gauge
Strain Gage Types
• Some of the most commonly used liquid-level
measurement methods are:
• RF capacitance
• Conductance (conductivity)
• Hydrostatic head/tank gauging
Level Measurement using Pressure Transmitter
P = ρgh
– Flow is measured by measuring velocity through a known
area.with this indirect method,the flow measured is the
volume flow rate Q .
Q = A x V
Where A is the cross sectional area of the pipe
V is the fluid velocity
Unit of low is m3
/hr or litres/hr
– Head Type Flowmeters
– Mechanical Flowmeters
– Electronic Flowmeters
– Mass Flowmeters
Different Type of Head Type
• Orifice Plate
• Flow Nozzle
• Pitot Tube
– Webster’s defines temperature as “the degree of hotness
or coldness measured on a definite scale.
Various units of temperature are related as
C = 5/9 (F – 32)
F = 9/5 (C ) + 32
K = 273 + C
R = 460 + F
• Types of Temp Measurement:
Temperature Control Loop
• Temperature Loop Issues:
– Fluid response slowly to change in input heat
– Requires advanced control strategies
• Feedforward Control
• Example: Thermistors
• RTD (discussed later)
• Semi-conductors made from specific mixtures of pure oxides of
nickel, manganese, copper, cobalt, and other metals sintered at very
• Used with Wheatstone Bridge which amplifies small change in
resistance - in a simple circuit with a battery and a micro-ammeter.
• Stability -
• Linearity -
• Slope of Output -
Change in RESISTANCE with response to change in
• What is an RTD ?
– RResistance TTemperature DDetector
Series 78, 88
Series 68, 58
Two common types of RTD elements:
Wire-wound sensing element
Thin-film sensing element
» Operation depends on inherent characteristic of metal
(Platinum usually): electrical resistance to current flow changes
when a metal undergoes a change in temperature.
» If we can measure the resistance in the metal, we know
• How does a RTD works?
– Resistance changes are Repeatable
– The resistance changes of the platinum wiring can be
approximated by an ideal curve -- the IEC 751
-400 -200 0 200 400 600 800
vs. Temperature Chart:
IEC 751 Constants are :- A = 0.0039083, B = - 5.775 x 10 -7
If t>=0°C, C=0, If t<0, C = - 4.183 x 10 -12
Example: RT = R0 [1 + At + Bt2
– Two dissimilar metals joined at a “Hot” junction
– The wires are connected to an instrument (voltmeter) that
measures the potential created by the temperature
difference between the two ends.
“40 millivolts!,” Tommy
Seebeck yelled in a heated
The junction of two dissimilar metals creates a
small voltage output proportional to
What is a Thermocouple ?
• How does a Thermocouple work ?
– The measured voltage is proportional to the temperaturetemperature
differencedifference between the hot and cold junction! (T2 - T1) =∆T.
Hot junction Cold junction
vs. Temperature Chart:
TYPE E THERMOCOUPLE
-500 0 500 1000
– Iron / Constantan
• White, Red
• 0 to 760 °C
• Least Expensive
Types of Thermocouple
– Chromel / Alumel
» Yellow, Red
» 0 to 1150 °C
» Most Linear
– Copper /
» Blue, Red
» -180 to 371 °C
» Highly resistant to
• Better Accuracy & Repeatability
– RTD signal less susceptible to noise
– Better linearity
– RTD can be “matched” to transmitter
(Interchangeability error eliminated)
– CJC error inherent with T/C’s; RTD’s lead wire
resistance errors can be eliminated
Why choose RTD over Thermocouple ?
– T/C drift is erratic and unpredictable; RTD’s drift
– T/C’s cannot be re-calibrated
– Special extension wires not needed
– Don’t need to be careful with cold junctions
• Applications for Higher Temperatures
• Above 1100°F
• Lower Element Cost
• Cost is the same when considering temperature point
• Faster response time
• Insignificant compared to response time for T-Well and
• Perceived as more rugged
• Rosemount construction techniques produce extremely
Why choose thermocouple over RTD ?
-200 to 500º C RTD
500 to 1100º C Thermocouple Type K
>1100º C Special Thermocouple R, S or B
Temperature SensorsTemperature Sensors
• What is a thermowell (T-well) ?
– A unit that protects a sensor from process
flow, pressure, vibrations, and corrosion
– Allows for sensor removal without process
– Slows response time (by 5 times)
Why are there different material types ?
– To handle different corrosive environments
– To handle different temperature and pressure limits
• The control valve manipulates a flowing fluid, such as gas, steam, water,
or chemical compounds, to compensate for the load disturbance and keep
the regulated process variable as close as possible to the desired set
• The control valve regulates the rate of fluid flow as the position of the
valve plug or disk is changed by force from the actuator.
• Control valves are valves used within industrial plants and elsewhere to
control operating conditions such as temperature, pressure ,flow, and
liquid level by fully or partially opening or closing in response to signals
received from controllers that compare a "set point" to a "process
variable" whose value is provided by sensors that monitor changes in such
• The opening or closing of control valves is done by means of electrical,
hydraulic or pneumatic systems.
They are basically pneumatically operated valves which require around 4
to 5 kg/cm2
of air pressure to operate the valve.
I / P Converter POSITIONER CONTROL VALVE
Valve Body Types
• Diff. types of Valve Body:
• Butterfly Valve
• Globe Valve
• Ball Valve
• Plug type Valve
• Needle Valve
Positioner & its accessories
• Pneumatically operated valves depend on a positioner to take
an input signal from a process controller and convert it to
• A pneumatic signal (usually 3-15 psig) is supplied to the
positioner. The positioner translates this to a required valve
position and supplies the valve actuator with the required air
pressure to move the valve to the correct position.
• Analog I/P Positioner—This positioner performs the same
function as the one above, but uses electrical current (usually
4-20 mA) instead of air as the input signal.
Automation (ancient Greek: = self dictated), roboticization or industrial automation
or numerical control is the use of control systems such as computers to control
industrial machinery and processes, replacing human operators.
The most commonly used automation systems are :
• DCS - Distributed Control System
• PLC - Programmable Logic Controller
• SCADA – Supervisory Control And Data Acquisition System
• Distributed control system (DCS) refers to a control system usually of a
manufacturing system, process or any kind of dynamic systemdynamic system, in which the
controller elements are not central in location (like the brain) but are distributed
throughout the system with each component sub-system controlled by one or
more controllers. The entire system of controllers are connected by a network for
communication and monitoring.
• DCS is a very broad term used in a variety of industries, to monitor and control
• A DCS typically uses computers (usually custom designed processors) as
controllers and uses both proprietary interconnections and protocols for
communication. Input & output modules form component parts of the DCS. The
processor receives information from input modules and sends information to
output modules. The input modules receive information from input instruments in
the process (a.k.a. field) and output modules transmit instructions to the output
instruments in the field. Computer buses or electrical buses connect the processor
and modules through multiplexers/demultiplexers. Buses also connect the
distributed controllers with the central controller and finally to the
Human-Machine Interface (HMI) or control consoles.
HIS HIS ENG
DCS : BASIC CONFIGURATION
BASIC TERMINOLOGIES OF DCS
HIS: Human Interface Station
The HIS is mainly used for operation and monitoring-it displays process variables,control parameters and
alarms necessary for users to quickly grasp the operating status of the plant.
NIU: Node Interface Unit
These are remote I/O units which all the Instruments are connected.these units in turn are connected to FCS
through RIO bus.
FCS: Field Control Station
It is the main control unit which controls the plant.there can be more than one FCS which then communicate
with each other and also communicate with the HIS from where the Operator is operating.
The Vnet real time control system BUS links station such as FCS,HIS,BCV andCGW.
Ethernet is used to link HIS,ENG and supervisory systems.it is also used for transferring data files to
supervisory computers and for HIS data equalization.
RL Bus: This a control system BUS(communication link) which connects Field control units,operators
CGW: Communication Gateway
This unit links the Vnet control system BUS to an ETHERNET BUS
BCV: Bus Converter
The communication bus of one version of DCS may not communicate with the newer versions so BUS
CONVERTER is used to convert the BUS to suitable mode.
In our plant our existing RL BUS is converted to newer system bus Vnet by the Bus Converter kept in
Engineering room near central control room.
•Programmable Logic Controller (PLC) is a microprocessor based system that uses programmable
memory to store instructions and implement functions such as logic, sequencing, timing, counting and
arithmetic in order to control machines and processes.
•Unlike Personal Computer, PLC does not contain peripherals, such as display or keyboard, that allow
user to directly interact with PLC. In order to facilitate interaction, separate computer is provided,
normally taking form of a standard PC. Through this external computer, operator can re-program PLC,
provide set-points and view trends of process variables that are controlled and manipulated by PLC.
PLC Actuator Process
Programmable Logic Controller Architecture
Output Module Actuator Process
PLC consists of the following components:
• Microprocessor – This is the brain of PLC. It reads input signals, executes control program
and communicates results (decisions) of control program as action signals to the outputs.
• Memory – It stores control program that is to be executed at a prescribed rate.
• Power Supply – This component is used to convert the mains AC voltage to the low DC
voltage (e.g. from 240V AC to 5V DC). This unit powers the processor and the circuits in
the input and output modules.
• Input Module – This component receives information from external devices (sensors). It
contains circuitry that provides electrical isolation and signal conditioning functionalities.
Input module can be analogue input (AI) or discrete input (DI) module. AI module receives
continuously changing signal whose amplitude is proportional to the current value of the
measured process variable. DI module receives discrete/digital (ON/OFF) information from
discrete sensors, for example push button (ON if button is pressed, OFF if button is not
pressed). Note that DI is much more frequently used than AI.
• Output Module – This module communicates control actions to external devices
(actuators). It contains circuitry required to interface PLC with actuators (e.g. digital-to-
analogue converter and power amplifier). Like input module, output module can be
analogue output (AO) or discrete output (DO) module depending on the type of actuator
• Communication Module – This component allows PLC to communicate with external
devices using sophisticated multiple-bit digital communication protocols (e.g. Ethernet).
• Ladder Diagram - most common
• Structure Text Programming (ST)
• Functional Block Programming (FB)
• Instruction List (IL)
• Sequential Function Chart (SFC)
• SCADA system performs the following tasks
• Collection of data from field devices, which can be sensors, actuators and
• Transfer of field devices’ information via communication link to the central
site (master station)
• Execution of any necessary analysis and supervisory control calculations,
all of which are taking place at the master stations.
• Display process information on a number of operator screens.
• Convey any required supervisory control actions back to the field devices.
Supervisory Control and Data