Introduction to Instrumentation I-Control and Instrumentation-Lecture Slides, Slides for Electronic Measurement and Instrumentation

Introduction to Instrumentation I-Control and Instrumentation-Lecture Slides, Slides for Electronic Measurement and Instrumentation

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This lecture was delivered by Prof. Sonu Vamsi at Agra University. This lecture is part of lecture series for course Control and Instrumentation. Its main points are: Introduction, Instrumentation, Industrial, Process, C...
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Introduction to Instrumentation

Introduction • Instrumentation is used in almost every industrial process and generating system, where consistent and reliable operations are required.

• Instrumentation provides the means of monitoring, recording and controlling a process to maintain it at a desired state.

• A typical industrial plant such as an electric generating station yields many process variables that have to be measured and manipulated.

• Variables such as boiler level, temperature,

pressure, turbine speed, generator output and

many others have to be controlled prudently to

ensure a safe and efficient operation.

• Because of the continuous interactive nature of

most industrial process systems, manual control

is non-feasible and unreliable.

• With instrumentation, automatic control of such

processes can be achieved.

• Before any process can be controlled, its current

status must be known.

• Specific instrumentation can be selected to

measure, and to indicate process conditions so

that a corrective action could be initiated if required

• An instrument could be mounted at the process

location to indicate the process state to the plant

personnel. This form of data display is referred to

as local or field indication.

Local indication is useful in many applications,

but has the disadvantage that someone must

travel throughout the plant in order to

determine the system status.

• To bring all of these indicators into one single

location (i.e. the central control room), would

mean transporting the actual process quantity

to that location.

– This would result in hazardous conditions due to the

presence of high pressure steam, high voltage, toxic

gases, corrosive liquids, etc., in the control room.

• Instead of simply indicating the process status

locally, or transporting the actual process to the

control room, it would be desirable to be able

to transmit a representative signal corresponding

to the process status, to the central control


• By measuring and displaying this signal in the

control room, the field process state can be


• The read-out device mounted in the control

room panel can be adjusted or calibrated to

indicate the process value directly.

• Another advantage of being able to transmit a

signal is that some signals may be analyzed by

controllers or computers so that an automatic

corrective action will be initiated if the process

deviates from the desired operating point, called

the set point.

• Also, if abnormal conditions arise, alarm units

which are activated by these signals can be used

to trigger annunciations in the control room or to

cause a process to shut down safely.

Transmitting a Process Parameter

• There are two standard methods of

transmitting a signal :

– Pneumatic

– Electronic

• Electrical signals can be further categorized

into • continuous

• digital

Pneumatic Signal

• A pneumatic process sensor coupled to a

transmitter is used to monitor a process

variable; such as level in a tank or pressure in a


• The output signal of the pneumatic transmitter is air

pressure, the magnitude of which is directly

proportional to the process variable being


• The standard industrial range for pneumatic

signals is 20 to 100 kPa(g) (kPa(g) = kPa above

atmospheric), which corresponds to a 0% to

100% process condition.

Pneumatic Signal vs Percent Process

•Note that the transmitter signal output starts at

20 kPa(g) – not 0 kPa(g).

•This 20 kPa(g) output is called a live zero.

Live Zero

• A live zero allows control room staff to

distinguish between a valid/invalid process


– Process of 0% (a 20 kPa(g) reading) is a valid

– and a disabled transmitter or interrupted

pressure line (a 0 kPa(g) reading).

Advantages and Disadvantages of

Pneumatic Transmission

• One advantage of a pneumatic system (over

an electronic system) is that sparks will not be

produced if a transmitter malfunction occurs,

making it much safer when used in an explosive


• There is no electric shock hazard.

• No interference problems (EMF, RFI etc)

• On the other hand, a pressurized system can be

dangerous if a line ruptures.

• Pneumatic signal lines are bulky and difficult to

install. (Increase in Cost)

• The biggest problem with pneumatic systems is

that air is compressible. (Slow Response) This means that a pressure transient representing a process

change will only travel in the air line at sonic velocity

(approximately 300 m/sec.). Long signal lines must

therefore be avoided to prevent substantial time delays

a serious drawback when you consider the size of nuclear

generating stations.

• Air compression is not precise, so precise control is not


Electronic Signals • For large industrial process applications such

as generating stations where central control

rooms are used, electronic signals are

preferred and in many cases are used


• The process condition is monitored and an

electronic signal that is proportional to that

process condition is produced by an electronic


• The accepted industrial standard for electronic

signals is now a 4 to 20 mA current signal that

represents the 0% to 100% process condition.

• The relationship between the process

condition and electronic transmitter signal

output is shown below.

• Again, a live zero (4 mA) is used to

distinguish between 0% process (4 mA) and

an interrupted or faulted signal loop (0 mA).

Advantages and Disadvantages of

Electronic Transmission

• When an electronic signal is used instead of a

pneumatic signal the pressurized fluid

transmission delay is eliminated.

• The electronic current signals travel at speeds

which approach the speed of light.

• These current signals can be transmitted over

longer distances without the introduction of

unnecessary time delays.

• Interfere with EMF, RFI.

• Need separate power to function.

Why 4 - 20 mA Current Loop ?

• The 4-20mA current loop shown in figure is a

common method of transmitting sensor information

in many industrial process-monitoring applications.

• Transmitting sensor information via current loop is

particularly useful when the information has to be

sent to a remote location over long distances (1000

feet, or more).

• The loop’s operation is straightforward:

– a sensor’s output voltage is first converted to a

proportional current, with 4mA normally representing the

sensor’s zero-level output, and 20mA representing the

sensor’s full-scale output.

– Then, a receiver at the remote end converts the 4-20mA

current back into a voltage which in turn can be further

processed by a computer or display module.

• Sending a current over long distances produces

voltage losses proportional to wires length.

However those losses do not reduce 4-20mA

current as long as the transmitter and loop supply

compensate for that.

The 4 - 20 mA Current Loop

• A big benefit of the current loop is its simple wiring of just the two wires.

• The supply voltage and measuring current are supplied over the same two wires.

• Zero offset of the base current (ie. 4mA) makes cable break detection simple:

– if the current suddenly drops to zero, you have a cable break.

– In addition, the current signal is immune to any stray electrical interference, and a current signal can be transmitted over long distances.

A Simple 4-20mA Current Loop

Rw = Wire Resistance

R= 250 Ohm =

Voltage is monitored here


ISA Symbols

• To simplify drawings and flow sheets

(instrumentation schematics) and therefore make

process loops more easily understandable we

use ISA symbols. [Standard Symbols in Process


• Instruments on drawings which show the

location and function of different devices are

represented by standard symbols.

Line Symbols

• Transmission lines which link different instruments

are shown in Figure below

Instrument Symbols

• Instruments are identified by circles with lettered

codes (two or three letters) inserted. This lettered

code shows the instrument type and function.

• In general, the process that is to be monitored by

the instrument is indicated by the first letter of the

coding, for example: – F = Flow

– L = Level

– P = Pressure

– T = Temperature

• The second letter in the coding indicates the function of

the instrument, for example:

– FI = Flow Indicator

– FC = Flow Controller

– LA = Level Alarm

– LR = Level Recorder

– PT = Pressure Transmitter

– TE = Temperature Element

• In some cases, when the instrument is used for two

purposes or when the function of the instrument has to

be more clearly specified, a third letter is used, for


– C = Flow Indicating Controller

– AH = Level Alarm High

– AL = Level Alarm Low

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