Search in the document preview
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 :
• Electrical signals can be further categorized
into • continuous
• 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.
• 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
• 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
• 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
• 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
• 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.
• Transmission lines which link different instruments
are shown in Figure below
• 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
Local or Field Mounted Instruments • To distinguish control room mounted instruments from
local or field mounted instruments, a horizontal line
drawn across the symbol diameter of the circle is used.
• A circle with lettered code only represents a field or
locally mounted instrument.
• A circle with a solid line across its diameter and the
lettered code above this line indicates a control room
panel mounted instrument.
• A circle with a dotted line across its diameter
indicates a control equipment room rack, (i.e. usually
behind the control panel), mounted instrument.