Applied Electronics & instrumentation Engg.
Haldia Institute of Technology
A transmitter is a transducer - device that
associated with a sensor and converts the
sensor output(MEASURAND) into a
standardized transmission signal.
The transmitter, a transducer usually
incorporates with various signal conditioning
and processing circuits for filtering,
amplification/scaling, linearization, isolation
etc. of the sensor signal.
In a process industry, a large number of
physical/chemical variables have to measure
for the purpose of indication, monitoring,
recording and control.
Measuring variables :
⇨ pH, conductivity, humidity, gas/liquid
composition, density, viscosity, etc.
So we have a large number of sensor measuring
different variables(order of thousands), which are
spread (or distributed) over a large area.
In normal practice, bring all the information to a
centralized location(control room) in the plant .
Total information about the status of the process
variables in the entire plant will have in a room
though the sensor is located in the field, at the
point of measurement.
we need a device that converts the sensor output
into a suitable form that can be conveyed over
large distances, to the control room.
A pneumatic transmitter translates the measured
value (from sensors) into an air pressure that is
transmitted to various receiver devices for
indication, recording, alarm, and control.
• safe and can be used in hazardous atmospheres.
• cheap (air costs nothing).
⇨ Pneumatic components are slow in response and
incompatible with modern microprocessor based
transmitter in terms of their ability to perform
sophisticated signal processing and conditioning and
their communication capability.
The detector detects the process variable and the
transducer converts this into mechanical movement
of flapper of a flapper-nozzle system present in
the pneumatic measuring unit.
It produces a pneumatic signal in form of nozzle
feedback pressure in the range 0.2-1 kg/cm2 and
the pneumatic relay amplifies this pneumatic signal
and produces an output signal in the range 0.2-1
It is then sent to the receiver pressure gauge place
in the central control loop through long PVC or SS
A pneumatic control system operates with air.
The signal is transmitted in the form of
variable air pressure (often in the range of 0.2
to 1.0 bar (3-15 psi)) that initiates the control
One of the basic building blocks of a
pneumatic control system is the flapper
nozzle amplifier. It converts very small
displacement signal (in order of microns) to
variation of air pressure.
Orifice dia- 0.25mm
Nozzle dia- 0.6mm
Typical change in pressure is 1.0 psi for a change in displacement of 0.0001 inch
∎ Constant air pressure is supplied to one end of the
pipeline. There is an orifice at this end. At the other
end of the pipe, there is a nozzle and a flapper. The
gap between the nozzle and the flapper is set by the
∎ As the flapper moves closer to the nozzle, there will
be less airflow through the nozzle and the air
pressure inside the pipe will increase. On the other
hand, if the flapper moves further away from the
nozzle, the air pressure decreases.
∎ At the extreme, if the nozzle is open (flapper is far
off), the output pressure will be equal to the
If the nozzle is blocked, the output pressure will be
equal to the supply pressure.
It is used after the flapper nozzle amplifier to enhance the
volume of air to be handled.
It can be seen that the air relay is directly connected to
the supply line (no orifice in between).
The output pressure of the flapper nozzle amplifier (p2) is
connected to the lower chamber of the air relay with a
diaphragm on its top.
The variation of the pressure p2 causes the movement (y)
of the diaphragm. There is a double-seated valve fixed on
the top of the diaphragm.
When the nozzle pressure p2 increases due to decrees
in xi, the diaphragm moves up, blocking the air vent line
and forming a nozzle between the output pressure line
and the supply air pressure line. More air goes to the
output line and the air pressure increases.
When p2 decreases, the diaphragm moves downwards,
thus blocking the air supply line and connecting the
output port to the vent. The air pressure will decrease.