• effect conveyance of, pass on,
communicate, serve as medium for
the passage of.
• involves a transmitter, a
transmission medium and a
• Combination of sensor and
conditioning device which together
enable process parameter to be
measured remotely from actual site of
• The transmitter enables Process
Variable (PV) to be obtained at the
receiving end (for display, recording or
• the general configuration of a
measurement transducer consists
of a sensing element combined with
a driving element (transmitter).
• Transducers for process measurements
convert the magnitude of a process variable
(e.g., flow rate, pressure, temperature, level,
or concentration) into a signal that can be
sent directly to the controller.
• The sensing element is required to convert
the measured quantity, that is, the process
variable, into some quantity more
appropriate for mechanical or electrical
processing within the transducer.
Standard Instrumentation Signal-Levels
• Before 1960-70, instrumentation in the process
industry utilized pneumatic (air pressure)
signals to transmit measurement and control
information almost exclusively.
• These devices make use of mechanical forcebalance elements to generate signals in the
range of 3 to 15 psig (0.2 to 1.0 kg/cm 2).
• Since about 1960-70s, electronic
instrumentation has come into widespread use.
• converts the physical variable
measured to a standard pressure
signal such as in the range 0.2 kg/cm2
to 1.0 kg/cm2 (3-15 psig).
• The pressure signal is transmitted
through pipes / tubes to gauges and
chart recorders and also used as
feedback signals in control
• internal and external leakages,
• pressure drops,
• availability of clean, dry compressed
air (at proper supply pressures),
• difficulty in transmitting pressure
signals to longer distances,
• noise, mechanical vibrations,
• stuck-up faults, etc.
• In control applications:
– quite a number of moving parts and
– hysteresis, non-linearity,
– low response times,
– difficulty in attaining repeatability of
– high frequency of service and associated
maintenance costs and inventory costs.
linearity and repeatability,
greatly simplified measurement &
• reduced operation and maintenance
• improved transmittability to greater
Analog - Digital/hybrid (SMART)
• Analog transmitters convert the process
variable measured into an equivalent
electrical signal voltage or current that is
conveyed by means of cable/wires from
measurement site to the control room for
interfacing to display or process controller.
• voltage transmitters
– wire-resistance/impedance poses a significant
problem, especially long transmission lengths
– signal attenuated along the line
– suitable only for short distances
– Typical signal ranges (corresponding to PV from
0% to 100%)
• 0 to 10V
• 0 to 5V
• -10V to +10V
• -5V to +5V, etc.
• current transmitters
– transmitter is in essence a current source
(current generator) that has very high output
– wire-resistance has no effect on the flowing
current in the current loop;
– signal ranges 0-20ma, 4-20mA, 0-50 mA and 1050 mA
– 4-20mA current signal is the industry standard.
The 4 to 20mA represents an entire span with
zero stimulus (0%) corresponding to 4mA and
maximum (100%) corresponding to 20mA.
Analog Conventional Transmitter
• A transmitter usually converts the sensor
output to a signal level appropriate for input to
a controller, such as 4 to 20 mA.
• Transmitters are generally designed to be
• In addition, most commercial transmitters
have an adjustable input range (or span). For
example, a temperature transmitter might be
adjusted so that the input range of the
platinum resistance element (the sensor) is 50
to 150 °C.
20 mA − 4 mA
Tm ( mA ) =
T − 50 oC + 4 mA
150 C − 50 C
= 0.16 o ÷T C − 4 mA
• This instrument (transducer) has a lower
limit or zero of 50 °C and a range or span of
• For the temperature transmitter discussed
above, the relation between transducer
output and input is
Four wire transmitters
• To connect sensors of relatively low resistance
such as piezo-resistors, RTDs, etc. to remotely
located interface circuits
• connecting wires resistance pose a serious
• 4-wire method allows measurement of resistance
of connecting conductors.
• 2 wires connect to current source, and another 2
wires to high input impedance voltage
• Pulse transmission
• Serial data transmission
• Instruments /devices are incorporated
with a sensing mechanism that produces
pulse output in proportion to the variable
being measured (generally, motion).
• The frequency of pulses is proportion to
• signals are normally weak, of low
amplitude, improper / non-uniform shapes,
unsuitable for transmission
• Conditioning: level shift, pulse-shaping,
• typical output peak-to-peak amplitudes 5V,
12V, 24V or bipolar ± 5V, ± 12V
• Usually output wave-shape is square, or of
fixed pulse-width, or sinusoidal;
Pulse Outputs are Popular
• frequency of pulses is proportional to
instantaneous value of process
variable - speed, flow etc.
• integration of process variable over a
period of time is simple
totalising/counting of the pulses
Switch/contact closure transmitters
• Similar to pulse transmitters; but
frequency of pulse or switch/contact
closure is very small.
• Used to convey position, level, alarm
status etc. to remote indicators,
anunciators and display systems.
• Control applications, ON/OFF, etc.
Serial data transmission
• communicates serially between Computers,
peripheral devices, Instruments, etc.
• introduced in 1962 ,
• widely used throughout the industry.
• single-ended data transmission at relatively
slow data rates (20 kBaud)
• short distances (upto 50 ft.)
TRANSMITTERS Part II
Coverage : Smart Transmitters
what is “smart” in transmitters
Benefits / Advantages
Specifying for Procurement
SMART or INTELLIGENT
The term is simply used to indicate
presence of a microprocessor.
This microprocessor in the field-device
adds new or extra features into the
device, over and above what are present
in a conventional non - microprocessor
based Process Transmitter.
WHAT FEATURES TO BE
Digital communication over the same
two wires used for analog
The digital communication is twoway between the transmitter and
the configurator - A hand-held
communicator, a microprocessor
based system or a computer.
HART Communication Protocol
• Bell-202 standard Frequency-shiftkeying (FSK)
• bit ‘1’ : 1200 Hz
• bit ‘0’ : 2200 Hz
• Transfer rate :
• 1200 bit/s
• Signal structure:
• 1 start bit
• 8 data bits
• 1 bit for odd parity
• 1 stop bit.
Software : Configuration Options
• User can select from various options
on Ranges, response, EU, Display
info, Outputs, etc….
Memory (EPROM) permits Storage
Store (and transmit when required) info such as :
– tag - for identification of transmitter
– date modified - date of last or next
calibration or installation
– message - name of person or some special
– information on flange type, flange material Oring, seal type, sensor range etc.
Sensor Characterisation Data
• Sensor linearization Coefficients or
interpolation points for different T, P
• stored in the memory (EPROM)
• Sensor behaviour at various operating
conditions is tested, and used for
• Drastically reduces drifts
Processor: Enables Computations
and Output Signal Options
• linear (for pressure, differential
pressure, level measurement. ..),
• square root (for flow measurement with
differential pressure meters),
• square root of third and fifth powers (for
flow measurement in open channels..)
• use values stored in table in the memory
to calculate the value of process
• Adjust or Change the zero / span
• Send Command from Handheld/remote
computer to the Process Transmitter
• Re-ranging can be performed without
applying reference (pressure /
temperature); uses Characterization
• Reranging done with reference will
actually be more accurate calibration.
Limits / Alarm values
high rate of change,
low rate of change , etc.
depending on Make/Model
• Set output signal to 3.5mA or 21mA
Network More than one smart
transmitter using same two-wire
Each transmitter configured a unique
Address (Non-zero) “1 to 15”.
Each can be individually read,
configured, reranged or calibrated.
Each transmitter draws, outputs 4mA Only
In conventional analog mode,
address set to “0”
• diagnostic to determine conditions of
sensor, communication line, power
supply, configurations, etc.
• helps reduce trouble shooting efforts,
• Newer transmitters can sense impulse
line plugging conditions.
Common Smart Transmitter
• Rosemount : HART (Highway
Addressable Remote Transducer)
• Honeywell : DE (Digitally Enhanced)
• Improved Safety
Reranging, calibration, etc., can be
done remotely without going to the
actual transmitter site which may be
in an hazardous or unsafe location.
• Time Savings
• Remote communication implies
facility to rerange, reconfigure, etc.
for one or more smart transmitters
using the hand-held communicator
means fewer trips to the field.
• Self - diagnostics,
implies lesser time spent for
troubleshooting, repairs etc.
• High Accuracy
• The process of analog-to-digital and
digital-to-analog conversion of the 420 signal are eliminated by the use of
• Functions like sensor output
compensation for drifts due to
changing operating conditions,
• output linearisation
• or other computations, etc.
enable high accuracy of transmitted
• Reduced Inventory
facility to rerange the transmitter
without loss of accuracy,
facility to configurate the transmitter
when using a different process
computational abilities like squareroot extraction, etc.,
imply that only one type of smart
transmitter need to be purchased or
maintained as spare for a wide
• Smart Transmitter Manufacturers:
• Foxboro, Honeywell, Moore Products,
• Rosemount, Emerson Process Management
• SMAR, ABB, Yokogawa, FUJI,
HART Communication between
master and slave
• The master sends messages with
requests for actual/specified values,
and/or any other data/parameters
available from the slave device.
• The slave interprets these
instructions as defined in the HART
• The slave responds with status
information and data for the master.
• Universal commands
• Common practice commands
• Device-specific commands
• HART follows the Open Systems
Interconnections (OSI) model of
the International Organization for
• The HART protocol uses a reduced
OSI model, implementing only
layers 1, 2 and 7
• Layer 1, physical layer
• Layer 2, link layer
• Layer 7, application layer
How do we make the
What signals can I
How do I address a
When can I send a
Plugs, sockets, cable
Voltage, current, frequency
None (point-to-point), numerical
Access rules: master-slave, tokenpassing, collision-detection
Coding: bits, characters, parity
What messages can I
What does a message
Data types: bits, integers, floating point, Applic
Function blocks, Device Descriptions