Chemical Engineering September 2010 - 38

Cover Story
technical Committee) industry
standard for platinum RTDs
is 0.12%, making an RTD
more than twice as accurate
as a thermocouple. RTDs also
have lead wires, but they can
be configured in two-, three- or
four-wire circuits. A two-wire
lead circuit has problems with
lead wire compensation and
should only be used for short
connections. The best solution is a fourwire
circuit, which is used when a high
degree of accuracy is required.
RTDs and thermocouples are availFIGURE
5. High density transmitters, such
as these, can accept up to eight thermocouple
or RTD inputs each, thus replacing many
individual wire runs in applications such as
columns and reactors
FIGURE 6. A wireless temperature transmitter
can be easily installed in remote or inaccessible
areas, where running wire would be
too impractical
able in various versions to suit virtually
any industrial application. As a
general rule, RTDs are more accurate
and drift less but are more expensive
than thermocouples. RTDs are useful
up to more than 600°C, while thermocouples
can go up to several thousand
degrees Celsius.
Signal conversion, transmission
The millivolt signal from a thermocouple
or the milliohm signal from
an RTD must be converted to a form
suitable for use by the control system.
Typically, this function is performed
by an analog-to-digital (A/D) converter
obtained via one of the following:
* Wiring directly to an analog input
card in a programmable controller
(PC), programmable logic controller
(PLC) or a distributed control
system (DCS)
* A multiplexer that converts it to a
digital value that can be transmitted
over a network
* A temperature transmitter, which
converts the sensor signal to a
4-20-mA signal that can be sent
over long distances on a twisted pair
of wires or wirelessly
Wiring the sensor signals directly to
a PC, PLC or DCS analog-input card
works only if the control device is
nearby. The cost of running lead wires
is extremely expensive, and long distances
can be affected by electrical
noise. The lead wires themselves act
as antennas that pick up noise.
A multiplexer is typically mounted
in a field enclosure centrally located
among several sensors. A multiplexer
might accommodate signals from dozens
of nearby sensors, with each sensor
attached to an analog input channel.
Multiplexers, along with remote terminal
units (RTUs), data acquisition
systems and data concentrators are
widely used, especially in rehabilitation
or retrofit applications where it is
too expensive to run additional sensor
wire or twisted pairs through existing
conduit or underground passages.
A temperature transmitter is an
industrially hardened device that can
be installed near the sensor, even in
hazardous areas (Figure 4). Twistedpair
copper wires (or wireless communications)
from each transmitter
convert and carry the signal from the
field to the controller. Using transmitters
can be economically justified due
to the lower cost of copper versus thermocouple
lead wire and the improved
reliability and performance of transmitters.
The recent advent of so-called
high-density transmitters combines
the multiplexing technology (the ability
to receive multiple sensor inputs)
with reliable transmitter technology
to provide an option that is both robust
and effective from a cost-per-point
basis in applications communicating
data from large quantities of sensors.
High-density transmitters complement
traditional single-point transmitters to
provide the plant designer with measurement
communications options for
different plant applications.
The choice of how to convert the signal
depends on several factors:
* Distance to the converting device
(long distances affect accuracy)
* Electrical noise in the area, which
may interfere with lead wires
* The number of nearby sensors,
which influences whether a high
density transmitter or a multiplexer
RTU can be used
In recent years, additional factors
have entered the equation. CPI decision
makers are now much more
aware of the high cost of installing
30 CHEMICAL ENGINEERING WWW.CHE.COM SEPTEMBER 2010
wires. Estimates for installing wires
currently say that parts and labor add
up to a cost of $50 to $200 per foot,
and up to $2,000 per foot in hazardous
areas. With such high costs, wiring
sensor signals directly to a controller
is rarely an option, unless the controller
is in the immediate vicinity of the
process unit.
Instead, chemical plants are turning
to transmitter technology (analog,
HART, fieldbuses and wireless)
because these methods drastically
reduce wiring costs while improving
measurement reliability.
A high-density transmitter can
save a project a considerable amount
of money, because it replaces up to
eight individual transmitters with
a single device. For example, a glass
manufacturer re-instrumented 12
vapor-deposition furnaces. Each furnace
had 16 temperature monitoring
points, so using conventional transmitters
would require 192 devices.
Instead, they installed high-density,
eight-input transmitters (Figure 5),
which accept eight sensor signals.
The savings amounted to $10,000
in transmitter costs, and $21,000 in
labor costs. The labor savings associated
with the installation arose from
reduced wiring and terminations, as
well as the transmitter's ability to
detect when a thermocouple fails or
needs to be recalibrated, eliminating
unnecessary maintenance.
Improved transmitters
Another factor entering the equation
is the increased capability of transmitters.
While they were once limited
to making the 4-20-mA conversion,
transmitters now perform a variety
of functions that make them more attractive
and cost effective.
Not too many years ago, a transmitter
was considered an expensive
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Chemical Engineering September 2010

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