Chemical Engineering June 2015 - 43

options are numerous, the optimum
configuration for a particular application
is not always clear. however,
there continues to be a steady shift
toward the use of RtDs in lieu of
t/cs across the board.
concerns about maintenance,
cost and accuracy are the primary
drivers in the shift to the use of RtDs
from t/cs. although, in many cases,
t/cs will have a lower initial cost
when compared to RtDs, users
may find that the value of the accuracy
and stability offered by RtDs
exceeds the initial cost savings of
installing a t/c. the shift away from
thermocouples is also reinforced by
a trend toward the use of thin-film
RtDs from wire-wound designs,
since the cost difference is much
less than with wire-wound devices.
additionally, with recent advances
in technology, users may find that
the benefits of an RtD can be obtained
at a lower price premium than
in the past.
still, applications do exist where
the use of a t/c makes sense, including
those where the process
temperature exceeds the limit of an
RtD (1,200°F), or when a very fast
response is needed. however, there
are some fast-response RtD designs
available that may also negate
the use of t/cs in the latter case.
thin-film RtDs are typically limited
to temperatures of 500°F, while wirewound
elements can withstand temperatures
up to 1,200°F. also, due
to the construction of the sensing
element, thin-film RtDs do not perform
as well in environments where
high levels of vibration or severe mechanical
shock occur.
Transmitters versus direct wiring
the majority of process temperature
measurements are still wired directly
to the control system or to the recorder
in use. this is usually done
when the controller is relatively inexpensive
and is located close to the
measurement point. transmitters
can provide functionality without direct
wiring, instead interacting with
the RtD directly, and there are several
benefits to their use with RtDs,
detailed in the sections below.
Decreasing wiring costs. standard
4-20-ma wiring is far less expensive
than RtD or t/c
wiring. this difference
can be verified by calculating
the distance
of the wire runs and
the cost of the t/c
wire, and comparing
this value with the
cost of the standard
signal wire plus the transmitter.
the cost of three- or
four-wire RtD cable makes
the decision easier as well,
although a four-wire design
allows the use of a lighter
grade of wire.
Protecting signals from
noise and grounding problems.
operating with bad
input data can cause off-specification
product and increase
material and energy usage.
transmitter circuits are designed
to minimize the impact
of noise that can affect input data.
this is particularly useful when the
temperature measurement is occurring
near electrical equipment that
may generate high radio-frequency
interference (RFi) or electromotiveforce
(emF) levels. the transmitter
also provides electrical grounding.
Reducing hardware and stocking
costs. Field sensors are connected
to the distributed control system
(Dcs) and programmable logic
controller (plc) via input and output
cards. each input/output (i/o)
card has a fixed footprint, so these
cards will often limit the number of
field-temperature devices that are
connected, since they may require
up to four wires versus two for standard
4-20-ma devices. the Dcs or
plc direct-temperature input card
is usually more expensive per point,
and sometimes has a lower density
(fewer points per card) that can increase
the overall i/o cost. using a
universal input transmitter also reduces
the variety of transmitters that
must be stocked as spares.
Enhancing accuracy and stability.
transmitters can be ranged to
only view a narrow span (say, 50
to 250°F), whereas a direct input
must be able to process the full
range of sensors used. the smaller
range also provides better resolution
and accuracy.
ChemiCal engineering www.Chemengonline.Com june 2015
Figure 2. New integrated designs
provide a thermowell, RTD, transmitter
and local LCD display
within a single device
Simplifying engineering
and preventing
miswiring.
with only standard
4-20-ma input cards
on the system, there is
less need to group and
segregate the temperature
inputs. maintenance of the
i/o system is also made easier,
and fewer i/o card spares
will be needed.
Easing future upgrades. when
a temperature instrument has
been upgraded from a t/c to
an RtD, it is much easier to
make the change when only a
short run of t/c wire needs to
be replaced.
Transmitters versus switches
For safety applications, there is a
growing recognition of the value of
using a transmitter instead of a switch.
this is because a transmitter has a
" live zero " functionality, meaning that
the 4-20-ma signal range indicates
that the unit is working at a 0% output
(4 ma). For a switch, the output possibilities
are " zero " or " one, " with the
" one " value normally indicating that
the switch's trip point has been exceeded.
a " zero " output could result
from a good measurement or a failed
switch. this means that the information
supplied by this measurement
is less reliable than that provided
by a transmitter.
the improved diagnostics available
with smart devices make this position
even more obvious. engineers are
increasingly using international standards,
such as the international electrical
commission's (iec; geneva,
switzerland; www.iec.ch) iec 61508,
iec 61511, and the international society
for automation's (isa; Research
triangle park, n.c.; www.isa.org) isa
s84 for designing their safety systems.
users even have the option to purchase
temperature transmitters that
have been tested and rated for safetyintegrity
levels (sil) 2 or 3, as required
for the process. this methodology
is recommended.
43
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Chemical Engineering June 2015

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