A two-wire transmitter is an ideal
solution for many remote
temperature measurement
applications. Transmitters have
definite advantages over
conventional temperature
measuring devices, but must be
selected with caution in order to
avoid “ground loop” problems.
PURPOSE
In many cases, the temperature of a
remote process must be monitored.
Common temperature sensing
devices such as thermocouples and
RTD’s produce very small “signals.”
These sensors can be connected to
a two-wire transmitter that will
amplify and condition the small
signal. Once conditioned to a usable
level, this signal can be transmitted
through ordinary copper wire and
used to drive other equipment such
as meters, dataloggers, chart
recorders, computers or controllers.
OPERATION
A two-wire transmitter draws current
from a remote dc power supply in
proportion to its sensor input. The
actual signal is transmitted as a
change in the power supply current.
Specifically, a thermocouple input
transmitter will draw 4 mA of current
from a dc power supply when
measuring the lowest temperature
of the process. Then, as the
temperature rises, the two-wire
transmitter will draw proportionally
more current, until it reaches 20 mA.
This 20 mA signal corresponds to
the thermocouple’s highest sensed
temperature. The transmitter’s
internal signal-conditioning circuitry
(powered by a portion of the
4-20 mA current) determines the
temperature range that the output
current signal will represent.
Physically, only two copper wires
are necessary to connect the
transmitter output signal in a series
circuit with the remote power supply
and the process equipment. This is
made possible since the signal and
the power supply line are combined
(one circuit serves a dual function).
ADVANTAGES
Two-wire transmitters offer
numerous advantages over the
more traditional ways of measuring
temperature.
1.ac power is not needed at the
remote location to operate a twowire
transmitter.
Since transmitters
are powered by a low level 4-20 mA
output current signal, no additional
power has to be supplied at the
remote location. In addition, the
usual 24 Vdc signal necessary for
operation is standard in plants that
have large amounts of
instrumentation.
2.Electrical noise and signal
degradation are not a problem for
two-wire transmitter users.
The
transmitter's current output signal
lends itself to a high immunity when
it comes to ambient electrical noise.
Any noise that does appear in the
output current is usually eliminated
by the common-mode rejection of
the receiving device. In addition, the
current output signal will not change
(diminish) with distance as most
voltage signals do.
3.Wire costs drop significantly
when using two-wire
transmitters.
Low voltage signals
produced by thermocouples almost
always require the use of shielded
cable when they are sent any
significant distance. Ambient
electrical noise from arcing electrical
relays, motors and ac power lines
can raise havoc with these signals
that are transmitted in an
unshielded cable. In addition,
expensive, heavy gage wire is often
installed in applications that call for
long cable runs (since it reduces
errors from signal voltage drops
caused by line resistance).
Ordinary copper wire can be used to
connect all the pertinent equipment in
a two-wire transmitter system. The
4-20 mA current output signal is
relatively immune to ambient electrical
noise and is not degraded by long
distance transmission, even over a
small diameter wire. Adding a
two-wire transmitter to a system
eliminates the problem of having to
provide long runs of costly wire and an
extensive amount of shielding.
GROUND LOOP PROBLEMS
If a grounding rod was driven into the
earth at two different points and a
voltmeter was connected between
them, a voltage difference would be
detected between the two. This
difference in potential exists between
practically any two points along the
earth’s surface. When one tries to
measure a process that is at a remote
location, this voltage difference will
induce an error current along the line,
which is referred to as a “ground loop”
signal. Its result will be an error at the
display.
To prevent “ground loop” errors of this
type, select an isolating two-wire
transmitter for your system. This type
of transmitter will optoelectronically
isolate the sensor signal from the
output current loop. This will allow the
user to ground both the sensor and
one side of the current loop.
TRANSMITTER FEATURES
Transmitters provide a two-wire output
with the same wiring used for power
and output. The load resistance is
connected in series with a dc power
supply, and the current drawn from
the supply is a 4-20 mA or output
signal which is proportional to the
input signal.
Two-wire transmission permits remote
mounting of the transmitter near the
sensor to minimize the effects of noise
and signal degradation to which low
level sensor outputs are susceptible.
A rugged metal enclosure, suitable for
field mounting, offers environmental
protection and screw terminal input
and output connections. This
enclosure may be either surface or
standard relay track mounted.
Most two-wire transmitters are
linearized to the voltage signal
produced by the thermocouple or
RTD, although there are new models
now available that are linearized to the
actual temperature.
The two-wire transmitters convert the
thermocouple or RTD signal to a
4-20 mA output signal. Some models
will convert to an RS-232C output.
Transmitters are available with dip
switch selection for several
thermocouple types per model, as
well as thermocouple and RTD
selection on a single model. Two-wire
transmitters are available in either
isolating or non-isolating models, and
they also feature output ranging
adjustments with zero and span
adjustments over 80 to 100%
(depending on model) of the sensor
range.
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